11institutetext: INAF-Osservatorio Astronomico di Capodimonte, via Moiariello
16, 80131 Napoli, Italy
11email<EMAIL_ADDRESS>22institutetext: Konkoly Observatory,
HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly-Thege Miklós
út 15-17, 1121 Budapest, Hungary 33institutetext: CSFK, MTA Centre of
Excellence, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
44institutetext: ELTE Eötvös Loránd University, Institute of Physics, Pázmány
Péter sétány 1/A, 1117 Budapest, Hungary 55institutetext: Institute for
Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, A-1180 Vienna,
Austria 66institutetext: Max Planck Institute for Astronomy, Königstuhl 17,
69117 Heidelberg, Germany 77institutetext: Institut de Recherche en
Astrophysique et Planétologie, Université de Toulouse, UT3-PS, OMP, CNRS, 9
av. du Colonel Roche, 31028 Toulouse Cedex 4, France 88institutetext:
Grantecan S.A., Centro de Astrofísica de La Palma, Cuesta de San José, E-38712
Breña Baja, La Palma, Spain 99institutetext: Instituto de Astrofísica de
Canarias, Avenida Vía Láctea, E-38205 La Laguna, Tenerife, Spain
1010institutetext: INAF-Osservatorio Astronomico di Roma, via di Frascati 33,
00078, Monte Porzio Catone, Italy 1111institutetext: Max-Planck-Institut für
Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany 1212institutetext:
Scottish Universities Physics Alliance (SUPA), School of Physics and
Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
1313institutetext: MTA-ELTE Lendület ”Momentum” Milky Way Research Group,
Hungary 1414institutetext: Mount Suhora Astronomical Observatory, Cracow
Pedagogical University, ul. Podchorazych 2, 30-084 Kraków, Poland
1515institutetext: Adiyaman University, Department of Physics, 02040 Adiyaman,
Turkey 1616institutetext: Astrophysics Application and Research Center,
Adiyaman University, Adiyaman 02040, Turkey 1717institutetext: Astronomical
Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
1818institutetext: Institute of Astronomy, Faculty of Physics, Astronomy and
Informatics, Nicolaus Copernicus University in Toruń, Grudziadzka 5, 87-100
Toruń, Poland

# The Enigma of Gaia18cjb: a Rare Hybrid of FUor and EXor?
††thanks: Based on observations collected at the Large Binocular Telescope
under LBT programme IT-2019B-008. ††thanks: Based on observations collected at
the European Southern Observatory under ESO/NTT programmes 105.203T.001 and
105.203T.003. ††thanks: Based on observations collected at the Gran Telescopio
Canarias under GTC program GTC29-22B.

Eleonora Fiorellino 112233 Péter Ábrahám 22334455 Ágnes Kóspál 22334466 Mária
Kun 2233 Juan M. Alcalà 11 Alessio Caratti o Garatti 11 Fernando Cruz-Sáenz de
Miera 223377 David García-Álvarez 8899 Teresa Giannini 1010 Sunkyung Park 2233
Michał Siwak 2233 Máté Szilágyi 223344 Elvira Covino 11 Gabor Marton 2233
Zsófia Nagy 2233 Brunella Nisini 1010 Zsófia Marianna Szabó 223311111212
Zsófia Bora 223344 Borbála Cseh 22331313 Csilla Kalup 223344 Máté Krezinger
223344 Levente Kriskovics 2233 Waldemar Ogłoza 1414 András Pál 2233 Ádám Sódor
2233 Eda Sonbas 15151616 Róbert Szakáts 2233 Krisztián Vida 223344 József
Vinkó 2233 Lukasz Wyrzykowski 1717 Pawel Zielinski 1818

(Received ; accepted )

###### Abstract

Context. Gaia18cjb is one of the Gaia-alerted eruptive young star candidates
which has been experiencing a slow and strong brightening during the last 13
years, similar to some FU Orionis-type objects.

Aims. The aim of this work is to derive the young stellar nature of Gaia18cjb,
determine its physical and accretion properties to classify its variability.

Methods. We conducted monitoring observations using multi-filter optical and
near-infrared photometry, as well as near-infrared spectroscopy. We present
the analysis of pre-outburst and outburst optical and infrared light curves,
color-magnitude diagrams in different bands, the detection of near-IR spectral
lines, and estimates of both stellar and accretion parameters during the
burst.

Results. The optical light curve shows an unusually long (8 years) brightening
event of 5 mag in the last 13 years, before reaching a plateau indicating that
the burst is still on-going, suggesting a FUor-like nature. The same outburst
is less strong in the infrared light curves. The near-infrared spectra,
obtained during the outburst, exhibit emission lines typical of highly
accreting low-intermediate mass young stars with typical EXor features. The
spectral index of Gaia18cjb SED classifies it as a Class I in the pre-burst
stage and a Flat Spectrum young stellar object (YSO) during the burst.

Conclusions.  Gaia18cjb is an eruptive YSO which shows FUor-like photometric
features (in terms of brightening amplitude and length of the burst) and EXor-
like spectroscopic features and accretion rate, as V350 Cep and V1647 Ori,
classified as objects in between FUors and EXors.

###### Key Words.:

Accretion, accretion disks - Techniques: imaging spectroscopy - Stars:
formation - Stars: low-mass - Stars: protostars - Stars: pre-main sequence

##  1 Introduction

Low mass ($<2M_{\odot}$) young stellar objects (YSOs) exhibit photometric
variability in the optical and infrared bands on timescales spanning from
minutes to centuries (Carpenter et al. 2001; Megeath et al. 2012; Cody et al.
2014; Siwak et al. 2018). Their photometric variability can be caused by
changes in accretion rate, varying line-of-sight extinction, or rotating
accretion hot or cold spots (see Fischer et al. 2022, for a review).

Among variable sources, the so called eruptive young stars (EYSs) show the
largest brightness increases. Historically, we classified EYSs on the
amplitude of the brightness change. EX Lupi-type objects (EXors, Herbig 1989)
present an EXor burst of about $1-2.5$ mag within timescales ranging from
weeks up to 1 year in the framework of the magnetospheric accretion scenario.
Their spectra are very similar to Class II pre-main sequence stars (PMSs) or
Classical T Tauri stars (CTTSs). FU Orionis-type objects (FUors, Herbig 1977),
on the other hand, present a stronger brightening, of $2.5-6$ mag, and it
takes months to years to reach the peak. Spectroscopically, FUors show
absorption-line profiles, formed because the disc atmosphere is heated from
the mid-plane, and their spectral types depend on the observed wavelengths:
F$-$G type in optical and K$-$M type in near-infrared (NIR; Hartmann & Kenyon
1996; Audard et al. 2014; Connelley & Reipurth 2018; Fischer et al. 2022).
Most importantly, the nature of the FUor outburst cannot be explained by
magnetospheric accretion and it is likely to be related to other accretion
mechanisms, as for example the boundary layer accretion, where the mass
accretion rate ($\dot{M}_{\rm acc}$) is so high that shrinks the stellar
magnetic field down to the stellar radius. Whichever the mechanism and the
order of magnitude, both EXor and FUor outbursts fuel the build-up of the
protostellar mass, increasing dramatically the mass accretion rate from the
disk to the central forming star. Such strong bursts are thought to be a
possible explanation to the protostellar luminosity spread (Kenyon et al.
1990; Evans et al. 2009; Fischer et al. 2022), consisting in the fact that
YSOs observed luminosity is larger by about an order of magnitude than what
was expected by the standard steady-state collapse mode by Shu (1977).

Recent works show that some outbursting YSOs have peculiar spectroscopic
properties and light-curves that do not fit any of the aforementioned two
classical categories (see Fischer et al. 2022, and references therein).
Detailed studies of individual objects are important to understand the
physical mechanism, the evolution, and the properties of very different EYSs,
and to understand how eruptive accretion impacts the protostellar mass
building process and the interplay with the steady magnetospheric accretion
process.

In the last decade, also thanks to the whole-sky monitoring of the Gaia space
observatory and its Gaia Photometric Science Alerts
Program111http://gsaweb.ast.cam.ac.uk/alerts/home (Hodgkin et al. 2021), many
EYSs have been discovered. These new EYSs have been classified as FUors, such
as Gaia17bpi (Hillenbrand et al. 2018), Gaia18dvy (Szegedi-Elek et al. 2020),
Gaia21bty (Siwak et al. 2023), and Gaia21elv (Nagy et al. 2023); EXors, like
Gaia18dvz (Hodapp et al. 2019), Gaia19fct (Park et al. 2022), and Gaia20eae
(Cruz-Sáenz de Miera et al. 2022; Ghosh et al. 2022); and in between, such as
Gaia19ajj (Hillenbrand et al. 2019a), and Gaia19bey (Hodapp et al. 2020).

Our study of Gaia18cjb enters in this context. An alert was issued for this
source by Gaia Photometric Science Alerts Program on 2018 August 21. After the
alert, we started monitoring this source with photometric and spectroscopic
follow-up observations.

Based on information from Gaia DR2, the Wide-field Infrared Survey Explorer
(WISE), and Planck measurements, Marton et al. (2019) evaluated that the
probability of Gaia18cjb being a YSO is 83%. It is located at (R.A., Dec.) =
(06h39m07.54s, 00d08m54.49s). The Gaia DR3 measured a parallax of $p=0.83\pm
0.69$ mas, corresponding to a distance range between 660 pc and 7140 pc.
Bailer-Jones et al. (2021) proposed a prior-dominated distance estimate for
sources with large uncertainties on their parallax, finding Gaia18cjb can be
located at a distance of $d=1.03^{+0.20}_{-0.19}$ kpc. However, this
determination uses a prior for the absolute G-band which is dominated by Main-
Sequence stars. The issue of Gaia18cjb’s distance will be further discussed in
Sect. 4.1.

In this work we present the data we collected (Sect. 2), and our analysis of
these data (Sect. 3). We discuss the distance, the YSO nature of Gaia18cjb and
its outburst on the basis of our results in Sect. 4. Conclusions are
highlighted in Sect. 5.

##  2 Observations and data reduction

To probe the eruptive nature of Gaia18cjb, we collected a plethora of
photometric and spectroscopic new observations, and archival data with several
telescopes. In this section we describe the dataset and its reduction.

###  2.1 Photometry

Technical details of the optical, infrared, and archival photometric data are
listed in Table 1. In the following subsections we describe in details the
observations and the data reduction of the collected photometry.

Table 1: Summary of photometric observations of Gaia18cjb. Telescope | Dates | Camera | Bands | Pixel Scale | FoV |
---|---|---|---|---|---|---
RC80 | 2020 - 2023 | FLI PL230 CCD |  $B$,$V$,$g^{\prime}$,$r^{\prime}$,$i^{\prime}$ | 0$\aas@@fstack{\prime\prime}$55 | $18\aas@@fstack{\prime}8\times 18\aas@@fstack{\prime}8$ |
Suhora | 2021 | Apogee Aspen-47 |  $B$,$V$,$g^{\prime}$,$r^{\prime}$,$i^{\prime}$ | 1$\aas@@fstack{\prime\prime}$116 | $19\aas@@fstack{\prime}0\times 19\aas@@fstack{\prime}0$ |
ADYU60 | 2020 - 2021 | Andor iKon-M934 |  $g^{\prime}$,$r^{\prime}$,$i^{\prime}$ | 0$\aas@@fstack{\prime\prime}$673 | $11\aas@@fstack{\prime}5\times 11\aas@@fstack{\prime}5$ |
NTT | 2021 | EFOSC |  $B$,$V$,$R$ | 0$\aas@@fstack{\prime\prime}$24 | $4\aas@@fstack{\prime}1\times 4\aas@@fstack{\prime}1$ |
NTT | 2021 | SOFI |  $J$,$H$,$K$ | 0$\aas@@fstack{\prime\prime}$273 | $4\aas@@fstack{\prime}5\times 4\aas@@fstack{\prime}5$ |
GTC | 2022 | EMIR |  $Y$,$J$,$H$,$K$ | 0$\aas@@fstack{\prime\prime}$1945 | $6\aas@@fstack{\prime}67\times 6\aas@@fstack{\prime}67$ |
ZTF | 2017 - 2023 | Cryogenic CCD |  $g^{\prime}$,$r^{\prime}$ | 1$\aas@@fstack{\prime\prime}$0 | $47^{\circ}\times 47^{\circ}$ |
PanSTARRS | 2010 - 2014 | PS1 GPC1 |  $g^{\prime}$,$r^{\prime}$,$i^{\prime}$,$z$,$Y$ | 0$\aas@@fstack{\prime\prime}$11 | $7^{\circ}\times 7^{\circ}$ |
2MASS | 2000 | 3 NICMOS3 |  $J$,$H$,$K_{s}$ | 2$\aas@@fstack{\prime\prime}$0 | $8\aas@@fstack{\prime}5\times 8\aas@@fstack{\prime}5$ |
UKIDSS | 2007 - 2010 | WFCAM |  $J$,$H$,$K_{s}$ | 0$\aas@@fstack{\prime\prime}$4 | $0.21^{\circ}\times 0.21^{\circ}$ |
WISE | 2010 - 2023 | HAWAII 1RG |  $W1$,$W2$ | 2.75′′ | $47^{\prime}\times 47^{\prime}$ |
Table 2: Summary of spectroscopic observations of Gaia18cjb. Telescope | Dates | Spectrograph | Spectral Range | Resolution
---|---|---|---|---
|  |  |  $\mu$m |
LT | 2020 Nov | SPRAT | 0.4020 - 0.7994 | 350
LBT | 2021 Jan | LUCI | 0.96 - 2.44 | 2000
NTT | 2021 May | SOFI | 0.95 - 2.52 | 1000
GTC | 2022 Oct | EMIR | 1.17 - 2.37 | 4000-5000

####  2.1.1 Optical Photometry

We have been monitoring Gaia18cjb since October 2020 with an approximately
monthly cadence using the 80 cm Ritchey–Chrétien (RC80) telescope at the
Piszkéstető mountain station of Konkoly Observatory (Hungary). The telescope
is equipped with an FLI PL230 CCD camera and Johnson $BV$ and Sloan
$g^{\prime}r^{\prime}i^{\prime}$ filters. The aperture photometry for
Gaia18cjb was obtained using 40 comparison stars located within a
$12^{\prime}\times 12^{\prime}$ box around our science target. We used an
aperture radius of 5 pixels ($2\aas@@fstack{\prime\prime}$75) and sky annulus
between 20 and 40 pixels ($11^{\prime\prime}$ and $22^{\prime\prime}$). The
instrumental magnitudes were converted to the standard system using the APASS9
magnitudes (Henden et al. 2016) of the comparison stars and fitting a linear
color term.

Gaia18cjb was also observed on three nights between 2021 January and February
at the Mount Suhora Observatory of the Cracow Pedagogical University (Poland)
with the 60 cm Carl–Zeiss telescope equipped with an Apogee Aspen-47 camera
(Johnson $BV$ and Sloan $g^{\prime}r^{\prime}i^{\prime}$ filters). We obtained
aperture photometry using the same aperture radius and sky annulus (in
arcseconds) as for the RC80 data, and same 40 comparison stars for the
photometric calibration.

Further observations were collected on six nights between 2020 November and
2021 February at Adiyaman University using ADYU60 Application and Research
Center using ADYU60, a PlaneWave 60 cm f/6.5 corrected Dall–Kirkham Astrograph
telescope. This telescope is equipped with an Andor iKon-M934 camera (Sloan
$g^{\prime}r^{\prime}i^{\prime}$ filters). Aperture photometry and calibration
was done the same way as described above.

On 2021 May 9, we further observed Gaia18cjb with the 3.6 m New Technology
Telescope (NTT) located in La Silla Observatory (Chile). We used the ESO Faint
Object Spectrograph and Camera (EFOSC, Arnaboldi et al. 2016, Bessell $BVR$
filters). Aperture photometry was obtained as above. For the photometric
calibration, we used five stars from the set above that were included in the
smaller FoV of the detector without fitting a color term. We calculated their
Bessell $R$ magnitudes by interpolating in their spectral energy distribution
using their APASS9 $BVg^{\prime}r^{\prime}i^{\prime}$ fluxes.

In all the images that we acquired, the target looked point-like and there
were no issue with extented emission.

####  2.1.2 Infrared Photometry

On 2021 May 9, we observed Gaia18cjb with the ESO New Technology Telescope
(NTT) infrared spectrograph and imaging camera, i.e. SofI (Arnaboldi et al.
2016). The data reduction of the photometry was done as prescribed by SOFI
manual222https://www.eso.org/sci/facilities/lasilla/instruments/sofi/tools/reduction.html.
We first constructed a sky frame by taking the median of the 5 dither
positions after scaling them to the same mean level. The flatfield frame was
constructed by subtracting a lamp off image from a lamp on frame and
correcting for the residual shade pattern. The resulting flat frames show
vertical stripes on the left hand quadrants. To remove the stripes in the SW
quadrant we applied an empirical correction, by taking the horizontal lines
one-by-one from the affected upper-right quadrant, subtracted from each 512
pixel long horizontal line its median value; and added back to it the median
of the remaining three quadrants of the whole image. After that, we divided
the sky-subtracted and stripe-corrected frames by the flatfield. From the 5
dither positions we constructed a mosaic per filter. Finally, we performed
photometry on the mosaics. We used a 9 pixel radius aperture, with a sky ring
between 7.8′′ and 10.4′′. For photometric calibration we used 30 2MASS sources
with quality flag ‘A’.

We also obtained $JHK$-band photometry of Gaia18cjb on 2022 October 10th and
26th using the 10.4m Gran Telescopio Canarias (GTC) installed at the Spanish
Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de
Canarias, on the island of La Palma, with the wide field Espectrógrafo
Multiobjeto Infra-Rojo (EMIR) Imager (Garzón et al. 2022). The images were
reduced using dedicated python routines customised by the GTC staff for EMIR
photometric data. The images were flat-fielded and the sky background was
eliminated. The astrometric solution was also calculated. The final reduced
image is the average of all the available images for each filter. We performed
photometry on the reduced data using the same aperture selected for SOFI
photometry (2.6′′). For photometric calibration we used 30 2MASS sources with
quality flag “A”.

####  2.1.3 Archival Photometry

We complemented our monitoring data with public domain photometry data. For
optical bands we used Gaia G-band from the Gaia Photometric Science Alerts
database, the Zwicky Transient Facility (ZTF, Masci et al. 2019) DR17
$g^{\prime}$\- and $r^{\prime}$-band from the ZTF archive and the Panoramic
Survey Telescope & Rapid Response System (PanSTaRRS).

We used the ZTF data with “catflags = 0”, i.e., perfectly clean extracted, to
filter out bad-quality images. We downloaded the multi-epoch 2010-2014 Pan-
STaRRS images obtained in $g^{\prime}r^{\prime}i^{\prime}zY$
bands333http://ps1images.stsci.edu/cgi-bin/ps1cutouts. We performed aperture
photometry of Gaia18cjb using only the good quality images in 2.5′′ aperture.
Then, to calibrate the results to absolute magnitudes, we used the magnitude
zero points provided in the headers for each image and filter. While this
approach may result in increased light losses during poor seeing conditions,
we estimated that it does not exceed 0.2 mag in the $g$-band, and $0.05-0.1$
mag for the other bands, which is sufficient for checking the light
variability history before Gaia. As a consequence, we use 0.2 dex of
uncertainty for the $g^{\prime}$-band photometry, and 0.1 dex for the other
bands photometry. In addition, the smaller aperture increases the number of
useful images, as their certain fraction suffers from detector imperfections.
We compare our results to APASS9 $g^{\prime}r^{\prime}i^{\prime}$ magnitudes
of stars in the same field determined by Henden et al. (2016) finding that
their measurements are reproduced with our method within 0.05 mag.

For infrared wavelengths, we used $JHK_{S}$ photometric results from the Two
Micron All Sky Survey (2MASS, Cutri et al. 2003), and downloaded processed
$JHK_{S}$ images at two epochs from the UKIRT Infrared Deep Sky Survey
(UKIDSS, Lucas et al. 2008). For UKIDSS images, in order to obtain results
consistent with our SOFI and GTC infrared photometry results (Sect. 2.1.2), we
performed aperture photometry using the same aperture and sky radii (in
arcseconds) as for the SOFI and GTC data. Photometric calibration was
performed via comparison with 15–25 2MASS sources, having quality flag ‘A’. We
also downloaded 3.35$\,\mu$m (W1) and 4.60$\,\mu$m (W2) photometry from the
AllWISE Multiepoch Photometry Table and the NEOWISE-R Single Exposure (L1b)
Source Table (Wright et al. 2010; Mainzer et al. 2011, 2014), available at the
NASA/IPAC Infrared Science
Archive444https://irsa.ipac.caltech.edu/Missions/wise.html. After filtering
out bad photometry, e.g., due to smeared images, images affected by the South
Atlantic Anomaly or by scattered light from the moon, we averaged the
magnitudes taken in each observing season (WISE scans the sky and takes a few
frames of a certain area once every 6 months). Saturation correction was not
necessary.

###  2.2 Spectroscopy

To probe the eruptive nature of Gaia18cjb, we observed it in the visible in
one epoch and in the near-IR in three epochs, with different telescopes.
Technical details on the spectroscopic data are listed in Table 2.

####  2.2.1 LT SPRAT

On 2020 November 11 we observed Gaia18cjb in the visible at low resolution
(R=350) with the SPectrograph for the Rapid Acquisition of Transients (SPRAT,
Piascik et al. 2014), mounted on the Liverpool Telescope (LT) 555ProgID:
XOL20B01, PI: Pawel Zielinski. The spectrum was reduced and calibrated to
absolute flux units by means of the dedicated SPRAT pipeline. The low
resolution and high noise of the spectrum prevent us from using it for any
quantitative analysis, however, it clearly shows the presence of the H$\alpha$
line in emission (see Figure 1).

Figure 1: H$\alpha$ line of Gaia18cjb from LT SPRAT spectrum.

####  2.2.2 LBT LUCI

We observed Gaia18cjb at the Large Binocular Telescope (LBT Observatory,
Arizona) with LUCI1 and LUCI2 intruments in the $zJ$ and $HK$ filters on 2021
January 17. We reduced the data by using the SIPGI pipeline (Gargiulo et al.
2022), following the recipes indicated in the SIPGI
documentation666http://pandora.lambrate.inaf.it/docs/sipgi/ for the flat field
correction, sky subtraction, wavelength calibration, and telluric correction.
We then averaged the LUCI1 and LUCI2 spectra, to maximize the signal-to-noise
ratio (SNR). Since we do not have contemporary $zJHK$ photometry, we flux
calibrate the spectrum using SOFI continuum as both optical and NIR light
curves suggest that the photometry does not vary much (e.g. $\Delta m_{r}=0.2$
mag and $\Delta m_{W1}=0.1$ mag) between January and May 2021 (see Figure 2).

Figure 2: The visible (top) and infrared (bottom) light curves of Gaia18cjb.
The red circle highlights the Gaia Alert trigger. The error bars smaller than
the symbol size are not presented. Grey dashed lines correspond to the epochs
for which we collected NIR spectroscopy.

####  2.2.3 NTT SOFI

The blue part (0.95 – 1.64$\,\mu$m) of NTT/SOFI spectrum was observed on the
2021 May 5 and 8, while the red part (1.53 – 2.52$\,\mu$m) only on the 8th.
For the observed data, flat-fielding, bad pixel removal, sky subtraction,
aperture tracing, and wavelength calibration were applied with the Image
Reduction and Analysis Facility (IRAF, Tody 1993). For the wavelength
calibration, a Xenon spectrum was used. The hydrogen lines in the telluric
standard stars were removed by Gaussian fitting. The spectra were flux
calibrated using the contemporary photometry (see Sect. 2.1). Technical
details are reported in Tab. 2 and spectra are presented in Sect. 3.

####  2.2.4 GTC EMIR

We obtained $JHK$-band spectroscopy of Gaia18cjb with the GTC/EMIR medium-
resolution multi-object spectrograph (Espectrógrafo Multiobjeto Infra-Rojo,
Garzón et al. 2022) in long slit mode on 2022 October 26. EMIR is equipped
with a 2048$\times$2048 Teledyne HAWAII-2 HgCdTe NIR-optimised chip with a
pixel size of 0.2$\arcsec$. The total exposure time on source was 1920 seconds
per grism. One grism per each band ($J$, $H$ and $K$) was used. A typical ABBA
nodding pattern was applied. The seeing during the observations was
FWHM$\sim$1.2$\arcsec$.

The spectra were reduced using several python routines customised by GTC staff
for EMIR spectroscopic data. The sky background was first eliminated using
consecutive A-B pairs. They were subsequently flat-fielded, calibrated in
wavelength and combined to obtain the final spectrum. To correct for telluric
absorption, we observed a telluric standard star with the same observing set
up as the science target, right after the Gaia18cjb observations and at
similar airmass. To apply the correction we used a version of Xtellcor (Vacca
et al. 2003) specifically modified to account for the atmospheric conditions
of La Palma observatory (Ramos Almeida et al. 2009). We calibrated the flux
scale of GTC spectra using the GTC contemporary photometry, see Sect. 2.1.2.

##  3 Results

Our photometric results, used for the optical and infrared light curves (see
Sect. 3.1), are shown in Appendix A. Uncertainties are computed as the
quadratic sum of the uncertainty of the aperture photometry, the scatter of
the individual exposures, and the uncertainty of the photometric calibration.

Normalized and flux calibrated spectra are presented in Sect. 3.4 and Appendix
B, respectively. They show emission lines typical of YSOs such as
magnetospheric accretion features (HI), winds/outflows (H2), and CO/NaI from
the disk.

###  3.1 Light curves

Figure 2 shows the optical (top panel) and IR (bottom panel) light curve of
Gaia18cjb.

Figure 3: Color–magnitude diagrams of Gaia18cjb based on data from PanSTaRRS
data (yellow filled circles), ZTF, Konkoly, Mt Suhors, and WISE data before
LUCI epoch (orange filled circles), and after LUCI epoch (blue filled
circles). The arrows show the the reddening vector, assuming extinction law
from Cardelli et al. (1989) with $R_{V}=3.1$ (black arrow) and $R_{V}=5.5$
(red arrow), respectively.

####  3.1.1 Optical Light Curve

The optical light curve shows the Gaia $G-$band brightening event of about 4.6
mag from 2015 February to 2020 May (black stars). Afterwards, the brightness
still increased, although slower, by only 0.3 mag, up to the last epoch in
March 2023. The overall brightening in the Gaia light curve is of about 4.9
mag. The outburst is still ongoing, and there is no indication that the peak
of the outburst has been reached. However, it looks like Gaia18cjb has reached
a constant plateau in 2022-23. The photometric data, collected from other
observatories after 2016, cover timescales shorter than Gaia. They confirm the
brightening and the current “plateau” phase. Indeed, the $r-$band data (orange
stars, filled circles, triangles, diamonds, and crosses, depending on the
telescope) show a strong increase of brightness consistent with the Gaia data
acquired with the $G$ filter. The variability within 13 years is $\sim$5.0
mag, a typical value for FUors variability (Fischer et al. 2022). However, the
rise time (several years) is atypical for such kind of objects, although there
are FUors-like objects whose light curve has been increased for decades (e.g.
V1515 Cyg, Hartmann & Kenyon 1996; Szabó et al. 2022). The ZTF $g-$band shows
brightening of 2.8 mag within 4.75 years, and only of 0.5 mag after 2021
January (LUCI Epoch). NTT, RC80, Suhora, and ADYU60 light curves are in
agreement within each other when the filter is the same and confirm the fact
that since 2020 May the brightness of Gaia18cjb is still increasing but with a
smaller amplitude than before (about 0.1 mag) in the $i-$, $r-$, $V-$, $g-$,
and $B-$ bands.

PanSTaRRS monitored Gaia18cjb prior to Gaia in $g-$, $r-$, and $i-$ bands from
2010 to 2013. During these years the variation in $r-$ band is only 0.65 mag,
suggesting a quiescent phase before the on-going outburst. The same behavior
is also seen in other bands. We note that the last data points from PanSTaRRs
$r-$band are brighter than the first data points of Gaia, suggesting the
presence of some possible pre-burst variability. We also note that PanSTaRRS
$r$ and $g$ variability is larger in a certain epoch (May 2012) than in the
overall three years. This can be a hint of ordinary small-amplitude accretion
events as seen in CTTSs (see e.g. Fischer et al. 2022, and references
therein).

####  3.1.2 Infrared Light Curve

We studied the variability also in the infrared using WISE, UKIDSS, and
PanSTaRRS archival data together with our acquired photometry (bottom panel of
Figure 2). The W1 channel at 3.4 $\mu$m (yellow filled dots) varies by 0.6
mag, while the variability is only 0.4 mag at 4.6 $\mu$m (W2 channel, blue
dots) from 2010 March to 2022 October. The variability of both W1 and W2
channels reduced down to 0.1 since 2021 January. The W2 variability does not
seem to follow the variability we see in the optical bands and it waves around
a mean value between the pre-burst phase and the burst phase. We cannot
confirm this trend in the W1 band since there were no W1 observations in the
pre-burst phase. The IR light curve is similar to V1057 Cyg, one of the
classical FUors (Szabó et al. 2021) after 1995 showing some kind of minor
modulation.

The variability in the near-IR between UKIDSS (2007 April) and SOFI (2021 May)
observations decreases with increasing wavelength: 2.70 mag, 2.14 mag, and
1.35 mag in $J$, $H$, and $K$ bands, respectively. Interestingly, PanSTaRRS
light curves, observed from 2010 to 2014, invert the trend: in the $Y-$band
the variability is larger (0.89 mag) than in the $z-$band (0.39 mag). This can
be explained by the large variability in the first epoch (2010 February),
which can be due to the presence of a small burst or to a sudden increase of
the light curve, as it is often the case before a strong FUor-like burst
(Audard et al. 2014). However, this could also be the result of the different
sampling for the PanSTaRRS filters and, in turn, not being associated with the
intrinsic variability of the star.

We highlight that the photometric variability in the infrared is moderate in
general and it does not reflect the strong burst we observe in the visible.
Indeed, the infrared variability is as small as in typical non eruptive
sources (see, for example, Lorenzetti et al. 2012; Zsidi et al. 2022;
Fiorellino et al. 2022). However, this result is puzzling since it suggests
that the burst is restricted to the star and its immediate circumstellar
environment and does not affect the dusty disk at all. We will discuss
possible interpretations of this result in Sect. 4.

###  3.2 Color-magnitude diagrams

To investigate the photometric properties of Gaia18cjb in the visible and
infrared bands, we built color–magnitude diagrams which are in Figure 3. The
top-left panel corresponds to the $g^{\prime}$ versus $g^{\prime}-r^{\prime}$
diagram. It shows extinction related to color variations, and a linear
decreasing trend. This trend suggests that $g^{\prime}$\- and
$r^{\prime}$-band variations are correlated, but $g^{\prime}$-band variation
is larger than $r^{\prime}$-band variations (see light curves on Figure 2). A
similar trend is also present in the infrared color-magnitude diagram in the
bottom-right panel, where W1 and W1$-$W2 strongly correlate following the
direction of the reddening vectors. It appears that the source is redder when
fainter, hence the downward trend in the color–magnitude diagram. The top-
right panel and the bottom-left panel present the $V$-band photometry versus
$V-i^{\prime}$ and $V-r^{\prime}$, respectively. In both cases, the
distribution appears to be vertical, suggesting that the variability amplitude
is similar in the $V$ and $i^{\prime}$ and in the $V$ and $r^{\prime}$ bands.
In all the color-magnitude diagrams in Figure 3 we note a temporal trend: the
variation of the colors decreases with time as the source brightens. This
produces a trend on the opposite direction of the reddending vectors. In
general, the difference between the reddening law with $R_{V}=3.1$ (black
arrow) and $R_{V}=5.5$ (red arrow) is small, and it is hard to distinguish
which one of the two is followed better from the data points. In the $g$ vs.
$g-r$ plot this difference is more evident and the black arrow better
reproduces the variation of extinction from 2018 to 2020 (orange-blue filled
dots), while the steeper slope of the red arrow better reproduces $A_{V}$
variations in the quiescent phase and in the latest epochs (from 2021 onward,
yellow-blue filled dots). We discuss possible interpretations of this result
in Sect. 4.

Figure 4: NIR color–color diagram of Gaia18cjb. Purple and red circles are
pre-outburst data from 2MASS and UKIDSS observations in 2000 and 2010,
respectively, while the brown and pink dots correspond to SOFI and GTC
photometry in the burst phase (2021 and 2022, respectively). The blue dashed
line and light blue region represent the CTTS locus and its uncertainty (Meyer
et al. 1997), and the tiny blue dotted line its extension toward larger color
indices. The orange solid and green dotted–dashed lines correspond to the
colors of zero-age main sequence and giant branch stars, respectively. The
black arrow shows the reddening vector for a source with 1 mag of extinction
(Cardelli et al. 1989, $R_{V}=3.1$).

Figure 4 shows the $J-H$ vs. $H-K$ diagram, which can be used to classify a
YSO and to estimate its extinction. We used UKIDSS and 2MASS archival data and
the newly acquired SOFI and GTC data points. We also plot in the same figure
the giant branch locus (green dot-dashed line), the Zero Age Main Sequence
(ZAMS, orange line), and the Classical T Tauri Stars (CTTS) locus (Meyer et
al. 1997), where CTTS are not extincted ($A_{V}=0$ mag, blue dashed line). By
following the extinction vector by Cardelli et al. (1989) and $R_{V}=3.1$
(black arrow), it is possible to compute the extinction of a CTTS. The 2MASS
and UKIDSS data of Gaia18cjb lie within the CTTS locus, suggesting negligible
extinction, while recent data points lie below the CTTS locus, making not
possible to compute its extinction with this method and suggesting that
Gaia18cjb is less evolved than a typical CTTS. However, in the case of GTC
data, considering the error bars, it is possible that the Gaia18cjb colors lie
on the CTTS locus, suggesting low extinction. This movement along the CTTS
locus from redder to bluer is similar to what was found in Kóspál et al.
(2011, see Figure 7 therein) for HBC 722. In general, it is known that highly
accreting stars become bluer in burst with respect to quiescence (see e.g.
Lorenzetti et al. 2007, 2011; Hillenbrand et al. 2018; Szegedi-Elek et al.
2020), suggesting the accreting nature of Gaia18cjb.

###  3.3 Spectral Energy Distribution

Figure 5: Spectral Energy Distribution. Circles and stars are observations
taken before and after 2020, respectively.

From an observational point of view, YSOs are usually classified on the basis
of their spectral energy distribution (SED, Lada & Wilking 1984; Lada 1987;
André 1995; Greene et al. 1994). Figure 5 shows the SED of Gaia18cjb, in the
low brightening state (dots, before 2020), resembling the SED of an embedded
object as a Class I YSOs, and in the outburst state (after 2020, star
symbols), which is similar to a Flat Spectrum source. The data we used to
build up the SEDs were observed at different periods and are listed in
Appendix A.

Note that, based on the light curve, it is challenging to determine when the
outburst begun. The Gaia alert was triggered in 2018, but the luminosity had
already increased since 2015 (when first Gaia data are available), while the
PanSTARRS monitoring between 2010 and 2014 does not provide definitive
conclusions about the quiescent phase of this object, even if we can
tentatively assume the average PanSTARRS magnitudes as values for the
quiescent phase ($m_{g}=21.8\pm 0.4$, $m_{r}=20.0\pm 0.2$, $m_{i}=19.1\pm
0.1$, $m_{z}=18.5\pm 0.1$, and $m_{Y}=18.2\pm 0.2$). However, we distinguish
here between the first part of the optical light curve, where the luminosity
increases by 4.6 mag in about 10 years, and the second part, after LUCI epoch,
where the luminosity moderately increases by only 0.3 mag in three years (see
the top panel of Figure 2 and Sect. 3.1). We refer to the first one as the
brightening or low state phase and to the latter one as the bursting or high
state phase. We stress that this nomenclature is only useful to distinguish
between the amplitude in the variability while in both the phases the burst is
on, with the luminosity increasing.

We computed the spectral index
$\alpha=\frac{d\log(F_{\lambda}\lambda)}{d\log(\lambda)}$ between 2.16 $\mu$m
and 22.1 $\mu$m using 2MASS and WISE data, respectively. For the pre-outburst
phase, we obtain $\alpha=0.55\pm 0.28$, which classifies Gaia18cjb as a Class
I protostar ($\alpha>0.3$). We note that the spectral index is also compatible
with a Flat Spectrum object ($-0.3<\alpha<0.3$) within the error. We conclude
Gaia18cjb is most likely a Class I YSOs, with a small chance that it is a Flat
Spectrum. We also note that the classification we use computes the spectral
index using $\lambda F_{\lambda}$ at 2 $\mu$m and 24 $\mu$m (not 22 $\mu$m as
we do) but, based on the SED shape, we do not expect a large difference
between the flux at 22 $\mu$m and at 24 $\mu$m.

We also computed the spectral index using SOFI $K-$band and WISE W4 data, to
get an estimate of $\alpha$ for the eruptive phase. SOFI observations were
taken during the eruptive phase, while WISE observations are not, but since
the low variability at long wavelengths and the lack of observations around 20
$\mu$m during the outburst, we use the same value as we did considering the
brightening SED. We obtain $\alpha=0.09\pm 0.02$, compatible with a Flat
Spectrum.

In both the low level and high level phases, the spectral index we compute
does not describe Gaia18cjb as a Class II PMS young star. In the scenario
where the empirical classification of YSOs based on the SED spectral index
corresponds to an evolutionary classification, Gaia18cjb seems to be in
between the protostellar phase, where the star is embedded in its envelope and
the accretion is very high (Class 0 and I YSOs), and the pre-main sequence
phase, where most of the stellar mass is accessed and the star is approaching
the ZAMS (Class II and III YSOs). Estimates of Class 0 lifetimes are between
0.2 and 0.6 Myr, while, based on Spitzer timescales, lifetimes of Class I and
Flat Spectrum YSOs are 0.54 Myr and 0.40 Myr, respectively (Evans et al.
2009). Thus, if the classification based on SED spectral index corresponds to
evolutionary stages and our classification is correct, we can assume for
Gaia18cjb an age of about 1 Myr.

###  3.4 Emission Lines

Figure 6: Normalized LBT zJHK normalized spectrum. Emission lines reported in
Table 3 are labelled.

Figure 7: Normalized SOFI spectrum. Emission lines reported in Table 3 are
labelled.

Figure 8: Normalized GTC JHK spectrum. Emission lines reported in Table 3 are labelled. Table 3: Equivalent width of Gaia18cjb emission lines for the three epochs we observed. The equivalent width unit is Å. Line | $\lambda$ | W${}_{eq}^{\rm LUCI}$ | W${}_{eq}^{\rm SOFI}$ | W${}_{eq}^{\rm GTC}$
---|---|---|---|---
| [$\mu$m] |  |  |
Pa$\beta$ | 1.281 | 18.41$\pm$6.22 | 14.62$\pm$1.66 | 21.99$\pm$3.01
${\rm Pa\gamma}$ | 1.093 | 5.99$\pm$0.96 | 7.20$\pm$1.98 | $-$
${\rm Pa\delta}$ | 1.004 | 4.58$\pm$0.97 |  $<$2.01 | $-$
${\rm Br\gamma}$ | 2.166 | 11.90$\pm$1.36 | 10.49$\pm$3.08 | 10.51$\pm$0.88
Br10 | 1.736 | 3.89$\pm$0.66 | $<3.93$ | 3.12$\pm$0.7
Br11 | 1.681 | 4.1$\pm$1.12 | $<6.99$ | 2.06$\pm$0.56
Br12 | 1.641 | 4.83$\pm$1.33 | $<4.77$ | 1.61$\pm$0.2
Br13 | 1.611 | 2.17$\pm$0.56 | $<8.94$ | 1.14$\pm$0.43
Br14 | 1.588 | 3.20$\pm$1.12 | $<1.60$ | 2.77$\pm$0.31
Br15 | 1.570 | $<2.90$ | $<2.07$ | 1.08$\pm$0.12
Br16 | 1.556 | $<3.81$ | 1.16$\pm$0.37 | 1.41$\pm$0.16
Br17 | 1.544 | $<2.28$ | $<2.31$ | 1.04$\pm$0.27
Br18 | 1.534 | $<4.87$ | 1.80$\pm$0.42 |  $<$2.73
Fe I | 1.188 | 1.02$\pm$0.15 | $<5.39$ | 2.3$\pm$0.36
Fe I | 1.197 | 2.84$\pm$0.97 | $<8.05$ | 2.06$\pm$0.65
Ca II | 1.195 | $<2.76$ | $-$ |  $<$1.05
Si I | 1.203 | $<0.67$ | $<7.39$ | 1.71$\pm$0.64
Mg I | 1.502 | $-$ | $<1.61$ | $-$
Mg I | 1.504 | $-$ | 3.45$\pm$1.11 | $-$
Mg I | 1.576 | $<2.72$ | $<10.94$ | 0.96$\pm$0.18
He I | 2.033 | $-$ | $<0.85$ | 1.73$\pm$0.51
Ca I | 2.101 | $<1.35$ | 4.20$\pm$1.10 | 0.57$\pm$0.22
Na I | 2.206 | 2.08$\pm$0.43 | 2.21$\pm$0.49 |  $<$1.35
Na I | 2.208 | 2.18$\pm$0.40 | 2.21$\pm$0.49 | 0.92$\pm$0.32
H2 | 2.127 | 3.59$\pm$0.71 | 2.71$\pm$0.57 | 3.57$\pm$0.45

The spectra of young solar-type stars show many features tracing the star-disk
interaction as, for example, accretion/ejection. Our NIR spectra of Gaia18cjb
covers several emission lines, and in particular some accretion tracers, such
as $\mbox{Br}\gamma$ and $\mbox{Pa}\beta$. We describe here how we have
measured the fluxes of these lines to be used for the analysis.

We measured the equivalent width ($W_{eq}$) and the observed line flux
($F_{obs}$) from the normalized and flux calibrated spectra, respectively, as
follows. We fitted a linear curve to the local continuum in a wavelength range
of $\Delta\lambda=2\,nm$, centered on the emission line wavelength
$\lambda_{0}$. We slightly modified this range, if needed, taking the one most
suitable for each line; for example, by avoiding other emission lines, if
present, or telluric absorption lines. The line flux was determined by
subtracting the local continuum from the spectra and integrating it over the
line. We computed the noise of the line by multiplying the standard deviation
of the local continuum ($rms$) for the wavelength element between two pixels
$\Delta\lambda$, and multiplying this by the square root of the number of
pixels within the wavelength range ($N_{pix}$). We considered a line to be
detected when its $S/N>3$. For those lines that were detected in at least one
epoch, we estimated the upper limits in the other epochs as three times the
noise:

$F_{line}^{upp}=3\times(\sqrt{N_{pix}}\times rms\times\Delta\lambda)$ (1)

For LUCI spectra, the flux calibration with contemporary photometry was not
possible. Therefore, we computed the observed flux by multiplying the
equivalent width of each line by its local continuum in the SOFI spectrum. We
stress that the acquisition of the LUCI spectrum was contemporaneous to that
of the SOFI data. Equivalent widths and fluxes are reported in Tabs. 3 and 4,
respectively.

LBT spectrum: 2021 January. Figure 6 reports the LBT normalized spectrum and
the emission lines detected. This spectrum shows several accretion tracers
such as Pa$\beta$, Pa$\gamma$, Pa$\delta$, Br$\gamma$, Br10, Br11, Br12, Br13,
Br14, and the Na I doublet. The H2 line in emission suggests the presence of
some shocked material in ejection, as winds. We also detect Fe I in emission.

SOFI spectrum: 2021 May. Figure 7 shows the SOFI normalized spectrum. It
confirms the presence of accretion (Pa$\beta$, Pa$\gamma$, Br$\gamma$, Br16,
Br18, Na I doublet) and ejection (H2) tracers. Emission lines of Mg I and Ca I
are also detected.

GTC spectrum: 2022 October. The GTC spectrum shows up to 9 lines of the
Brackett series, but, due to the restricted wavelength range in the blue part
of the spectrum, only the brightest line of the Paschen series (Pa$\beta$ see
Figure 8). Fe I, Mg I, Ca I, and Na I lines are detected. The H2 emission is
confirmed.

Table 4: Observed fluxes of the emission lines in the Gaia18cjb spectra. The
observed flux unit is $10^{-14}$ erg s-1 cm-2.

Line | $\lambda$ | F${}_{obs}^{\rm LUCI}$ | F${}_{obs}^{\rm SOFI}$ | F${}_{obs}^{\rm GTC}$
---|---|---|---|---
| [$\mu$m] |  |  |
Pa$\beta$ | 1.281 | 1.08$\pm$0.07 | 1.96$\pm$0.16 | 0.664$\pm$0.046
${\rm Pa\gamma}$ | 1.093 | 0.73$\pm$0.12 | 0.72$\pm$0.10 | $-$
${\rm Pa\delta}$ | 1.004 | 0.57$\pm$0.12 |  $<$0.25 | $-$
${\rm Br\gamma}$ | 2.166 | 0.88$\pm$0.10 | 1.05$\pm$0.11 | 0.680$\pm$0.045
Br10 | 1.736 | 0.30$\pm$0.07 |  $<$0.25 | 0.078$\pm$0.016
Br11 | 1.681 | 0.60$\pm$0.08 |  $<$0.29 | 0.051$\pm$0.013
Br12 | 1.641 | 0.49$\pm$0.18 |  $<$0.29 | 0.041$\pm$0.005
Br13 | 1.611 | 0.21$\pm$0.05 |  $<$0.43 | 0.029$\pm$0.01
Br14 | 1.588 | 0.32$\pm$0.11 |  $<$0.40 | 0.072$\pm$0.007
Br15 | 1.570 | $<$ | $-$ | 0.029$\pm$0.003
Br16 | 1.556 |  $<$0.36 | 0.12$\pm$0.04 | 0.037$\pm$0.004
Br17 | 1.544 |  $<$0.37 |  $<$0.33 | 0.028$\pm$0.007
Br18 | 1.534 |  $<$0.37 | 0.20$\pm$0.05 |  $<$0.07
Fe I | 1.188 | 0.08$\pm$0.01 |  $<$0.14 | 0.074$\pm$0.01
Fe I | 1.197 | 0.35$\pm$0.12 |  $<$0.84 | 0.068$\pm$0.018
Ca II | 1.195 |  $<$1.02 |  $<$0.80 | 0.032
Si I | 1.203 |  $<$0.25 |  $<$0.80 | 0.055$\pm$0.019
Mg I | 1.502 | $-$ |  $<$0.56 | $-$
Mg I | 1.504 | $-$ | 0.39$\pm$0.13 | $-$
Mg I | 1.576 |  $<$0.85 |  $<$3.44 | 0.026$\pm$0.005
He I | 2.033 | $-$ |  $<$0.54 | 0.118$\pm$0.032
Ca I | 2.101 |  $<$0.27 | 0.24$\pm$0.05 | 0.037$\pm$0.040
Na I | 2.206 | 0.18$\pm$0.04 | 0.19$\pm$0.04 |  $<$0.087
Na I | 2.208 | 0.18$\pm$0.03 | 0.19$\pm$0.04 | 0.062$\pm$0.021
H2 | 2.127 | 0.25$\pm$0.05 | 0.67$\pm$0.18 | 0.232$\pm$0.026

> Notes. Observed fluxes are not corrected for the extinction. GTC and SOFI
> lines flux is computed from the flux calibrated spectra, while LUCI lines
> flux is computed from the $W_{eq}$ using the SOFI continuum for that line.

##  4 Discussion

In this section we discuss evidences supporting that Gaia18cjb is a YSO, and
put into context its accretion properties with respect to other erupting YSOs.

###  4.1 On the distance of Gaia18cjb

Measuring the distance of an unknown object is a complex task. In particular,
the case of Gaia18cjb presents its own set of challenges, given the large
parallax uncertainty derived by Gaia. In this session we discuss strenghts and
limitations of the photogeometric distance by Bailer-Jones et al. (2021).

The Gaia DR3 parallax for Gaia18cjb is $p=0.83\pm 0.69$ mas. With such large
uncertainty, the parallax method does not yield accurate results. Indeed, the
main DR3 catalog does not provide a distance for sources whose parallax has
low SNR, including our target. For this reason, Bailer-Jones et al. (2021)
developed a probabilistic approach to estimate stellar distances. This method
relies on the prior using a full range of populations in the mock catalogue,
based on the Besancon Galaxy model. For each source they computed two
posterior probability distributions over distance: a geometric distance, based
only on the prior and the parallax, and the photogeometric distance, based
also on the G magnitude and the BP$-$RP color. Gaia18cjb’s geometrical
distance is $d_{g}=3.20_{1.62}^{2.66}$ kpc, while the photogeometrical
distance is $d_{pg}=1.03^{+0.20}_{-0.19}$ kpc. The recommendation of Bailer-
Jones et al. (2021) is to adopt the photogeometric distance for sources whose
parallax has low SNR. This is the case of Gaia18cjb.

The point of the distance inference in Bailer-Jones et al. (2021) is to
provide a gradual transition from data-dominated to prior-dominated distance
estimates. While the populations used by Bailer-Jones et al. (2021) contained
YSOs, their results are not optimized for intrinsically red sources with high
circumstellar extinction, such as YSOs. While a more appropriate photometric
prior constructed solely for young stars would provide more accurate
photometric distances, this is not the primary objective of our study. To be
conservative, we here discuss how our main results change, considering the
spread in the distance suggested from Gaia parallax $d\sim
1/p=660\,\mbox{pc}\,-7140$ pc.

In the following, we discuss results assuming $d_{min}=660$ pc, $d_{BJ}=1030$
pc, and $d_{max}=7140$ pc.

Table 5: Interstellar extinction and bolometric luminosity at different distances. Distance | A${}_{V}^{B19}$ | A${}_{V}^{STILIsM}$ | A${}_{V}^{mean}$ | $L_{\rm bol}^{pre}$ | $L_{\rm bol}^{pre\\_corr}$ | $L_{\rm bol}^{burst}$ | $L_{\rm bol}^{burst\\_corr}$
---|---|---|---|---|---|---|---
pc | mag | mag | mag | $\mbox{L}_{\odot}$ | $\mbox{L}_{\odot}$ | $\mbox{L}_{\odot}$ | $\mbox{L}_{\odot}$
660 | $0.3\pm 0.1$ | $0.5\pm 0.2$ | $0.4\pm 0.2$ | $1.4\pm 0.2$ | $1.4\pm 0.2$ | $3.0\pm 0.6$ | $3.1\pm 0.6$
1030 | $0.8\pm 0.2$ | $1.0\pm 0.2$ | $0.9\pm 0.3$ | $3.4\pm 0.2$ | $3.5\pm 0.2$ | $8.0\pm 0.6$ | $8.3\pm 0.6$
7140 | $2.9\pm 0.4$ | $3.1\pm 0.4$ | $3.0\pm 0.6$ | $162\pm 31$ | $173\pm 31$ | $353\pm 31$ | $534\pm 31$

####  4.1.1 Interstellar Extinction

To compute the reddening of Gaia18cjb due to the interstellar extinction, we
use the 3D dust maps based on Gaia parallaxes and stellar photometry from Pan-
STaRRS 1 and 2MASS (Green et al. 2019). We run the dustmaps python
package777http://argonaut.skymaps.info/usage, using the most updated version
Bayestar 2019, assuming $R_{V}=3.1$, and the reddening law by Cardelli et al.
(1989). We performed the same exercise using the STructuring by Inversion the
Local Interstellar Medium (STILIsM, Capitanio et al. 2017; Lallement et al.
2018) with the on-line tool888https://stilism.obspm.fr/. In both cases, we
consider these values as lower limits for the extinction towards the source,
because they are values averaged in a certain area and, most importantly, not
sensitive to local circumstellar extinction. We repeate the calculation for
$d_{min}$, $d_{BJ}$, and $d_{max}$. In Tab. 5 $A_{V}$ results are listed,
ranging from 0.4 mag to 3.0 mag depending on the distance. We note that
extinction estimates obtained with STILIsM show slightly higher values, but
compatible within the errors.

####  4.1.2 Bolometric Luminosity

Using the fluxes reported in Appendix. A, we computed the bolometric
luminosity by integrating the SED fluxes ($F_{\lambda}$) with a dedicated
python procedure, following the same method already used in literature (e.g.
Antoniucci et al. 2008; Fiorellino et al. 2021, 2023). We integrated the SED
from the shortest wavelength, interpolating with straight lines in the
$\log\lambda-\log F_{\lambda}$ plane between the available SED points. We
denote the value found with this method $L_{\rm bol}^{pre}$, to highlight that
this is the bolometric luminosity corresponding to the pre-burst phase,
obtained using the photometry observed before 2015 (filled circles in Fig. 5).

We added optical and NIR photometry acquired during the outburst (i.e. after
2015, filled stars). For the high level phase, we computed the bolometric
luminosity using Suhora, NTT/EFOSC, NTT/SOFI, and W1 photometry (blue, yellow,
pink, and purple stars in the figure) instead of SDSS, PanSTARR, and 2MASS
photometry (brown, green, and blue filled circles in the figure). As there is
little or no variability in W2, we assumed that the source was constant within
about 0.4 mag at 4.5 $\mu$m, and used the ALLWISE and AKARI points reward
these wavelengths for the outburst SED, too. We name the value found with this
method $L_{\rm bol}^{burst}$, to highlight that this is the bolometric
luminosity computed using the photometry observed during the ourtburst (see
Fig. 5).

To build the SED and to measure the bolometric luminosity of Gaia18cjb we used
observed fluxes, not corrected for the extinction. We note that the longest
wavelength data point we have is AKARI at 90 $\mu$m, and we assume a decrease
in the emission as $1/\lambda^{2}$ after that. Even applying this correction,
we are aware that the bolometric luminosity derived so can be underestimated
due to the lack of information at longer wavelengths. We also assume that
beyond 3.6 $\mu$m the SED did not change during the burst.

It is desirable to correct the flux for extinction and compute the bolometric
luminosity corrected for the extinction ($L_{bol}^{corr}$). Following Evans et
al. (2009), since the spectral index suggest Gaia18cjb can be classified as a
Class I/Flat Spectrum YSO, we used the interstellar $A_{V}$ to remove
foreground extinction, but not local extinction from the surrounding envelope,
which will be reradiated in the far-infrared. We used the mean value between
the values we obtain with the dustmaps and STILIsM softwares. Results are
listed in Tab. 5.

The lowest possible distance and the BJ distance both results in pre-outburst
luminosities ($1.4-3.5\mbox{L}_{\odot}$) typical of a low-mass young
protostar, and the outburst luminosities ($3.1-8.3\mbox{L}_{\odot}$) are
reasonable for modest accretion outbursts (cf. HBC 722 or some EXors). The
luminosities obtained with the largest possible distance are in the several
hundred $\mbox{L}_{\odot}$ range both in quiescence ($173\mbox{L}_{\odot}$)
and in outburst ($534\mbox{L}_{\odot}$). While such a large outburst
luminosity is possible in some of the most luminous and most highly accreting
FUors (e.g., V1057 Cyg or FU Ori Audard et al. 2014, and refs therein), the
quiescence luminosity is definitely too large for a low-mass YSO.

####  4.1.3 Stellar parameters and Accretion Rates

In this section, the stellar parameters (spectral type, extinction,
luminosity, radius and mass) and accretion properties (accretion luminosity
and accretion rate) of Gaia18cjb are determined.

An estimate of the total extinction can be done by assuming that the accretion
luminosity, $L_{\rm acc}$, as derived from different accretion tracers, is the
same. In order to derive $L_{\rm acc}$, we used the Pa$\beta$ and Br$\gamma$
lines and the empirical relationships between $L_{\rm acc}$ and the line
luminosity, $L_{\rm line}$, by Alcalá et al. (2017). By minimizing the
quantity $|L_{\rm acc}(Pa\beta)-L_{\rm acc}(Br\gamma)|$ it is possible to
estimate the circumstellar extinction, $A_{V}$, as follows: the observed
fluxes of the Pa$\beta$ and Br$\gamma$ lines were dereddened with a series of
$A_{V}$ values ranging from 0 to 200 mag in steps of 0.1 mag. The $A_{V}$
values minimizing the $|L_{\rm acc}(Pa\beta)-L_{\rm acc}(Br\gamma)|$ quantity
are given in Table 6 and shown graphically in Figure 9. For these, we
determine values of $\log(L_{\rm acc}/\mbox{L}_{\odot})$ between $-0.07$ and
2.63 dex, depending on the chosen distance (see Tab. 6). A robust value of
$A_{V}$ for the eruptive phase can be then obtained by computing the average
from the three individual estimates, the standard deviation being the error.
This value ranges from $A_{V}=10.5\pm 2.7$ mag to $A_{V}=15.0\pm 0.3$ mag,
depending on the distance, see Tab. 6.

Assuming that $L_{\rm bol}=L_{\star}+L_{\rm acc}$, we get a rough estimate of
the stellar luminosity for Gaia18cjb by knowing the dereddened bolometric
luminosity for the eruptive phase $L_{\rm bol}^{burst\\_corr}$ and the
accretion luminosity $L_{\rm acc}$. We thus obtain $L_{\star}=1.9\pm
1.2\,\mbox{L}_{\odot}$ for a $d_{min}=660$ pc, $L_{\star}=6.0\pm
1.2\,\mbox{L}_{\odot}$ for $d_{BJ}=1030$ pc, and $L_{\star}=107\pm
62\,\mbox{L}_{\odot}$ for $d_{max}=7140$ pc.

Based on the spectral features, we discard the possibility for Gaia18cjb to be
a high mass star, since high excitation lines, such as He I are not detected.
The stellar luminosity we obtain if assuming a distance of 7140 pc
($L_{\star}=107\pm 62\,\mbox{L}_{\odot}$) is, therefore, not compatible with
information contained in our spectra. Our spectra also discard the possibility
that the spectral type of Gaia18cjb could be later than K7. Indeed, M-type and
later type sperctra show photospheric absorption features such as CO and VO
bands. One may argue that the molecular bands could be veiled by the strong
continuum emission, but if that were the case then the accretion luminosity
should be extremely high, and the Weq we obtain suggest this is not the case.

Then, we used the evolutionary tracks by Siess et al. (2000), assuming an age
of $\sim 1$ Myr (see Sect. 3.3) and considering only spectral type earlier
than K7. We found that our results for $L_{\star}$ at 660 pc are compatible
with a young star with stellar mass $M_{\star}=(0.7-1.6)\,\mbox{M}_{\odot}$, a
radius $R_{\star}=(3.5-1.4)\,\mbox{R}_{\odot}$, and $T_{\rm eff}=4000-4600$ K,
corresponding to SpT$=$K7-K3. The same exercise for $L_{\star}$ at 1030 pc
leads to a young star with stellar mass
$M_{\star}=(0.7-2.2)\,\mbox{M}_{\odot}$, a radius
$R_{\star}=(4.4-2.6)\,\mbox{R}_{\odot}$, and $T_{\rm eff}=4000-5080$ K,
corresponding to SpT$=$K1-K7.

From the derived $L_{\rm acc}$ and stellar parameters, we computed the mass
accretion rate ($\dot{M}_{\rm acc}$) in the bursting phase using the relation
from Hartmann et al. (1998) in the framework of magnetospheric accretion
scenario:

$\dot{M}_{\rm acc}=\Bigg{(}1-\frac{R_{\star}}{R_{\rm
in}}\Bigg{)}^{-1}\frac{L_{\rm acc}R_{\star}}{GM_{\star}}$ (2)

where $R_{in}$ is the inner-disk radius which we assume to be $R_{in}\sim
5R_{\star}$ as discussed by Hartmann et al. (2016, see their Eq. 3) and for
consistency with previous similar studies (e.g. Park et al. 2022; Cruz-Sáenz
de Miera et al. 2022, 2023). We obtain $\dot{M}_{\rm acc}=1.8\times
10^{-7}-1.6\times 10^{-7}\,\mbox{M}_{\odot}$yr-1, for $d_{min}$; and
$\dot{M}_{\rm acc}=1.0-5.3\times 10^{-7}\,\mbox{M}_{\odot}$yr-1, for $d_{BJ}$.
Both the estimates are similar to the values found in highly accreting CTTSs
and in EXors-like objects, e.g. Giannini et al. (2022).

Figure 9: Extinction estimates of Gaia18cjb computed from the empirical relations between the accretion luminosity and Br$\gamma$ and Pa$\beta$ lines. Different colors represent $A_{V}$ estimates at different distances as described in the legend.  Table 6: Circumstellar extinction and accretion luminosity estimates for Gaia18cjb using empirical relations from Alcalá et al. (2017). Epoch | AV | $\log(L_{\rm acc}/\mbox{L}_{\odot})$ | $\log(L_{\rm acc}/\mbox{L}_{\odot})$
---|---|---|---
| [mag] | from Pa$\beta$ | from Br$\gamma$
|  | 660 pc |
LUCI | $11.0\pm 0.3$ | $-0.03\pm 0.21$ | $-0.04\pm 0.24$
SOFI | $7.9\pm 0.3$ | $-0.11\pm 0.17$ | $-0.11\pm 0.24$
GTC | $12.5\pm 0.3$ | $-0.08\pm 0.17$ | $-0.08\pm 0.24$
mean | $10.5\pm 2.7$ | $-0.07\pm 0.11$ | $-0.08\pm 0.14$
|  | 1030 pc |
LUCI | $11.8\pm 0.3$ | $0.39\pm 0.21$ | $0.39\pm 0.31$
SOFI | $8.7\pm 0.3$ | $0.34\pm 0.21$ | $0.34\pm 0.32$
GTC | $13.3\pm 0.3$ | $0.33\pm 0.12$ | $0.33\pm 0.32$
mean | $11.3\pm 0.3$ | $0.35\pm 0.21$ | $0.35\pm 0.21$
|  | 7140 pc |
LUCI | $15.5\pm 0.3$ | $2.68\pm 0.12$ | $2.67\pm 0.18$
SOFI | $12.4\pm 0.3$ | $2.59\pm 0.12$ | $2.59\pm 0.18$
GTC | $17.0\pm 0.3$ | $2.62\pm 0.12$ | $2.62\pm 0.18$
mean | $15.0\pm 0.3$ | $2.63\pm 0.07$ | $2.63\pm 0.10$

###  4.2 On the YSO Nature of Gaia18cjb

Figure 10: The background of the figure is the Planck 857 GHz map, 10 degrees
in size, centred on Gaia18cjb, the red star symbol in the plot. Blue and
orange dots are sources with H$\alpha$-excess from Fratta et al. (2021) whose
distances are $0.95\,{\rm pc}<d<1.05\,{\rm pc}$. The orange dots are sources
whose proper motions are within 3 mas/yr from Gaia18cjb proper motions, see
Figure 11. The regions closes to Gaia18cjb are highlighted. Figure 11: Left:
Color-Absolute Magnitude diagram not corrected for the extinction. The red
star symbol corresponds to Gaia18cjb. The black solid and dashed lines show
the ZAMS and the 1 Myr isochrone, respectively (Dell’Omodarme & Valle 2013).
Dots are sources which show H$\alpha$-excess from Fratta et al. (2021) catalog
whose distance is $0.95\,{\rm kpc}\,<d<1.05$ kpc. Orange dots are sources with
proper motions consistent within $3\sigma$ the propoer motion of Gaia18cjb
(see right panel). The error bars smaller than the symbol size are not
presented. The purple arrow represents an extinction vector of 1 mag. However,
the $A_{V}$ vector depends on the color index and on the position in the
diagram, see Prisinzano et al. (2022). Right Proper motions diagram of the
H$\alpha$-excess sample. Symbols are as in the Left pannel.

Besides the photometric and spectroscopic features presented in the previous
sections, another important aspect supporting the PMS nature of Gaia18cjb is
the enviroment in which the object is located. We investigated the region
nearby Gaia18cjb looking for its possible association with a star-forming
region (SFR). Figure 10 shows the environment in the general direction of
Gaia18cjb, with the nearby known star-forming regions highlighted. Gaia18cjb
is located three degrees South-West of the Rosette SFR in the Monoceros
constellation. Three degrees angular distance corresponds to $\sim 50$ pc at 1
kp. The distance to the Rosette complex is $1489\pm 32$ pc (Mužić et al.
2022). Thus it is unlikely that Gaia18cjb is connected with the Rosette SFR if
its distance is $d\leq d_{BJ}$. Contrary, if Gaia18cjb distance is similar to
the Rosette SFR, then it might be part of the star-forming complex. The VDB 85
cluster is about $\sim$1.5 degrees away from Gaia18cjb, and its measured
distance is about $\sim$1.69 kpc (Cantat-Gaudin & Anders 2020), so, for the
same reasoning applied to the Rosetta Complex, it is unlikely Gaia18cjb
belongs to this cluster. According to the 3D extinction map STILIsM, in
correspondence of the TGU 1475 dark cloud position ($l$, $b=210.87^{\circ}$,
$-3.28^{\circ}$), there is one dark cloud at a distance of $\sim 0.9-1.0$ kpc
which we may suspect that corresponds to TGU 1475. The ellipse in Fig. 10
shows its nominal size from Dobashi et al. (2005). Although TGU 1475 and
Gaia18cjb could have similar distances under the assumption that $d\leq
d_{BJ}$, looking at Fig. 10, we note that our target is outside the dark
cloud. Therefore, we may conclude that Gaia18cjb is not a member of any of the
known SFRs.

Recent investigations (Fratta et al. 2021; Prisinzano et al. 2022), based on
H$\alpha$ surveys and on Gaia DR3, show evidence of populations of YSO
candidates distributed in large volumes around the previously known SFRs. May
Gaia18cjb be part of such population in the general area of Monoceros?
Overplotted in Figure 10 are the sources showing H$\alpha-$excess as selected
by Fratta et al. (2021) and whose Gaia distance is between 0.95 kpc and 1.05
kpc, similar to the Gaia18cjb distance proposed by Bailer-Jones et al. (2021).
The lack of H$\alpha$ emitting objects in the southern part is due to the
latitude limit of the INT Photometric H$\alpha$ Survey of the Northern
Galactic Plane, IPHAS. We also distinguish in this plot objects having proper
motions consistent, within $3\sigma$,to the proper motion of Gaia18cjb (orange
dots) and those with inconsistent proper motion (blue dots). These two samples
are also plotted in the proper motion diagram in Figure 11 (right panel).
Gaia18cjb, shown as a red star symbol in the figure, is not associated with
any source in this catalog.

Figure 11 (left panel) shows a color-magnitude diagram of the sample with
H$\alpha$ excess, where the absolute magnitude computed from the Gaia $G-$band
as a function of the $G_{BP}-G_{RP}$ color (blue and orange colors are as in
Figure 10). The black lines correspond to the ZAMS (solid) and to the 1 Myr
isochrone (dashed, see Dell’Omodarme & Valle 2013; Randich et al. 2018;
Tognelli et al. 2018, 2020). Models have been converted into the Gaia observed
plane using the recent EDR3 filters passbands999GAIA EDR3 passbands are
available at the url https://www.cosmos.esa.int/web/gaia/edr3-passbands. with
the MARCS 2008 (Gustafsson et al. 2008) synthetic spectra for
T${}_{\mathrm{eff}}\leq 8000\leavevmode\nobreak\ $K and Castelli & Kurucz
(2003) spectra for higher Teff (see e.g., Prisinzano et al. 2022). About 97%
(91/94) of the H$\alpha$-excess sample lie above the ZAMS (blue dots), while
only three lie below the ZAMS line. Yet, many of these objects, in particular
those piled-up close to the ZAMS, may be evolved H$\alpha$ emitting stars,
like novae and symbiotic stars (see Munari et al. 2022). Note, however, that
symbiotic stars have giant companions and so are located near the giant
sequence, and quiescent novae resemble nova-like cataclismatic variables which
lie between the white dwarf and main sequences (see, for example Merc et al.
2020). Classical novae in outburst would probably be much brighter than the
main sequence. Note that basically all the objects with proper motion
compatible with those of Gaia18cjb (orange dots) lie well above the ZAMS
hence, it is likely they are PMS stars. However, wide-band spectroscopy of
these objects is necessary to definitely establish their evolutionary status.

The red star symbol lying immediately above the ZAMS corresponds to the
Gaia18cjb mean values of absolute magnitude and G${}_{\rm BP}-{\rm G}_{\rm
RP}$ color provided by the Gaia archive: $G=19.751\pm 0.010$ mag, G${}_{\rm
BP}=20.65\pm 0.77$ mag, and G${}_{\rm RP}=18.16\pm 0.77$ mag. These value,
reported in Gaia EDR3 are averaged on the period 2014 July $-$ 2017 May, when
Gaia18cjb has already started brightening, but was still relatively faint,
probably closer to quiescent than to outburst properties. We stress that the
magnitudes of all these sources are not dereddened, implying that after
correcting each source for its extinction they will move up and left in the
color-magnitude diagram. For what concerns Gaia18cjb, we estimated an
accretion luminosity in burst which is about 27% of the bolometric luminosity
and $A_{V}\sim 8-12$ mag. With these values, the contribution of the
extinction is larger than the contribution of the accretion, ending up moving
Gaia18cjb up in the $M_{\rm abs}\,{\rm vs.}\,{\rm G}_{\rm BP}-{\rm G}_{\rm
RP}$ plot. Therefore, we can speculate that these sources lie above the ZAMS,
and are young stars. The study of the spatial distribution of YSO candidates
within 1.5 kpc by Prisinzano et al. (2022), based on Gaia DR3, highlighted a
population of distributed YSO candidates surrounding the previously known SFRs
in the Monoceros region, but Gaia18cjb was not selected by their criterion,
probably because too faint to be included in their catalog.

In summary, we selected 20 emission line stars (19 + Gaia18cjb) that: 1) lie
above the ZAMS in the Gaia color-magnitude diagram; 2) are located at
approximately the same distance ($0.95\,{\rm kpc}<d<1.5\,{\rm kpc}$); 3) have
coherent proper motion. We therefore conclude that these likely YSOs,
including Gaia18cjb might be part of the distributed population of the
Monoceros complex. This results may indicate that the $d_{BJ}$ could be a good
estimate of the real distance of Gaia18cjb. However, as discussed in Sect.
4.1, the photogeometric distance $d_{BJ}$ may be strongly affected by the
prior for the absolute G-band mag, being dominated by the sample of main
sequence stars.

Several facts suggest, that Gaia18cjb is a young star: (i) the high
probability of Gaia18cjb to be a YSO: 83%, computed by Marton et al. (2019);
(ii) the similarity between Gaia18cjb SED and the SED of a young accreting
object (Fig. 5); (iii) the spectral index which classifies Gaia18cjb as Class
I YSO during the pre-burst phase and as a Flat Spectrum YSO during the burst
(Sect. 3.3); (iv) the detection of CTTSs typical features, as accretion and
ejection tracers in the NIR spectra (see Sect. 3.4); and (v) the environment
in which the object is projected on the sky. All these facts together lead us
to conclude that Gaia18cjb is an accreting young star.

###  4.3 On the Outburst of Gaia18cjb

The optical light curve (Figure 2) presents a typical amplitude of a FUor-like
object, with timescales compatible with EYOs, longer than typical EXors and
smaller than typical FUors. On the contrary, the WISE light curves in the MIR
show no evidence of such FUor-like strong burst and the variability is smaller
than expected for a FUor-like object, while it is compatible with the EXor-
like YSOs or even CTTs variability.

If a burst is present, as the optical light curves suggest, it is not clear
why it is less pronounced at NIR wavelengths and does not affect the SED at
(and possibly beyond) the WISE band 2 (4.6 $\mu$m), since eruptive objects
usually show similar variability at optical and infrared wavelengths (see for
example Gaia19fct, Park et al. 2022). Naively, one could conclude that the
large optical and smaller amplitude in the infrared for Gaia18cjb is due to
variability in extinction. Looking at the color-magnitude diagrams in Figure
3, we see that the extinction could only partially explain the variability of
Gaia18cjb and, in any case, its low level and high level colors.

For example, if the observed NIR variability ($\Delta W1\sim 0.6$ mag, $\Delta
m_{K}\sim 0.7$ mag, $\Delta m_{H}\sim 2.2$ mag, $\Delta m_{J}\sim 2.9$ mag)
were caused only by extinction, this would correspond to a visible amplitude
of $\Delta m_{V}\sim 10-12$ mag, that is too large in comparison with the
observed visual variability ($\Delta m_{r}\sim 5$ mag). This suggests that at
least some of the observed bluening is intrinsic and not just linked to
reddening. On the other hand, the accretion burst has to be relatively mild,
as the continuum at 4.6 $\mu$m is barely or not affected at all. It is thus
possible that the observed photometric variability of Gaia18cjb is a
combination of enhanced accretion and reduced extinction, as inferred in
Gaia19ajj (Hillenbrand et al. 2019b). The reduced extinction might possibly
rise from winds triggered by the enhanced accretion. These would partially
clear the dust in the outflow cavities. This scenario is compatible with the
H2 line seen during the burst (see Sect. 3.4).

The burst weakness is also supported by the analysis of the spectra. The
latter do not show typical FUors-like features like the triangular shape in
the $H-$band, the undetected NIR HI and CO lines in absorption (Greene et al.
2008). On the contrary, accretion tracers are detected in emission in all the
epochs we investigated, confirming this source is an accreting young star and
strengthening the EXor nature of Gaia18cjb burst.

Figure 12: Top: $L_{\rm acc}-L_{\star}$ diagram. The red and black star
symbols correspond to Gaia18cjb and EX Lup, respectively. The blue dashed line
represents the $L_{\rm acc}=L_{\star}$ locus. The grey line is the best fit
for the CTTSs of Lupus from Alcalá et al. (2017) and the grey region
corresponds to the standard deviation of the ﬁt. Bottom: $\dot{M}_{\rm
acc}-M_{\star}$ diagram. Star symbols are as in the top panel. The grey line
is the best fit for the CTTSs of Lupus from Alcalá et al. (2017) and the grey
region corresponds to the standard deviation of the ﬁt. The pink area
corresponds to $\dot{M}_{\rm acc}$ ranges for embedded Class I+Flat Spectrum
YSOs from Fiorellino et al. (2023).

In Sect. 4.1.3, using empirical relations, we find that $L_{\rm acc}$ ranges
from 0.85 $\mbox{L}_{\odot}$ to 2.24 $\mbox{L}_{\odot}$, and $\dot{M}_{\rm
acc}$ ranges from $2.8\times 10^{-8}\,\mbox{M}_{\odot}/$yr to $1.0\times
10^{-7}\,\mbox{M}_{\odot}/$yr if $d_{min}\leq d\leq d_{BJ}$.

Calculating $L_{\rm acc}$ using empirical relationships from Alcalá et al.
(2017) is an indirect method that has shown success with CTTSs but some
evidence has recently emerged that it may not be always applicable to eruptive
sources, even if we expect magnetospheric accretion, such as for EXors
objects. For instance, recent analysis of EX Lupi revealed that several
emission lines indicated lower values of $L_{\rm acc}$ compared to the slab
model, the direct way to determine $L_{\rm acc}$ in the framework of
magnetospheric accretion (Cruz-Sáenz de Miera et al. 2023). They suggest that
this discrepancy may arise from the challenge of identifying the continuum in
a highly accreting objects. To investigate whether this holds true for
Gaia18cjb, we would require wide-band spectroscopic data covering the Balmer
continuum and Balmer jump for application of a slab model fit. Since we lack
such spectroscopic data, and conversely, we can only calculate $L_{\rm acc}$
from the $\mbox{Pa}\beta$ and $\mbox{Br}\gamma$ lines in the NIR, the estimate
of $L_{\rm acc}$ obtained from empirical relationships using these lines is
currently the best estimate of $L_{\rm acc}$. However, we emphasize that due
to the likely overestimated value of Gaia18cjb’s stellar luminosity and the
uncertainty on the distance, there is a high probability that the accretion
luminosity exceeds $L_{\rm acc}=2.24\,\mbox{L}_{\odot}$. This needs to be
tested through the direct measurement of $L_{\rm acc}$ by studying the Balmer
continuum excess emission and Balmer jump, using wide-band spectra.

Figure 12 shows the $L_{\rm acc}\,{\rm vs.}\,L_{\star}$ (top panel) and
$\dot{M}_{\rm acc}\,{\rm vs.}\,M_{\star}$ (bottom panel) of Gaia18cjb at
$d_{min}=660$ pc and $d_{BJ}=1.03$ kpc (red star symbols), compared to EX Lup
(black stars, quiescent value from Cruz-Sáenz de Miera et al. (2023), and
burst value from Fischer et al. 2022) and typical distributions for CTTSs in
Lupus (grey, Alcalá et al. 2017), and Class I/Flat Spectrum protostars (pink,
Fiorellino et al. 2023).

The top panel of Figure 12 shows that Gaia18cjb is located in the Class I/Flat
Spectrum YSOs parameter space, right below the $L_{\rm acc}=L_{\star}$
threshold (dashed blue line). Therefore, during the epochs we investigated
with spectroscopy, i.e. during the burst, the ratio between the stellar and
the accretion luminosity was more similar to a YSO more evolved than a
protostar if $d_{min}\leq d\leq d_{BJ}$, and its accretion rate and stellar
parameters are compatible with a Class I/Flat Spectrum object, suggesting
Gaia18cjb evolutionary stage is in between the protostellar and the pre-main
sequence phases. This in agreement with the Flat Spectrum classification
suggested by the spectral index (Sect. 3.3). Note, however, that the
luminosity of Gaia18cjb is still compatible with that of typical CTTS if the
distance $d_{min}=660$ pc is adopted.

Regarding the $\dot{M}_{\rm acc}$ vs. $M_{\star}$ plot, the bottom panel of
Figure 12 shows that Gaia18cjb accretes more than typical CTTSs of the same
stellar mass. Gaia18cjb location in this plot is in agreement with bright
Class I and Flat Spectrum sources, and the mass accretion rate is similar to
what derived for EX Lup during the burst. Indeed, while the optical light
curve resemble a FUor, the spectroscopic features and the accretion rate
suggest an EXor-like nature of Gaia18cjb. Interestingly, there are at least
two young stars, V350 Cep and V1647 Ori, displaying photometric
characteristics of FUors in the visible bands and spectral characteristics of
EXors (Herbig 2008; Ibryamov et al. 2014; Aspin & Reipurth 2009). In both
cases, the conclusion was that the sources were probably intermediate objects
between FUors and EXors.

The similarities between V350 Cep, V1647 Ori, and Gaia18cjb, might suggest
that also Gaia18cjb is an intermediate object between FUors and EXors.
However, there are a few aspects to take into account. First, V350 Cep and
V1647 Ori also show P Cyg H$\alpha$ profile, and random fluctuations in
brightness with amplitudes of a few tenths of a magnitude in timescales of
several days that we do not see in Gaia18cjb. Second, the current discussion
about EYSs classification highlights that the classification in very tight
containers (e.g. FUor, EXor, V1647 Or) is not sufficient. Gaia18cjb is another
star suggesting instead a spectrum of classifications in the EYSs. Last, our
results about accretion rates and stellar parameters should be considered as
rough estimates, because our analysis is based only on NIR spectroscopy and
because of the large uncertainty on the distance. To provide accurate
parameters and hence, a robust characterization of Gaia18cjb, further
observations and analysis are needed:

  * •

Continuing monitoring optical and infrared light curves is crucial to follow
the evolution of the Gaia18cjb burst. Also, $JHK$ NIR infrared monitoring is
needed to study the light curve variability in between the FUor-like visible
light curve and the EXor-like infrared light curves.

  * •

By having a flux calibrated intermediate/high-resolution spectrum from the UVB
to the NIR (as ESO/VLT XShooter can provide), we will be able to study the Li
I ($\lambda$6708 Å) absorption line to reinforce the YSOs nature of Gaia18cjb,
to perform a direct spectral typing and determine in a more accurate way the
stellar parameters, and hence, measure the accretion luminosity from the
Balmer continuum excess emission. As a consequence, we will be able to provide
robust estimates of stellar and accretion parameters, and of the mass
accretion rate.

  * •

Millimeter interferometry will provide estimates for disk mass and radius
which are needed to prove the instability of the disk, believed to trigger the
eruptive accretion.

  * •

Accurate distance estimate, by using a pre-main-sequence dominated prior in
the Bailer-Jones et al. (2021) prescription, or a more accurate parallax
estimate from GDR4.

##  5 Conclusions

The goal of this discovery paper was to determine the nature of the Gaia
alerted object Gaia18cjb.

For this purpose, we collected all the available photometry in the archives
and monitored Gaia18cjb in optical bands. NIR spectroscopy was also performed
in three epochs between 2020 and 2022 and we studied the object’s variability,
stellar and accretion parameters, and inspected the sky region nearby
Gaia18cjb.

The large uncertainty on the Gaia parallax and the main-sequence dominated
prior of the Bailer-Jones et al. (2021) method, prevent us from adopting a
precise distance for this source. Because of this, we cannot derive precise
values but only ranges of stellar and accretion parameters based on the
comparison between analytical results and spectral features. Comparing the
Gaia stellar parameters and accretion rates with those of CTTSs, protostars,
FUors, and other similar eruptive objects as EX Lup, V350 Cep, V1647 Ori, and
Gaia19ajj, our results suggest Gaia18cjb is a young star experiencing eruptive
accretion and that it can be classified as in between FUors and EXors: the
length of the burst points to a FUor-like nature, whereas the strength of the
burst and detected spectral features point to an EXor. However, further
optical and NIR monitoring campaigns, at least one intermediate/high-
resolution spectrum from UVB to NIR, and millimeter interferometry, are needed
to determine accurate stellar and accretion parameters and, hence, provide
robust conclusions on the Gaia18cjb variability classification.

###### Acknowledgements.

We thank Dr. C. Bailer Jones for discussions on the parallax of the object and
Dr. L. Prisinzano for providing the isochrones in the Gaia photometric system.
We acknowledge Dr. András Ordasi for his contribution to this paper. This work
has been supported by the project PRIN‐INAF 2019 “Spectroscopically Tracing
the Disk Dispersal Evolution (STRADE)” and by the INAF Large Grant 2022 “YSOs
Outflow, Disks and Accretion (YODA)”. This project has received funding from
the European Research Council (ERC) under the European Union’s Horizon 2020
research and innovation programme under grant agreement No 716155 (SACCRED).
We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team
(http://gsaweb.ast.cam.ac.uk/alerts) This work is (partly) based on data
obtained with the instrument EMIR, built by a Consortium led by the Instituto
de Astrofísica de Canarias. EMIR was funded by GRANTECAN and the National Plan
of Astronomy and Astrophysics of the Spanish Government. This work is (partly)
based on data obtained at the Mount Suhora Observatory, Krakow Pedagogical
University, Poland. This work has been supported by the Hungarian National
Research, Development and Innovation Office grants OTKA K131508, OTKA K138962,
the Élvonal KKP-143986 and KKP-137523 ’SeismoLab’ grants of the Hungarian
Research, Development and Innovation Office (NKFIH). We acknowledge support
from the ESA PRODEX contract nr. 4000132054. KV, LK, ZN, and GM are supported
by the Bolyai János Research Scholarship of the Hungarian Academy of Sciences,
KV is supported by the Bolyai+ grant ÚNKP-22-5-ELTE-1093. Authors acknowledge
the financial support of the Austrian-Hungarian Action Foundation (112öu1). LK
is supported by the Hungarian National Research, Development and Innovation
Office grant PD-134784. G.M. acknowledges support from the European Union’s
Horizon 2020 research and innovation programme under grant agreement No.
101004141. This work is supported by the Polish MNiSW grant DIR/WK/2018/12 and
the European Union’s Horizon 2020 research and innovation program under grant
agreement No. 101004719 (OPTICON-RadioNet Pilot). Zs.M.Sz. acknowledges
funding from a St Leonards scholarship from the University of St Andrews. For
the purpose of open access, the author has applied a Creative Commons
Attribution (CC BY) licence to any Author Accepted Manuscript version arising.
BZs is supported by the ÚNKP-22-2 New National Excellence Program of the
Ministry for Culture and Innovation from the source of the National Research,
Development and Innovation Fund. The Liverpool Telescope is operated on the
island of La Palma by Liverpool John Moores University in the Spanish
Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de
Canarias with financial support from the UK Science and Technology Facilities
Council. We acknowledge the Hungarian National Research, Development and
Innovation Office grant OTKA FK 146023. FCSM received financial support from
the European Research Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (ERC Starting Grant “Chemtrip”, grant
agreement No 949278).

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##  Appendix A Photometry

Tables 7 and 8 collect the optical and NIR photometry of Gaia18cjb,
respectively.

Table 7: The optical photometry of Gaia18cjb we acquired and/or reduced. EFOSC
$r$ photometry was taken with Bessell $R$ filter.

Instr. | Date | HJD | $B$ | $g$ | $V$ | $r$ | $i$
---|---|---|---|---|---|---|---
|  |  | mag | mag | mag | mag | mag
PanSTaRRS | 2011 Oct 25 | 2455860.13 | $-$ | $-$ | $-$ | $20.2\pm 0.1$ | $-$
PanSTaRRS | 2011 Oct 25 | 2455860.14 | $-$ | $-$ | $-$ | $20.1\pm 0.1$ | $-$
PanSTaRRS | 2011 Nov 01 | 2455566.95 | $-$ | $22.1\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2011 Nov 01 | 2455566.96 | $-$ | $22.0\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2011 Nov 11 | 2455876.06 | $-$ | $-$ | $-$ | $-$ | $19.2\pm 0.1$
PanSTaRRS | 2011 Nov 11 | 2455876.07 | $-$ | $-$ | $-$ | $-$ | $19.4\pm 0.1$
PanSTaRRS | 2012 Jan 04 | 2455930.94 | $-$ | $-$ | $-$ | $-$ | $19.1\pm 0.1$
PanSTaRRS | 2012 Jan 04 | 2455930.96 | $-$ | $-$ | $-$ | $-$ | $19.3\pm 0.1$
PanSTaRRS | 2012 Oct 14 | 2456215.08 | $-$ | $21.8\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Oct 14 | 2456215.08 | $-$ | $21.7\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Oct 14 | 2456215.10 | $-$ | $22.4\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Oct 14 | 2456215.10 | $-$ | $21.7\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Oct 19 | 2456220.09 | $-$ | $-$ | $-$ | $20.1\pm 0.1$ | $-$
PanSTaRRS | 2012 Oct 19 | 2456220.10 | $-$ | $-$ | $-$ | $20.0\pm 0.1$ | $18.9\pm 0.1$
PanSTaRRS | 2012 Oct 19 | 2456220.11 | $-$ | $-$ | $-$ | $20.3\pm 0.1$ | $19.0\pm 0.1$
PanSTaRRS | 2012 Nov 08 | 2456240.09 | $-$ | $-$ | $-$ | $19.7\pm 0.1$ | $-$
PanSTaRRS | 2012 Nov 08 | 2456240.09 | $-$ | $-$ | $-$ | $20.1\pm 0.1$ | $-$
PanSTaRRS | 2012 Nov 08 | 2456240.10 | $-$ | $-$ | $-$ | $19.8\pm 0.1$ | $-$
PanSTaRRS | 2012 Nov 08 | 2456240.10 | $-$ | $-$ | $-$ | $20.1\pm 0.1$ | $-$
PanSTaRRS | 2012 Nov 09 | 2456241.05 | $-$ | $-$ | $-$ | $20.1\pm 0.1$ | $-$
PanSTaRRS | 2012 Nov 09 | 2456241.06 | $-$ | $-$ | $-$ | $20.0\pm 0.1$ | $-$
PanSTaRRS | 2012 Nov 11 | 2456243.04 | $-$ | $21.6\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Nov 11 | 2456243.05 | $-$ | $21.1\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Dec 23 | 2456284.96 | $-$ | $-$ | $-$ | $-$ | $19.0\pm 0.1$
PanSTaRRS | 2013 Mar 05 | 2456356.78 | $-$ | $21.9\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2013 Mar 05 | 2456356.78 | $-$ | $22.1\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2013 Mar 05 | 2456356.79 | $-$ | $22.6\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2013 Mar 05 | 2456356.90 | $-$ | $21.8\pm 0.2$ | $-$ | $-$ | $-$
PanSTaRRS | 2013 Dec 04 | 2456631.01 | $-$ | $21.4\pm 0.2$ | $-$ | $19.9\pm 0.1$ | $-$
PanSTaRRS | 2013 Dec 04 | 2456631.02 | $-$ | $21.5\pm 0.2$ | $-$ | $19.8\pm 0.1$ | $-$
PanSTaRRS | 2013 Dec 23 | 2456649.99 | $-$ | $-$ | $-$ | $-$ | $19.0\pm 0.1$
PanSTaRRS | 2013 Dec 24 | 2456650.00 | $-$ | $-$ | $-$ | $-$ | $19.1\pm 0.1$
Adiyaman | 2020 Nov 13 | 2459167.49 | – | 17.688$\pm$0.052 | – | 16.227$\pm$0.026 | 15.564$\pm$0.060
Adiyaman | 2020 Nov 27 | 2459180.51 | – | 17.617$\pm$0.065 | – | 16.149$\pm$0.025 | 15.495$\pm$0.054
Adiyaman | 2020 Nov 29 | 2459183.39 | – | 17.425$\pm$0.082 | – | 16.162$\pm$0.031 | 15.491$\pm$0.045
Adiyaman | 2021 Jan 10 | 2459225.28 | – | 17.498$\pm$0.073 | – | 16.141$\pm$0.036 | 15.504$\pm$0.047
Adiyaman | 2021 Jan 24 | 2459239.45 | – | 17.575$\pm$0.086 | – | 16.213$\pm$0.033 | 15.503$\pm$0.052
Adiyaman | 2021 Feb 10 | 2459256.40 | – | 17.613$\pm$0.058 | – | 16.174$\pm$0.025 | 15.503$\pm$0.056
EFOSC | 2021 May 09 | 2459344.49 | 18.220$\pm$0.022 | – | 16.778$\pm$0.030 | 16.028$\pm$0.021 | –
RC80 | 2020 Oct 22 | 2459145.56 | 18.32$\pm$0.18 | 17.597$\pm$0.035 | 16.903$\pm$0.032 | 16.242$\pm$0.014 | 15.516$\pm$0.012
RC80 | 2020 Nov 05 | 2459159.67 | 18.21$\pm$0.12 | 17.390$\pm$0.076 | 16.832$\pm$0.036 | 16.180$\pm$0.021 | 15.450$\pm$0.025
RC80 | 2020 Nov 22 | 2459175.65 | 18.50$\pm$0.15 | 17.562$\pm$0.038 | 16.880$\pm$0.041 | 16.213$\pm$0.012 | 15.470$\pm$0.011
RC80 | 2021 Feb 15 | 2459261.26 | 18.27$\pm$0.15 | 17.520$\pm$0.044 | 16.802$\pm$0.055 | 16.162$\pm$0.029 | 15.463$\pm$0.027
RC80 | 2021 Feb 18 | 2459264.27 | 18.17$\pm$0.11 | 17.511$\pm$0.079 | 16.830$\pm$0.051 | 16.167$\pm$0.031 | 15.461$\pm$0.030
RC80 | 2021 Feb 20 | 2459266.25 | 18.49$\pm$0.14 | 17.605$\pm$0.048 | 16.896$\pm$0.051 | 16.217$\pm$0.035 | 15.503$\pm$0.030
RC80 | 2021 Feb 23 | 2459269.25 | 17.80$\pm$0.25 | 17.29$\pm$0.15 | 16.54$\pm$0.14 | 16.20$\pm$0.13 | 15.296$\pm$0.088
RC80 | 2021 Feb 24 | 2459270.27 | 18.57$\pm$0.24 | 17.570$\pm$0.083 | 16.846$\pm$0.083 | 16.11$\pm$0.15 | 15.436$\pm$0.047
RC80 | 2021 Sep 08 | 2459465.61 | 18.30$\pm$0.13 | 17.530$\pm$0.053 | 16.785$\pm$0.032 | 16.137$\pm$0.016 | 15.410$\pm$0.018
RC80 | 2021 Oct 20 | 2459507.61 | 18.181$\pm$0.096 | – | 16.86$\pm$0.16 | – | –
RC80 | 2021 Oct 23 | 2459510.52 | – | 17.548$\pm$0.052 | – | 16.128$\pm$0.021 | 15.415$\pm$0.016
RC80 | 2021 Nov 24 | 2459542.61 | 18.204$\pm$0.080 | 17.514$\pm$0.037 | 16.781$\pm$0.037 | 16.118$\pm$0.015 | 15.414$\pm$0.016
RC80 | 2022 Jan 07 | 2459586.59 | 18.19$\pm$0.10 | 17.464$\pm$0.047 | 16.688$\pm$0.036 | 16.013$\pm$0.017 | 15.319$\pm$0.017
RC80 | 2022 Feb 05 | 2459616.25 | 18.168$\pm$0.075 | 17.331$\pm$0.030 | 16.637$\pm$0.029 | 15.983$\pm$0.016 | 15.265$\pm$0.011
RC80 | 2022 Sep 24 | 2459846.63 | 18.16$\pm$0.11 | 17.313$\pm$0.034 | 16.636$\pm$0.030 | 15.982$\pm$0.026 | 15.290$\pm$0.016
RC80 | 2022 Oct 25 | 2459878.49 | 17.910$\pm$0.091 | 17.304$\pm$0.034 | 16.588$\pm$0.030 | 15.944$\pm$0.015 | 15.250$\pm$0.012
RC80 | 2022 Nov 25 | 2459908.62 | 18.172$\pm$0.085 | 17.478$\pm$0.032 | 16.731$\pm$0.029 | 16.090$\pm$0.015 | 15.405$\pm$0.016
RC80 | 2022 Dec 24 | 2459938.47 | 18.09$\pm$0.13 | 17.316$\pm$0.039 | 16.649$\pm$0.038 | 15.998$\pm$0.019 | 15.290$\pm$0.013
RC80 | 2023 Feb 06 | 2459982.34 | 18.06$\pm$0.19 | 17.363$\pm$0.094 | 16.599$\pm$0.056 | 15.933$\pm$0.023 | 15.220$\pm$0.017
RC80 | 2023 Mar 22 | 2460026.36 | 18.10$\pm$0.16 | 17.261$\pm$0.058 | 16.517$\pm$0.044 | 15.911$\pm$0.026 | 15.210$\pm$0.015
Suhora | 2021 Dec 13 | 2459561.56 | 18.267$\pm$0.096 | 17.340$\pm$0.026 | 16.576$\pm$0.025 | 15.924$\pm$0.022 | 15.235$\pm$0.082
Suhora | 2021 Jan 03 | 2459218.46 | – | 17.499$\pm$0.045 | – | 16.092$\pm$0.024 | 15.422$\pm$0.067
Suhora | 2021 Jan 22 | 2459237.46 | – | – | 16.782$\pm$0.042 | – | –

Table 8: The near-infrared photometry of Gaia18cjb we reduced. Instr. | Date | HJD | $Y$ | $z$ | $J$ | $H$ | $K$
---|---|---|---|---|---|---|---
|  |  | mag | mag | mag | mag | mag
UKIDSS | 2007 Apr 15 | 2454205.5 | $-$ | $-$ | $-$ | $-$ | 12.620$\pm$0.040
UKIDSS | 2010 Oct 24 | 2455493.5 | $-$ | $-$ | 16.210$\pm$0.040 | 14.860$\pm$0.050 | 13.260$\pm$0.040
PanSTaRRS | 2010 Feb 15 | 2455242.8 | $18.7\pm 0.1$ | $-$ | $-$ | $-$ | $-$
PanSTaRRS | 2010 Feb 15 | 2455242.8 | $18.3\pm 0.1$ | $-$ | $-$ | $-$ | $-$
PanSTaRRS | 2010 Feb 15 | 2455242.8 | $18.6\pm 0.1$ | $-$ | $-$ | $-$ | $-$
PanSTaRRS | 2010 Feb 15 | 2455242.8 | $18.1\pm 0.1$ | $-$ | $-$ | $-$ | $-$
PanSTaRRS | 2010 Oct 22 | 2455492.1 | $18.2\pm 0.1$ | $-$ | $-$ | $-$ | $-$
PanSTaRRS | 2010 Oct 22 | 2455492.1 | $18.1\pm 0.1$ | $-$ | $-$ | $-$ | $-$
PanSTaRRS | 2011 Sep 18 | 2455823.1 | $18.2\pm 0.1$ | $-$ | $-$ | $-$ | $-$
PanSTaRRS | 2011 Sep 18 | 2455823.1 | $17.9\pm 0.1$ | $-$ | $-$ | $-$ | $-$
PanSTaRRS | 2011 Oct 22 | 2455856.8 | $18.1\pm 0.1$ | $18.6\pm 0.1$ | $-$ | $-$ | $-$
PanSTaRRS | 2011 Oct 22 | 2455256.8 | $-$ | $18.4\pm 0.1$ | $-$ | $-$ | $-$
PanSTaRRS | 2011 Oct 27 | 2455862.1 | $-$ | $18.5\pm 0.1$ | $-$ | $-$ | $-$
PanSTaRRS | 2011 Oct 27 | 2455862.1 | $-$ | $18.6\pm 0.1$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Feb 27 | 2455984.7 | $18.0\pm 0.1$ | $-$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Feb 27 | 2455984.7 | $18.1\pm 0.1$ | $-$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Feb 28 | 2455485.1 | $-$ | $18.4\pm 0.1$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Feb 28 | 2455485.1 | $-$ | $18.5\pm 0.1$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Mar 20 | 2456006.8 | $-$ | $18.5\pm 0.1$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Mar 20 | 2456006.8 | $-$ | $18.6\pm 0.1$ | $-$ | $-$ | $-$
PanSTaRRS | 2012 Oct 22 | 2456223.1 | $-$ | $18.3\pm 0.1$ | $-$ | $-$ | $-$
PanSTaRRS | 2013 Oct 20 | 2456586.1 | $18.0\pm 0.1$ | $18.2\pm 0.1$ | $-$ | $-$ | $-$
PanSTaRRS | 2014 Oct 08 | 2456939.1 | $18.0\pm 0.1$ | $-$ | $-$ | $-$ | $-$
SOFI | 2021 May 08 | 2459343.0 | $-$ | $-$ | 13.505$\pm$0.058 | 12.720$\pm$0.035 | 11.911$\pm$0.042
GTC | 2022 Oct 10 | 2459862.5 | $-$ | $-$ | 13.354 $\pm$0.080 | 12.672$\pm$0.073 | 11.952$\pm$0.106
GTC | 2022 Oct 26 | 2459878.6 | $-$ | $-$ | 13.396 $\pm$0.124 | 12.675$\pm$0.067 | 12.059$\pm$0.160
Table 9: The photometry used for the SED of Gaia18cjb. Instr. | Year | $\lambda$ | Flux
---|---|---|---
|  |  $\mu$m | mJy
WISE | 2013 | 11.55 | $93.1\pm 1.3$
WISE | 2013 | 22.09 | $145.0\pm 3.0$
PanSTaRRS | 2012 | 0.747 | $0.06580\pm 0.0014$
PanSTaRRS | 2012 | 0.865 | $0.1300\pm 0.0020$
PanSTaRRS | 2012 | 0.959 | $0.1620\pm 0.0040$
WISE | 2011 | 3.350 | $14.40\pm 0.30$
WISE | 2011 | 4.600 | $37.50\pm 0.70$
AKARI | 2010 | 8.610 | $114\pm 16$
AKARI | 2010 | 64.99 | $461\pm 76$
AKARI | 2010 | 90.00 | $506\pm 81$
2MASS | 2000 | 1.239 | $0.665\pm 0.052$
2MASS | 2000 | 1.649 | $1.430\pm 0.080$
2MASS | 2000 | 2.163 | $3.95\pm 0.15$
SDSS | 1998 | 0.477 | $0.00847\pm 0.00037$
SDSS | 1998 | 0.481 | $0.00849\pm 0.00037$
SDSS | 1998 | 0.612 | $0.03260\pm 0.00040$
EFOSC | 2021 | 0.44 | $1834\pm 38$
Suhora | 2021 | 0.52 | $0.373\pm 0.016$
EFOSC | 2021 | 0.5476 | $6331\pm 177$
EFOSC | 2021 | 0.6431 | $14823\pm 285$
Suhora | 2021 | 0.67 | $1.638\pm 0.036$
Suhora | 2021 | 0.79 | $3.226\pm 0.198$
SOFI | 2021 | 1.247 | $57.8\pm 3.1$
SOFI | 2021 | 1.653 | $75.7\pm 2.4$
SOFI | 2021 | 2.162 | $103.2\pm 4.0$

##  Appendix B Spectra

Figure 13 shows the NIR flux calibrated spectra of Gaia18cjb.

Figure 13: Flux calibrated NIR spectra of Gaia18cjb.