# An Unusual Transient in the Extremely Metal-Poor Galaxy SDSS
J094332.35+332657.6 (Leoncino Dwarf)

Mercedes E. Filho,1,2 and J. Sánchez Almeida,3,4
1Center for Astrophysics and Gravitation - CENTRA/SIM, Departamento de Física,
Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1,
P-1049-001 Lisbon, Portugal
2Departamento de Engenharia Física, Universidade do Porto, Faculdade de
Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, s/n, P-4200-465,
Oporto, Portugal
3Instituto Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
4Departamento de Astrofísica, Universidad La Laguna, 38206 La Laguna,
Tenerife, Spain E-mail<EMAIL_ADDRESS>(MEF)

(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We have serendipitously discovered that Leoncino Dwarf, an ultra-faint, low-
metallicity record-holder dwarf galaxy, may have hosted a transient source,
and possibly exhibited a change in morphology, a shift in the center of
brightness, and peak variability of the main (host) source in images taken
approximately 40 yr apart; it is highly likely that these phenomena are
related. Scenarios involving a Solar System object, a stellar cluster, dust
enshrouding, and accretion variability have been considered, and discarded, as
the origin of the transient. Although a combination of time-varying strong and
weak lensing effects, induced by an intermediate mass black hole (104 – 5
$\times$ 105 M⊙) moving within the Milky Way halo (0.1 – 4 kpc), can
conceivably explain all of the observed variable galaxy properties, it is
statistically highly unlikely according to current theoretical predictions,
and, therefore, also discarded. A cataclysmic event such as a
supernova/hypernova could have occurred, as long as the event was observed
towards the later/late-stage descent of the light curve, but this scenario
fails to explain the absence of a post-explosion source and/or host HII region
in recent optical images. An episode related to the giant eruption of a
luminous blue variable star, a stellar merger or a nova, observed at, or near,
peak magnitude may explain the transient source and possibly the change in
morphology/center of brightness, but can not justify the main source peak
variability, unless stellar variability is evoked.

###### keywords:

galaxies: photometry – galaxies: irregular – galaxies: peculiar – galaxies:
dwarf – galaxies: individual (SDSS J094332.35+332657.6)

††pubyear: 2017††pagerange: An Unusual Transient in the Extremely Metal-Poor
Galaxy SDSS J094332.35+332657.6 (Leoncino Dwarf)–B

##  1 Introduction

Time-domain astronomy, with an emphasis on transient astronomical events, is
one of the major focuses of synoptic, wide-field surveys (e.g., Djorgovski et
al. 2012). Transient astronomical events include sources such as
SN111supernovae of all types, FRBs222fast radio burst, GRBs333gamma-ray burst,
merger events (tidal disruption events, binary system mergers) and novae,
amongst others (Sect. 3). Such events are of particular astrophysical interest
since they are relatively rare. In some instances, the origin and physical
mechanisms that give rise to certain transients is still under debate, and
counterparts to many of the sources still remain unknown. Targeted surveys
have provided a plethora of new detections, and allowed systematic population
studies. In addition, surveys have also discovered new types of transient
sources, such as low-luminosity calcium-rich SN (e.g., Perets et al. 2010;
Sect. 3), hydrogen-poor superluminous SN (e.g., Quimby et al. 2011), and FRBs
(e.g., Lorimer et al. 2007). Recent and current on-going surveys include the
Gaia Science Alerts (Hodgkin et al. 2013), the SMT444SkyMapper Transient
survey (Scalzo et al. 2017), the CRTS555Catalina Real-Time Transient Survey
(Djorgovski et al. 2011), the CSP666Carnegie Supernova Project (Hamuy et al.
2006), the PTF777Palomar Transient Factory (Law et al. 2009; Rau et al. 2009)
and its sucessor the iPTF888Intermediate Palomar Transient Factory, and the
Pan-STARRS999Panoramic Survey Telescope and Rapid Response System (Chambers et
al. 2016). Examples of upcoming projects include the ZTF101010Zwicky Transient
Factory, the CRTS and Pan-STARRS successors, and the LSST111111Large Synoptic
Survey Telescope (Ivezić et al. 2008). Several interesting citizen science
projects, included, for example, within
Zooniverse121212https://www.zooniverse.org/, have also provided (or will
provide) significant results in the search for transients; these include
Astronomy Rewind, Supernova Sighting, Supernova Hunter and Galaxy Zoo
Supernova (Smith et al. 2011a).

This work was motivated by the serendipitous discovery of a Northern knot in
the blue POSS131313Palomar Observatory Sky Survey 1955 image of the extremely
metal-poor dwarf galaxy (XMP) SDSS J094332.35+332657.6 (Fig. 2; left-hand
column; row 4 – 5), a knot which is no longer present in the subsequent POSS
(Fig. 2; middle column), HST141414Hubble Space Telescope (Fig. 3; top) or
SDSS151515Sloan Digital Sky Survey (Fig. 3; bottom) images taken approximately
40 – 60 years later. SDSS J094332.35+332657.6 (with the epithet Leoncino Dwarf
used hereinafter), the optical counterpart to the HI source AGC 198691, was,
until recently (see J0811+4730 with 12+log(O/H) = 6.98$\pm$0.02; Izotov et al.
2017), the local, low-metallicity record-breaker at 12+log(O/H) = 7.02 $\pm$
0.03 (Hirschauer et al. 2016; hereinafter H16). The XMP category, to which
Leoncino Dwarf belongs, is generally composed of low-surface-brightness
(Sánchez Almeida et al. 2017), isolated (Filho et al. 2015) galaxies,
currently undergoing star formation processes under extreme low-metallicity
conditions (Filho et al. 2013, 2016), likely fueled by cosmological gas
accretion (Sánchez Almeida et a. 2015); hence, XMPs are excellent candidates
for the occurrence of extreme and variable events (Sect. 3), some of which may
be rare or unusual (e.g., Pustilnik et al. 2008; Izotov & Thuan 2009; Bomans &
Weis 2011; Pustilnik et al. 2017). Indeed, metal-poor environments appear to
be preferred hosts of long GRBs (Fruchter et al. 2006; Modjaz et al. 2008),
hydrogen-poor superluminous SN (Lunnan et al. 2014, 2015; Leloudas et al.
2015) and ultraluminous X-ray sources (Sutton et al. 2012). More recently, a
recurrent FRB, associated with a persistent radio source of unknown origin,
was detected in a metal-poor galaxy (Tendulkar et al. 2017; Bassa et al. 2017;
Chatterjee et al. 2017; Marcote et al. 2017; Kokubo et al. 2017). A star-
forming metal-poor dwarf galaxy was also found to host the unusual hydrogen-
rich SN iPTF14hls (Arcavi et al. 2017); interestingly, the POSS data was used
to verify a possible eruption in 1954, besides the 2014 event registered by
the iPTF, demonstrating the potential in using long-period-baseline archival
data for detecting transient events.

The present paper characterizes the observed properties of Leoncino Dwarf
utilizing all the available multi-wavelength data (Sect. 2), and attempts to
provide an interpretation for the observed transient phenomenon, as well as
the other time-dependent observables (Sect. 3).

Throughout, a cosmological model where $H_{0}=69.6$ km s-1 Mpc-1, $\Omega_{\rm
M}=0.286$, and $\Omega_{\rm vac}=0.714$ (Bennett et al. 2014), has been
adopted.

##  2 Data

Motivated by Leoncino Dwarf’s identification as one of the most metal-poor
dwarf galaxies in the local Universe, multi-wavelength information and data
for the target was procured via online database inquiries. A summary of the
general properties of Leoncino Dwarf, mainly reproduced from H16, is provided
in Table 1. It is to be noted that, although Leoncino Dwarf is too bright and
in the large size range, its stellar mass (M⋆) is within the realm of globular
clusters.

Table 1: General Properties of Leoncino Dwarf

Parameter | Value | Parameter | Value
---|---|---|---
RA (J2000) | 09 43 32.4 | $m_{\rm V}$ | 19.5 mag
Dec (J2000) | 33 26 58 | $M_{\rm V}$ | -10.0 mag
12+log(O/H) |  7.02 $\pm$ 0.03 | $m_{\rm B}$ | 19.8 mag
$D^{1}$ |  $\approx$8 Mpc | $M_{\rm B}$ | -9.8 mag
$d^{2}$ | 8.1 arcsec | SB${}_{\rm B}^{6}$ |  24.8 mag arcsec-2
$L({\rm H\alpha}$)3 |  6.4 $\times$ 1037 erg s-1 |  M⋆ |  1.6 $\times$ 105 M⊙
SFR4 |  5.1 $\times$ 10-4 M⊙ yr-1 | $V_{\odot}^{7}$ |  514 $\pm$ 2 km s-1
$W_{50}^{5}$ |  33 $\pm$ 2 km s-1 |  M${}_{\rm HI}^{8}$ |  8.0 $\times$ 106 M⊙

1Approximate minimum distance. Leoncino Dwarf is located in the direction of
the ’local velocity anomaly’, so that its distance is highly uncertain. This
distance is used to estimate absolute values in the table.

2Angular diameter, taking into account faint extended low surface brightness
emission lying beyond the central core of bright stars and the HII region,
obtained from the HST images.

3H$\alpha$ luminosity obtained from the WIYN narrow-band observations assuming
$D\simeq$ 8 Mpc.

4Star formation rate computed from the H$\alpha$ luminosity.

5ALFALFA HI line width at 50% of the peak flux density level.

6$B$-band surface brightness obtained using the $B$-band apparent magnitude
($m_{\rm B}$) and HST angular diameter ($d$).

7ALFALFA HI heliocentric velocity.

8ALFALFA HI mass assuming $D\simeq$ 8 Mpc.

###  2.1 POSS Data

There are first (I) and second (II) epoch POSS images of Leoncino Dwarf, taken
approximately 40 years apart (1955 and 1995 – 1998). These surveys were
carried out with the 48-inch (Oschin) Schmidt telescope on Mount Palomar, and
registered onto photographic plates, with plate scales of $\sim$1 arcsec
pix-1. The STScI161616Space Telescope Science Institute provides
photometrically uncalibrated, astrometrically calibrated (through the
GSC171717Guide Star Catalog 2.3; Lasker et al. 2008), digitized POSS images
with photometry in the blue (400 – 500 nm; POSS I-O and POSS II-J), red (600 –
750 nm; POSS I-E and POSS II-F) and infrared (750 – 1000 nm; POSS II-N),
available via several online catalogs. Table 2 contains the various catalog
entries for the POSS photometry of Leoncino Dwarf.

Astrometrically calibrated, photometrically uncalibrated, 15 arcmin $\times$
15 arcmin POSS I-O and POSS II-J (blue), POSS I-E and POSS II-F (red), and
POSS II-N (infrared) images were uploaded from the STScI, and processed
according to the procedure outlined in Section 2.1.1.

Table 2: Catalog POSS Photometry for Leoncino Dwarf

Catalog | Survey |  Band† |  $m$ [mag] | Date | Reference
---|---|---|---|---|---
USNO A2.0 | POSS I-O | O |  18.7$\pm$…a | 13/03/1955 | Monet (1998)
USNO B1.0 | POSS I-O | O |  19.5$\pm$…a | 13/03/1955 | Monet et al. (2003)
USNO B1.0 | POSS II-J |  BJ |  19.6$\pm$…a | 18/03/1996 | Monet et al. (2003)
GSC 2.2 | POSS II-J |  BJ |  19.0$\pm$0.4 | 18/03/1996 |  STScI (2001)b
GSC 2.3.2 | POSS II-J |  BJ |  19.0$\pm$0.4 | 18/03/1996 |  STScI (2006)b
GSC 2.3.2 | POSS II-J |  BJohn |  19.5$\pm$0.4 | 18/03/1996 |  STScI (2006)b
USNO A2.0 | POSS I-E | E |  18.9$\pm$…a | 13/03/1955 | Monet (1998)
USNO B1.0 | POSS I-E | E |  19.1$\pm$…a | 13/03/1955 | Monet et al. (2003)
USNO B1.0 | POSS II-F |  RF |  19.1$\pm$…a | 16/04/1998 | Monet et al. (2003)
GSC 2.3.2 | POSS II-F |  RF |  19.5$\pm$0.6 | 16/04/1998 |  STScI (2006)b
USNO B1.0 | POSS II-N |  IN |  18.9$\pm$…a | 23/02/1995 | Monet et al. (2003)

†Blue (top), red (middle) and infrared (bottom) bands.

aCatalog does not provide a photometric error.

bCatalog releases.

####  2.1.1 POSS Processing Techniques

The POSS photometric calibration, as implemented in the USNO181818United
States Naval Observatory Astrometric Standards Catalog A2.0 catalog (Monet
1998), utilizes the Tycho catalog for the bright end of the photometric
calibration, while the faint end is set by the USNO CCD191919charge-coupled
device parallax fields in the North and the Yale Southern Proper Motion CCD
calibration fields in the South202020http://tdc-
www.harvard.edu/catalogs/ua2.html#phot. The Tycho-2, GSPC212121Guide Star
Photometric Catalog II and the photometric data measured for the
NOFS222222Naval Observatory Flagstaff Station CCD parallax program are used
for the photometric calibration of the USNO B1.0 catalog (Monet et al. 2003),
while the Tycho and GSPC (I and II) catalogs are used as photometric
calibrators for the GSC 2.3.2 catalog (Lasker et al. 2008). The different
photometric calibrations, particularly at the faint end, may explain the
$\Delta m\lesssim$ 1 mag in the USNO A2.0 relative to the USNO B1.0/GSC
2.2/GSC 2.3.2 blue photometric values in Table 2. The photometry is optimized
for point-like sources; the photometric accuracy is $\approx$0.3 mag for
stellar-like objects, with additional errors for faint, extended objects
(Monet et al. 2003; Lasker et al. 2008). When compared with the SDSS early
data release, photometric offsets for extended, faint objects as large as 0.4
mag were found (Lasker et al. 2008).

The POSS astrometry, as provided by the GSC 2.3 catalog, is tied to the
ICRS232323International Celestial Reference System, defined by the
Tycho/Tycho-2 (and a subset of ACT242424Astrographic Catalog/Tycho) faint-end
stars, since the Hipparchus stars are heavily saturated on the plates (Lasker
et al. 2008). For this catalog, absolute astrometric errors are typically 0.2
– 0.3 arcsec for stellar-like sources, with $>$20% poorer errors for extended,
faint sources (Lasker et al. 2008). An initial comparison with the SDSS early
data release and UCAC252525United States Naval Observatory Charge-Coupled
Device Astrograph Catalog 2 showed astrometric offsets for extended, faint
objects as large as 0.7 arcsec (Lasker et al. 2008).

In order to further assess the astrometrical calibration of the POSS images
acquired via the STScI, large FoV262626field-of-view images (15 arcmin
$\times$ 15 arcmin) around the target were inspected. For the stars on the
individual images, the astrometric calibration appears to be satisfactory,
within the expected errors. Residual shifts observed in the stellar positions
in the subtracted images are observed, but these are caused by proper motions
of the stars over the approximate 40-year period. For several galaxies around
Leoncino Dwarf (Fig. 3), a simple subtraction procedure has performed well,
demonstrating that the astrometric calibration in the extended sources appears
also to be satisfactory. Hence, an additional amelioration of the astrometric
calibration was not attempted.

Conducive to evidencing any change in the POSS images of Leoncino Dwarf
between epochs, the images (POSS I-O and POSS II-J in the blue, and POSS I-E
and POSS II-F in the red) were adequately subtracted, a process which requires
matching the spatial resolution and the photometry per band in the two epochs.
Photometric calibration per band and subtraction of the POSS images was
performed according to the following standard prescription (e.g.,
Annunziatella et al. 2013). Firstly, the POSS images were properly registered,
i.e., they were matched in the WCS272727world coordinate system using
Python282828https://www.python.org/ 2.7.5. Aperture photometry of 10 – 20
stars in each POSS field was performed using
ds9292929http://ds9.si.edu/site/Home.html 7.1, and the average count ratio was
used to calibrate the POSS images, assuming that the stars did not vary
between epochs and neglecting (small) differences in the filters. The typical
normalized rms303030root mean square deviation of the stellar counts in all
four POSS images (red/blue POSS I/POSS II) was 0.3. In the blue and red bands,
the normalizations (POSS II to POSS I) are 1.06 and 1.12, respectively. The
PyRAF313131http://www.stsci.edu/institute/software_hardware/pyraf 2.1.6
package DAOPHOT, specifically the routines DAOFIND (finds the stars in the
image), PHOT (performs aperture photometry) and PSF (builds the PSF323232point
spread function model), was then used to extract the PSF in all four images
(red/blue POSS I/POSS II). The procedure provided almost circular Gaussian
PSFs of FWHM333333full-width half-maximum 4.7, 3.8, 4.4, and 3.7 arcsec for
the blue POSS I, blue POSS II, red POSS I and red POSS II image, respectively.
Assuming that the PSF characterizes the POSS images, in order to match the
image PSFs, the better resolution image (POSS II) needs to be degraded in
resolution by convolving ($\otimes$) it with a Gaussian 2-dimensional kernel
(G2DK)

${\rm POSS\,II}\otimes{\rm G2DK}={\rm POSS\,I}.$ (1)

If the PSFs are Gaussians, then G2DK is also a 2-dimensional Gaussian function
with a FWHM that follows from

${\rm FWHM}_{\rm POSS\,II}^{2}+{\rm FWHM}_{\rm G2DK}^{2}={\rm FWHM}_{\rm
POSS\,I}^{2}.$ (2)

This results in a blue and red G2DK FWHM of 2.7 and 2.4 arcsec, respectively
(Eq. [2]). Using Python, the (blue and red) POSS II images were convolved with
a G2DK of the corresponding FWHM. The convolved POSS II images (POSS II-J in
the blue and POSS II-F in the red), were then subtracted from the POSS I
images (POSS I-O in the blue and POSSI-E in the red). Figure 1 contains an
example, in the blue, of the PSF variation between epochs, and the effective
substraction procedure, on three unsaturated stars in the field, after they
have been shifted to account for proper motion between epochs. Figure 2
includes the multi-epoch, multi-band (WCS matched, photometrically calibrated)
POSS and POSS convolved images of the target, as well as subtracted and
convolved subtracted images.

Figure 1: Top: Blue POSS I, blue POSS II, and blue convolved POSS II image of
three unsaturated stars in the field, after they have been shifted to account
for proper motion. Bottom: Blue subtracted (POSS II – POSS I) and blue
convolved subtracted (POSS II – POSS I) image of the same stars. The images
illustrate the effectiveness of the subtraction procedure. All images show the
same (linear) brightness scale for clarity. All images are 1 arcmin $\times$ 1
arcmin. Figure 2: Row 1: Infrared POSS II image. Row 2: Red POSS I, red POSS
II, and red subtracted (POSS II – POSS I) image. Row 3: Red POSS I, red
convolved POSS II, and red convolved subtracted (POSS II – POSS I) image. Row
4: Blue POSS I, blue POSS II, and blue subtracted (POSS II – POSS I) image.
Row 5: Blue POSS I, blue convolved POSS II, and blue convolved subtracted
(POSS II – POSS I) image. Red/blue (convolved or unconvolved) and subtracted
red/blue (convolved or unconvolved) images show the same (linear) brightness
scale for clarity. All images are 1 arcmin $\times$ 1 arcmin. Indication of
the sources in the FoV is provided in the blue POSS images. Residual signals
in the subtracted blue images at the position of the main source are partly an
artifact, arising from the imperfect photometric calibration, differences in
the PSFs, and partly due to true flux variability. Notice the absence of the
transient source in the blue POSS II image.

####  2.1.2 Transient and Morphological Changes?

Inspection of the blue POSS I, blue POSS II and blue subtracted images (Fig.
2; row 4) shows that the most accented change is the disappearance, in the
second POSS epoch, of the bright blue knot to the North (hereinafter transient
source) of the main (host) source; the transient is clearly present in the
blue POSS I image (Fig. 2; left-hand column; row 4) at the 5$\sigma$ level
(see below), and absent in the blue POSS II higher resolution, higher
SNR343434signal-to-noise image (Fig. 2; middle column; row 4). It is
noteworthy that the blue subtracted image (Fig. 2; right-hand column; row 4)
mostly removes the main galaxy (see below), leaving a residual signal at the
position of the transient, which reinforces the absence of the transient in
the blue POSS II image. This variation occurs even when the change in spatial
resolution between the two epochs is taken into account (Fig. 2; row 5). The
transient source should not be confused with the companion Northern source in
Figure 2 (row 4), which is also marked in the HST (Fig. 3; top) and SDSS (Fig.
3; bottom) images. There is no clear evidence for the presence of the
transient source in the red POSS I image (Fig. 2; left-hand column; row 2 –
3), which has an average limiting magnitude of $m_{\rm lim}\approx$ 20.0
mag353535http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/dss/?pedisable=true
(see below).

The distance between the peak of the main source and the transient source
(blue POSS I) is $\approx$6 pixels or $\approx$6 arcsec ($\approx$0.3 kpc)
towards the North, at an angle of $\approx$$-$10∘ (Fig. 2; left-hand column;
row 4 – 5). From the PSF characteristics of the blue POSS I and blue
subtracted POSS images (Sect. 2.1.1), the transient source is slightly
resolved ($<$ 2 $\times$ PSF FWHM). Because of the faintness of the transient
and its proximity to the larger, brighter main source, an automated detection
algorithm such as SExtractor (Bertin & Arnouts 1996), implemented in
GAIA363636Graphical Astronomy and Image Analysis Tool 2016A, was unsuccessful
in deblending the two sources. Therefore, a more rudimentary method was
employed to determine the magnitude of the transient: using ds9, the transient
and the brightest Northwestern source (Sect. 2.2 and 2.3; Fig. 2; left-hand
column; row 4 – 5; Fig 3) were fit with ellipses to perform relative aperture
photometry on the photometrically calibrated unconvolved POSS images, after
sky subtraction (see below). This procedure shows that the transient source
corresponds to $\gtrsim$96% of the flux of the Northwestern source ($m\approx$
20.9 mag; USNO B1.0; Monet et al. 2003), which is equivalent to the transient
being $\Delta m\lesssim$ 0.05 mag fainter than the Northwestern source (Fig.
2; left-hand column; row 4 – 5). Similarly, the flux ratio between the main
source and the transient is approximately a factor of 6 (Sect. 2.1.4). Given
this very simplistic procedure to obtain the transient photometry, the
magnitude of the transient is quite uncertain. A magnitude of $m\approx$
21.0$\pm$0.2 mag will be adopted in the following discussion (Sect. 3), with
the confidence interval provided assuming a 20% error in the flux (detection
at a 5$\sigma$ level; see below), corresponding to the estimated photometric
error at the maximum intensity extended to the full source, and neglecting
unquantifiable systematics errors; the error estimation is included in
Appendix A. Favorably, changes in the magnitude as large as $\Delta m\approx$
1.5 will not significantly alter the conclusions of the present work (Sect.
3).

The magnitude of the transient puts the transient near the limiting magnitude
of the blue POSS I images commonly quoted in literature ($m_{\rm lim}\approx$
21.0 – 21.5 mag; e.g., Djorgovski et al. 2013), which appears associated with
the sky brightness level (e.g., Reid et al. 1991). A 9 arcsec $\times$ 13
arcsec elliptical annulus around the main source and transient was used to
estimate the local mean value and rms fluctuation of the sky signal. In the
case of the transient, the peak flux is found to be $\simeq$5 times the rms
fluctuation of the local sky signal. As a sanity check, a similar-sized
elliptical annulus around the main source and transient in the blue subtracted
image (Fig. 2; right-hand column; row 4) was used to estimate the local rms
fluctuation of the sky signal, providing a consistent $\simeq$5 transient
peak-to-rms fluctuation value. Therefore, the chance that the transient signal
is due to a random fluctuation of the sky signal is as low as 3 $\times$ 10-7,
assuming the sky noise follows a Gaussian probability density function
(similarly small probabilities are found when assuming other probability
density functions). Several other independent arguments support that the
transient signal is not due to a random fluctuation of the sky signal. For
instance, the peak signal of the transient is $\simeq$1.6 higher than the peak
of the Northwestern source, which has a (blue POSS I) USNO B1.0 entry (see
above), and is clearly detected in the HST (Fig. 3; top) and SDSS (Fig. 3;
bottom) images. In addition, the peak sky signals surrounding the galaxy are
$\approx$2.5 times smaller than the transient signal.

Because the SNR and resolution of the blue POSS I image is poorer than the
blue POSS II image, the transient should be more difficult to detect in the
first instance; hence, the fact that the transient is observed in the poorer
resolution, poorer SNR image strengthens the reliability of the transient.
However, it also raises concerns as to whether it may be an artifact. Various
possibilities will be examined and discarded, as follows. A large FoV around
the source (15 arcmin x 15 arcmin) was examined, and from the inspection of
tens of stars in the field, it was concluded that the transient was not a PSF
effect; although the PSF is poorer in the blue POSS I image, it is relatively
well-behaved across the field and almost circular (Sect. 2.1.1; Fig. 1). The
blue POSS I digitized image does show scratches in several places, but nothing
that could explain the transient. Plate distortions would also not produce
such a response. Dust on the plate would have created a pinhole effect. There
does not appear to be any anomalous optical aberration in the field, nor is
there a repeated pattern in other sources. Photographic plate emulsions, such
as those used for POSS, typically respond to illumination (i.e., sky) by
producing a granular noise structure (Fig. 1 and 2), but the transient signal
is clearly distinguishable from the sky signal (see above). Cosmic rays are a
possibility, but these photographic plate emulsions are sensitive to less
energetic particles that leave a dense signal track. In addition, emulsions
were generally thin at the time, so that particles would need to travel almost
perfectly parallel to the plane to leave a substantial track. Not only is the
transient source only slightly resolved (see above) and unlike a track,
inspection of the large FoV around the source does not reveal any such
discernible tracks. There is no obvious reason why an anomaly in the
telescope, plate, plate emulsion or densiometer would produce such an effect.
Hence, the evidence for a true transient is secure.

Residual signals in the blue subtracted images (Fig. 2; right-hand column; row
4 – 5), at the position of the main source, are partly an artifact, arising
from the imperfect photometric calibration (Sect. 2.1.1), differences in the
PSFs (Fig. 1), and partly due to true flux variability (Sect. 2.3.1). From the
comparison with the PSFs of the POSS images (Sect. 2.1.1), the main source
appears resolved. The infrared POSS II image shows only very faint emission at
the position of the main source (Fig. 2; row 1). The blue and red POSS I
images (Fig. 2; left-hand column; row 2 – 5) show the main source to be almost
circular. Elliptical fits (avoiding the transient source) using ds9 provide
axis ratios of $\approx$1\. However, the morphology of the main source appears
to change in the second epoch POSS images (Fig. 2; middle column; row 2 and 4)
to become similar to what is seen in the SDSS image (Fig. 3; bottom); the
emission appears to elongate towards the West-Southwest direction. Elliptical
fits to the blue and red POSS II images show an axis ratio of $\approx$1.5 and
PAs373737position angle of $\approx$220 and $\approx$1 deg, respectively. Such
a change in the morphology of the main source can not be due to effects from
the varying PSF (Sect. 2.1.1), nor is it likely due to a systematic
astrometric offset (Sect. 2.1.1). It is significant that the morphology change
is still apparent in the convolved red/blue POSS II images (Fig. 2; middle
column; row 3 and 5).

A Northwestern source is discernible in the blue POSS I (Fig. 2; left-hand
column; row 4 – 5), blue POSS II (Fig. 2; middle column; row 4 – 5) and red
POSS I images (Fig. 2; left-hand column; row 2 – 3), corresponding to the
brightest of two Northwestern sources apparent in the HST (Fig. 3; top) and
SDSS (Fig. 3; bottom) images. A very faint Northern source is apparent in the
blue POSS II image (Fig. 2; middle column; row 4 – 5), as well as in the HST
(Fig. 3; top) and SDSS (Fig. 3; bottom) images. A faint Southern source
appears in the second epoch blue POSS image (Fig. 2; middle column; row 4 –
5), discernible also in the HST (Fig. 3; top) and SDSS (Fig. 3; bottom)
images.

####  2.1.3 Proper Motion and Brightness Centroid Shift?

Leoncino Dwarf is documented as having a relative proper motion (in
RA383838Right Ascension) of 16 $\pm$ 5 mas yr-1 in the USNO B1.0 (Monet et al.
2003) catalog, and an absolute proper motion (in RA) of 7.5 $\pm$ 5 mas yr-1
in the PPMXL393939Positions and Proper Motions Extra Large catalog, relative
to a mean epoch of 1979.7 (Roeser, Demleitner & Schilbach 2010). The errors in
the proper motions in Dec404040Declination in both catalogs are of the order
of, or larger, than the proper motions themselves, and so, are not considered
further. The PPMXL absolute proper motion value corresponds to $\approx$0.3
arcsec in approximately 40 years, and represents superluminal motion at the
distance of Leoncino Dwarf (Table 1). Measuring proper motions beyond 1 Mpc is
highly unusual; typically, proper motions are measured only for the Local
Group (e.g., van der Marel et al. 2014), with values that are in the range of
tens of $\mu$arcsec yr-1 (e.g., Brunthaler et al 2007).

As was argued in Section 2.1.1, the PSF is relatively well-behaved, so that it
is likely not responsible for a proper motion effect. The astrometrical
calibration also appears to be satisfactory, within the expected errors, which
should be $\approx$0.3 arcsec for slightly resolved objects such as the
transient and main source (Sect. 2.1.1). In order to further investigate this
issue, relative astrometry between the main source, companion sources and
field sources (seven bright stars and two galaxies within 15 arcmin of the
target), was attempted using ds9 and Python. The conclusion is that there does
not appear to be any significant relative motion between POSS epochs of the
various sources, in the red and blue bands. Hence, it can not be excluded that
the apparent flagged proper motion in RA arises from astrometric calibration
errors (Sect. 2.1.1).

It is, however, noteworthy that, in the blue band, the peak of the main source
shifts by $\approx$1 pixel ($\approx$1 arcsec) towards the South between POSS
epochs (Fig. 2; row 4 – 5), which is larger than the expected astrometric
error in the POSS images (Sect. 2.1.1). This main source brightness centroid
shift is not related to PSF variations (Sect. 2.1.1), and no systematic
astrometric offset appears to be present (Sect. 2.1.1). It is also not an
effect brought on by the disappearance of the transient; the distance from the
transient to the main source peak is $\approx$6 arcsec, larger than the
typical astrometric error, and the main source-to-transient flux ratio is
approximately 6 (Sect. 2.1.1, 2.1.2 and 2.1.4).

####  2.1.4 Long-Term 40-year Baseline Variability?

Rejecting the USNO A2.0 entries, which may have faint-end photometric issues
(Sect. 2.1.1), the POSS photometry (Table 2) may suggest that some variation
in flux ($\Delta m$ $\lesssim$ 1 mag) has occurred between 1955 and 1998. The
photometric accuracy is $\approx$0.3 mag for stellar-like objects, with
additional errors for faint, extended objects, which typically may reach
$\approx$0.4 mag (Monet et al. 2003; Lasker et al. 2008).

In order to further investigate possible flux variation, the photometrically
calibrated unconvolved POSS images were used to investigate variability in the
main source using ds9. The comparison between the two epochs appears to
demonstrate that the peak of the main source shows a factor $\simeq$2
variation in flux ($\Delta m$ $\simeq$ 0.7 mag), in the blue band, over a
period of approximately 40 years, a variation larger than the typical
photometric error for faint, extended objects (Lasker et al. 2008). The main
source peak appears to have brightened in the blue band, a variation which can
also be observed in Figure 2 (middle column; row 4 – 5). The variation is not
an effect of the calibration procedure, as the magnitude of the variation is
larger than the calibration applied to the blue POSS I image (Sect. 2.1.1).
The brightness increase of the main source peak is also unrelated to the
disappearance of the transient; the transient is approximately 6 times lower
in flux, and $\approx$ 6 arcsec distant from the main source (Sect. 2.1.2. and
2.1.4).

###  2.2 H16 Data

H16 contains an ACS414141Advanced Camera for Surveys HST image of Leoncino
Dwarf in the (combined) $\sim$$V$ and $\sim$$I$ filters. The HST images (Fig.
3; top) reveal a blue cluster of bright stars, extended from the Northeast to
the Southwest, but more concentrated towards the Southwest, defining the main
source. There are also source companions (hereinafter companion sources), all
likely galaxies, to the North, Northwest and South of Leoncino Dwarf (Fig. 1;
top; Sect. 2.1 and 2.3). The fainter of the Northwestern sources appears to be
a red edge-on disk galaxy (Fig. 1; top). The Northern source also appears as
an edge-on disk galaxy, but the color is slightly bluer (Fig. 1; top). Both
the brighter Northwestern source and the Southern source appear more face-on,
and have similar colors to the Northern source (Fig. 1; top). An HI map
obtained with the WSRT424242Westerbork Synthesis Radio Telescope, although of
poor resolution ($\sim$22 arcsec $\times$ $\sim$13 arcsec beam), shows HI
emission centered on the optical galaxy, with a hint of an extension towards
the South/Southwest and possibly Northwest, towards the companion sources
(H16). Although it is not entirely clear from the HI image if there is some
association, it appears that, at least the brighter Northwestern and Southern
source may be similar redshift companions to Leoncino Dwarf (Sect. 2.1 and
2.3). There are also WIYN434343University of Wisconsin-Madison, Indiana
University, Yale University and the National Optical Astronomy Observatories
0.9 m observations (H16), which show unresolved H$\alpha$ emission to the
Southwest. It is this Southwestern HII emission region that was the target for
follow-up observations with the KPNO444444Kitt Peak National Observatory
KOSMOS454545Ohio-State Multi-Object Spectrograph and with the
MMT464646Multiple Mirror Telescope Blue Channel Spectrograph (H16). These
spectroscopic observations provide the low-metallicity value for Leoncino
Dwarf, as well as other relevant spectroscopic measurements (Table 1; see H16
for details).

Figure 3: Top: HST $V$-band (left) and $I$-band (right) sinh-scale images.
Bottom: SDSS composite image. Notice the companion sources to the North, South
and Northwest. The images are 50 arcsec $\times$ 50 arcsec.

###  2.3 SDSS Data

Leoncino Dwarf is included in the SDSS photometric catalog, but possesses no
SDSS spectroscopy. The SDSS photometric parameters are included in Table 3,
and the SDSS composite image is provided in Figure 3 (bottom).

Table 3: SDSS Photometry of Leoncino Dwarf

Parameter | Value |  Parameter† | Value
---|---|---|---
$m_{\rm u}$ |  19.79 $\pm$ 0.09 mag | $R_{\rm u}$ | 4.5 arcsec
$m_{\rm g}$ |  19.58 $\pm$ 0.04 mag | $R_{\rm g}$ | 3.6 arcsec
$m_{\rm r}$ |  19.55 $\pm$ 0.04 mag | $R_{\rm r}$ | 3.1 arcsec
$m_{\rm i}$ |  19.96 $\pm$ 0.12 mag | $R_{\rm i}$ | 2.4 arcsec
$m_{\rm z}$ |  19.86 $\pm$ 0.38 mag | $R_{\rm z}$ | 2.9 arcsec

†SDSS Petrosian radius, at 90% of the light, in the various bands.

The SDSS $u$\- and $i$-band images essentially probe the main source, which is
reproduced also in the $g$\- and $r$-band images, albeit as brighter emission
(Fig. 3; bottom; Table 3). The SDSS $u$\- and $i$-band image of the main
source appears to lack some of the extended emission seen to the West-
Southwest in the $g$\- and $r$-band (Fig. 3; bottom). The main source is
barely detectable in the $z$-band. It is to be noted that the $g$\- and
$r$-band have the least noise; missing weak signals in other SDSS bands are
likely due to their higher noise level.

The companion sources observed in the HST image (Fig. 3; top; Sect. 2.2) are
also detected in the SDSS image (Fig. 3; bottom). The Northern source
possesses no SDSS spectroscopy nor photometry, but shares the same reddish
color as the other companion sources. Despite not appearing in the SDSS
spectroscopic catalog, the remaining companion sources do appear in the SDSS
photometric catalog as galaxies, with magnitudes of $m_{\rm g}\simeq$ 22.5 mag
(Southern source), and $m_{\rm g}\simeq$ 21.2 and $m_{\rm g}<$ 22.0 mag
(Northwestern sources).

####  2.3.1 Long-Term 60-year Baseline Variability?

Section 2.1.4 discusses possible main source peak variability on an
approximately 40-year baseline. Flux variations on an approximately 60-year
baseline may be constrained using the color transformation from the SDSS $g$-,
$r$\- and $i$-bands to the POSS (I-O, I-E, II-J, II-F, II-N) survey filters,
transformations which typically have a dispersion of approximately 0.3 mag
(Monet et al. 2003). Comparison of the POSS magnitudes (Table 2) and SDSS
magnitudes transformed to the POSS filters (Table 4) appear to show some
possible variation in the infrared since 1998 ($\Delta m\lesssim$ 1 mag), but
a steady blue and red flux since 1998. If the blue POSS main source peak flux
variation is real (Sect. 2.1.4), this suggests that the event occurred between
1955 and 1998; however, if the POSS/SDSS infrared flux variation is real, this
suggests that the event may still persist (Sect. 2.4.1 and 3).

Table 4: POSS I/II Magnitudes Corresponding to SDSS Photometry for Leoncino
Dwarf

Survey |  Band† |  $m$ [mag]
---|---|---
POSS I-O | O | 19.7
POSS II-J |  BJ | 19.6
POSS I-E | E | 19.4
POSS II-F |  RF | 19.5
POSS II-N |  IN | 19.6

†See Table 2.

###  2.4 WISE Data

Using a 10 arcsec cone search, the WISE474747Wide-Field Infrared Survey
Explorer Post-Cryogenic Single Exposure Source Table reveals three targets
(Fig. 4; blue circles), all within $\lesssim$8 arcsec of the galaxy position
(Table 5). Four targets (Fig. 4; green circles), all within $\lesssim$8 arcsec
of the galaxy position, were also found in the NEOWISE-R484848Near-Earth
Object WISE Reactivation mission Single Exposure Source Table (Table 5). The
single exposure data were acquired several months to one year apart (Table 5).
One target (Fig. 4; magenta circle) was further found in the AllWISE Reject
Table (Table 5). AllWISE is a combination of WISE cryogenic and NEOWISE post-
cryogenic phase data, including a recalibration of the original photometry and
astrometric solutions integrating the proper motions of reference stars; for
high SNR sources in non-confused regions, the astrometric accuracy of AllWISE
is $\sim$50 mas radially, relative to the ICRS defined by quasars. The WISE
data at 3.35 and 4.6 $\mu$m possess angular resolutions of $\approx$6 arcsec.
The SNR of all the targets falls below the value of 5 generally required to be
a reliable detection (Table 5).

####  2.4.1 Short-Term Variability?

Cross-correlation of the WISE Post-Cryogenic/NEOWISE-R/AllWISE data with the
blue POSS I image (Fig. 4) shows that one of the NEOWISE-R targets (source 5)
is within $\simeq$3 arcsec of the transient source, and another two targets
(source 1 and 2) coincide with the main source peak position. There is,
however, a difference in magnitude (Table 5) between the NEOWISE-R (source 2)
and AllWISE (source 1) target, which exceeds the difference expected from pure
noise.

It is unclear whether, and how, the WISE data points and the galaxy are
related in terms of position and flux (Fig. 4). There are at least five
possibilities for these apparent short-term (several months – 1 year) changes
in magnitude and position: (a) they represent the same source/sources moving
across the galaxy (Sect. 2.1.3 and 3), (b) they demonstrate intrinsic
variability of the galaxy (Sect. 2.1.4, 2.3.1 and 3), (c) they represent one
weak infrared source, likely related to the galaxy itself, but because it is
at noise level, the peak will shift slightly between exposures (Sect. 2.1.3),
(d) they are spurious signals at noise level, and (e) they correspond to
astrometric/photometric shifts due to the new astrometric/photometric
calibrations. Given the evidence, it is likely that the WISE detections
correspond to astrometric/photometric shifts representing one weak (2 $<$ SNR
$<$ 5) infrared source associated with the galaxy.

Figure 4: Blue POSS I image (linear-greyscale) on which there is superimposed the positions (1 arcsec circles) of the WISE Post-Cryogenic (blue circles), NEOWISE-R (green circles) and AllWISE (magenta circle) targets. All detections have 2 $<$ SNR $<$ 5\. One of the NEOWISE-R (source 2) and AllWISE (source 1) targets coincides with the peak in the main source, while one of the NEOWISE-R (source 5) targets is within $\simeq$3 arcsec of the transient source. The image is 1 arcmin $\times$ 1 arcmin. Table 5: WISE Photometry of Leoncino Dwarf Survey | Source | Date |  3.35 $\mu$m | SNR |  4.6 $\mu$m | SNR
---|---|---|---|---|---|---
| [Fig. 4] |  | [mag] |  | [mag] |
ALLWISE Reject | 1 | … | 18.0 | 3.7 | … | …
NEOWISE-R | 2 | 03/05/2014 | 16.1 | 4.4 | … | …
| 3 | 10/11/2014 | 16.8 | 2.2 | … | …
| 4 | 10/11/2015 | 15.9 | 3.4 | … | …
| 5 | 30/04/2015 | … | … | 14.7 | 2.3
WISE Post-Cryo | 6 | 10/11/2010 | … | … | 15.3 | 2.7
| 7 | 09/11/2010 | 16.6 | 2.8 | … | …
| 8 | 10/11/2010 | 16.5 | 3.1 | … | …

###  2.5 Other Data

Other multi-wavelength data for the target was procured via online database
inquiries, with searches within a 10 arcsec cone/radius of the target position
(Table 1). The source has not been detected in the FIRST494949Faint Images of
the Radio Sky at Twenty-Centimeters, NVSS505050National Radio Astronomy
Observatory Very Large Array Sky Survey nor in the VLSS515151Very Large Array
Low-Frequency Sky Survey. There are no observations of Leoncino Dwarf in the
Herschel, Chandra or XMM525252X-ray Multi-Mirror Mission-Newton database.
There also does not appear to be a 2MASS535353Two Micron All-Sky Survey source
associated with the target position. The IAU545454International Astronomical
Union Central Bureau for Astronomical
Telegrams555555http://www.cbat.eps.harvard.edu/ was investigated for any
SN/nova/unusual variable events at the target position; none were found. The
source is not flagged in any of the NEOWISE565656Near-Earth Object Wide-Field
Infrared Survey Explorer mission catalogs as being associated with a Solar
System object (Sect. 2.4). The HORIZONS Web-Interface
tool575757https://ssd.jpl.nasa.gov/horizons.cgi#top, a limited interface to
Jet Propulsion Laboratory’s HORIZONS system, was used to generate ephemerides
for the Solar System planets; no planets were found on the observation date
and time at the position of the transient source.

##  3 Nature of the Observed Phenomena

It has been hypothesized that it is probable, from the multi-epoch imaging,
that the bright blue knot to the North of the main source is a transient,
since it is no longer visible in the blue POSS II (Fig. 2; iddle column; row 4
– 5) nor in the HST (Fig. 3; top) and SDSS (Fig. 3; bottom) images. During the
same approximate 40-year time period, the disappearance of this transient
source appears also accompanied by a variation in the main source morphology
(Sect. 2.1.2), a change in the brightness centroid of the main source (Sect.
2.1.3), and a peak flux variation of the main source (Sect. 2.1.4 and 2.3.1;
Table 2 and 4; Fig. 2; row 4 – 5). In principle, it could be argued that these
phenomenon are uncorrelated. However, several quasi-simultaneously variable,
but independent events, in one extragalactic source is highly improbable.
Hence, given the timeline and characteristics, it appears more likely that the
phenomena are correlated.

Below, various simultaneous explanations of these observables are attempted,
taking into consideration the following criteria, ordered by decreasing
reliability in the data: (a) transient source (Sect. 2.1.2), (b) change in the
morphology of the main source (Sect. 2.1.2), (c) brightness centroid shift of
the main source (Sect. 2.1.3), (d) long-term ($t\lesssim$ 40 year) peak
variability of the main source (Sect. 2.1.4 and 2.3.1), and (e) short-term (1
year $\gtrsim t\gtrsim$ several months) variability (Sect. 2.4.1). The
scenarios put forward to explain the observations have been ordered according
to their ability to account for an increasing number of observables. However,
it is to be stressed that this is not synonymous with an increasing
probability of the scenario to explain the observations; for example, despite
being able to explain most of the observables, the gravitational lens scenario
is statistically highly unlikely. Although some of the hypotheses may be
clearly discarded, several others are still viable under certain specific
conditions. Table 6 provides a compilation of the various scenarios that have
been explored, and the observables they are able to reproduce.

Throughout, a distance of $D\simeq$ 8 Mpc to Leoncino Dwarf (Table 1), and an
apparent magnitude for the transient of $m\approx$ 21.0 mag (Sect. 2.1.2),
have been adopted. However, the distance (H16) and magnitude (Sect. 2.1.2) are
uncertain. If the apparent magnitude of the transient is fainter by $\Delta
m\approx$ 1.5 mag, either due to doubling the distance to Leoncino Dwarf
($D\approx$ 16 Mpc) or due to a large error in the estimated transient
photometry, the following results are not substantially altered.

Table 6: Nature of the Observed Phenomena

Observable | Stellar | Dust | Stellar | Solar System | Accretion | Hypernova/ | Stellar | Nova | LBV | Gravitational
---|---|---|---|---|---|---|---|---|---|---
| Variability | Enshroudment | Cluster | Object | Variability | SN | Merger |  | Star | Lensing
Transient Source | ✗ | ✗ | ✓? | ✓? | ✓? | ✓ | ✓ | ✓ | ✓ | ✓
Morphological Changes | ✓ | ✗ | ✗ | ✗ | ✗ | ✗ | ✓? | ✓? | ✓? | ✓
Brightness Centroid Shift | ✓ | ✗ | ✗ | ✗ | ✗ | ✗ | ✓? | ✓? | ✓? | ✓
Long-term Variability | ✓ | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | ✓
Short-term Variability | ✓ | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | ✓?

’?’ Refers to an uncertain, improbable or highly contrived explanation.

Stellar Variability in the Main Source: As a low-surface-brightness galaxy
(Table 1), the shift in the brightness centroid (Sect. 2.1.3), the morphology
change (Sect. 2.1.2), and the (long- and short-term) flux variation in the
main source (Sect. 2.1.4, 2.3.1 and 2.4.1), may be due to the variability
($\Delta m\simeq$ 0.7 mag, the main source peak variability in the blue) of a
few individual massive stars in the galaxy, stars that are clearly resolved in
the HST images (Fig. 3; top). However, such a scenario requires an independent
explanation for the transient source (Sect. 2.1.2).

Dust Enshroudment by Feedback Processes: In galaxies there are two types of
feedback processes to consider: those related to massive black holes
(M${}_{\rm BH}\gtrsim$ 106 M⊙), and those related to star formation. There is
evidence for relatively strong SN-driven winds ($v\simeq$ 100 – 400 km s-1)
originating from HII regions (e.g., Olmo-García et al. 2017 and references
therein). Massive black holes, on the other hand, show a large range in
outflow velocities, ranging from $v\simeq$ 200 km s-1 in quiescent galaxies
(e.g., Cheung et al. 2016), to velocities of thousands of kilometers per
second in quasars (e.g., Rogerson et al. 2015). In principle, winds/outflows
can sweep up dusty gas around bright sources, thus producing optical
transients.

In approximately 40 years, a black hole-driven outflow ($v\simeq$ 10 000 km
s-1) can travel a radial distance of $r\approx$ 0.4 pc, while SN-driven winds
($v\simeq$ 100 km s-1) from an HII region can travel a radial distance of
$r\approx$ 0.004 pc. However, such distances are very small when compared with
the overall size of Leoncino Dwarf ($s\approx$ 0.4 kpc, for $D\simeq$ 8 Mpc;
Table 1), and with the distance between the transient source and peak of the
main source ($d\approx$ 0.3 kpc, for $D\simeq$ 8 Mpc; Table 1; Fig. 2 and 3).
Hence, dust-enshroudment by winds/outflows from massive black holes and/or by
SN is likely not the cause of the disappearance of the transient source (Sect.
2.1.2), nor the explanation for the change in morphology/brightness centroid
shift/variability (Sect. 2.1.2, 2.1.3, 2.1.4 and 2.3.1).

Fading Stellar Cluster: The transient source may be a stellar cluster that
fades with time. If the transient source is a young stellar cluster, commonly
associated with star-forming regions, the cluster ($m\approx$ 21.0 mag; Sect.
2.1.2) may contain anywhere from 10 to 25 O-type stars (e.g., Walborn et al.
2002). However, this scenario can be abondoned (e.g., Allen et al 2007); it is
inconsistent with the isolation, with the lack of a discernible HII region at
the position of the transient (Fig. 2 and 3), with the offset of the transient
relative to the main source ($d\approx$ 0.3 kpc, for $D\simeq$ 8 Mpc; Table
1), and with the (quasi-simultaneous) disappearance of a large number of stars
in approximately 40 years (Fig. 2; row 4 – 5), either due to their lifespan (a
typical O-type star lifetime is 3 – 6 Myr) or due to dust enshroudment (see
also Dust Enshroudment by Feedback Processes). In addition, it does not
provide an explanation for the other observables (Sect. 2.1.2, 2.1.3, 2.1.4
and 2.3.1).

The absolute magnitude of a globular cluster is $M\simeq-$10 – $-$5 mag,
which, for a distance of $D\simeq$ 8 Mpc (Table 1), translates into an
apparent magnitude of $m\simeq$ 19.5 – 24.5 mag (e.g., Brodie & Strader 2006);
hence, the optical magnitude of the transient ($m\approx$ 21.0 mag; Sect.
2.1.2) is consistent with a globular cluster. As globular clusters can be
found in halos (e.g., Brodie & Strader 2006), this scenario could explain the
isolation, the lack of a host HII region (Fig. 2 and 3) and the offset of the
transient relative to the main source ($d\approx$ 0.3 kpc, for $D\simeq$ 8
Mpc; Table 1). However, the size of the transient region appears inconsistent
with typical globular cluster sizes (10 – 30 pc; Brodie & Strader 2006). In
addition, such a scenario can be discarded based on the impossibility of the
globular cluster disappearance over a period of approximately 40 years (Fig.
2; row 4 – 5; see also Dust Enshroudment by Feedback Processes), as well as
not providing an explanation for the other time-dependent observables (Sect.
2.1.2, 2.1.3, 2.1.4 and 2.3.1).

Passing of a Solar System Object: The transient source may be associated with
the passing of a Solar System object near Leoncino Dwarf, providing an
explanation for the isolation, the lack of a host HII region (Fig. 2 and 3),
the transient/main source offset ($d\approx$ 0.3 kpc, for $D\simeq$ 8 Mpc;
Table 1) and the lack of a radio signature (Sect. 2.5). An asteroid at a
distance of $D\approx$ 1 AU, with an apparent magnitude of $m\approx$ 21.0 mag
(Sect. 2.1.2), and a typical albedo value (0.05 – 0.25), would possess a size
anywhere from $\approx$150 to $\approx$350
m585858https://cneos.jpl.nasa.gov/tools/ast_size_est.html. Generally, these
asteroids possess orbital periods between 3 and 6 years, and proper motions in
the range of several arcsec per minute. However, this scenario has several
severe issues. Firstly, it would imply that the blue POSS I image (Fig. 2;
left-hand column; row 4 – 5) was acquired during the small time window
(several minutes) of the object passage near the galaxy. Secondly, Solar
System objects are generally brighter in the red band than in the blue band
(e.g., Jurić et al. 2002), which contradicts the observational data for the
galaxy (Fig. 2; left-hand column; row 2 – 5; Table 2). Thirdly, Solar System
objects are periodical/quasi-periodical, so that they are fairly well-
documented; the fact that there is no registered object implies that it would
have to be a new, undocumented Solar System object (Sect. 2.5). Lastly, such a
scenario can not simultaneously explain the transient source/varying
morphology/brightness centroid shift/flux variability (Sect. 2.1.2, 2.1.3,
2.1.4 and 2.3.1) unless multiple objects/events are involved. Given the above
cumulative evidence, it is highly unlikely that the event is Solar System
object-related.

Variable Accretion Onto a Compact Object: Because dwarf galaxies are
chemically and dynamically unevolved, they possess the ideal conditions for
the presence of IMBHs595959intermediate mass black holes of masses M${}_{\rm
BH}\simeq 10^{4}$ – $10^{6}$ M⊙ (e.g., Izotov, Thuan & Guseva 2007; Izotov &
Thuan 2008; Reines et al. 2013), formed during the early phases of the
Universe. Black hole-related variability, occuring at optical wavelengths and
on different timescales, is associated with the accretion disk and may have
several origins, ranging from disk instabilities (e.g., Siemiginowska & Elvis
1997), to variations in the accretion rate/mode (e.g., Zuo, Lui & Jiao 2012),
the tidal disruption of nearby stars (e.g., Bogdanović et al. 2004), and X-ray
re-processing (e.g., McHardy et al. 2016). X-ray binaries are also accretion-
powered sources, where a donor (star) transfers material to an accretor,
generally a neutron star or a (stellar-mass) black hole (e.g., Casares, Jonker
& Israelian 2017). X-ray binaries typically show optical counterparts (the
accretion disk or the star itself), with absolute magnitudes ranging from
$M\approx$ $-$10 – 5 mag (LMXBs606060low-mass X-ray binaries and microquasars)
to $M\approx-$5 – 5 mag (HMXBs616161high-mass X-ray binaries), depending on
the mass of the donor (e.g., Casares, Jonker & Israelian 2017). They are also
variable on different (shorter) timescales, showing erratic light curves,
pulsations, quasi-periodic oscillations and transient accretion events (e.g.,
Casares, Jonker & Israelian 2017).

If the observed transient source is associated with an accreting object in
Leoncino Dwarf, an upper limit to its mass (Macc) and accretion rate
($\dot{\rm M}_{\rm acc}$) can be estimated for a thin-disk approximation and
an 0.1 efficiency, assuming the accretion is Eddington-limited. The observed
magnitude of the transient source ($m\approx$ 21.0 mag; Sect. 2.1.2) then
provides an accretor mass of M${}_{\rm acc}<$ 50 M⊙ and an accretion rate of
$\dot{\rm M}_{\rm acc}<$ 10-7 M⊙ yr-1; such values are within the realm of
LMXBs (e.g., Casares, Jonker & Israelian 2017). Indeed, a subgroup of LMXBs
are transient sources, as a result of accretion instabilities; they are
generally undetectable in the optical (and X-rays), but become observable for
a period of a few days (e.g., Casares, Jonker & Israelian 2017). As LMXBs can
be commonly found in globular clusters (e.g., Casares, Jonker & Israelian
2017), which can, in turn, occur in halos (see also Fading Stellar Cluster),
such a scenario is not inconsistent with several of the characteristics
(magnitude, isolation, lack of a host HII region and transient/main source
offset; Fig. 2 and 3; Sect. 2) of the transient source. However, this would
imply that the blue POSS I image (Fig. 2; left-hand column; row 4 – 5) was
acquired during that brief, bright period. In addition, an accreting object at
the position of the transient does not provide an explanation for the other
observables (Sect. 2.1.2, 2.1.3, 2.1.4 and 2.3.1). Therefore, this scenario is
deemed highly unlikely.

Hypernova/SN: Hypernovae are generally associated with stellar explosions,
corresponding to some of the most catastrophic events in the Universe. Models
for hypernova are diverse and include black holes, magnetars, pair-
instabilities in massive, low-metallicity stars and stellar systems in unusual
configurations (e.g., Nomoto et al. 2004). Hypernovae are also known to be
associated with long-duration GRBs, which appear to be predominantly hosted by
metal-poor, actively star-forming, subluminous galaxies (e.g., Modjaz et al.
2008; Lyman et al. 2017). Hypernovae generally peak at an absolute magnitude
of $M\simeq-$21 – $-$20 mag, which corresponds to an apparent magnitude of
$m\simeq$ 8.5 – 9.5 mag, for a distance of $D\simeq$ 8 Mpc (Table 1; e.g.,
Maeda et al. 2003). There is a quick drop-off of $\Delta m\approx$ 1 mag in
the first 30 – 50 days (e.g., Maeda et al. 2003).

Type Ia SN occur in binary systems in which one star is a white dwarf, as a
result of the explosion of the white dwarf via the accretion mechanism; these
SN constitute a hetergeneous class of sources (e.g., Maoz, Mannucci & Nelemans
2014). Type Ia SN are typically hosted by normal field galaxies, showing no
preference for an association with star-forming regions (e.g., Childress et
al. 2013). The light curve for a typical type Ia SN shows a peak at an
absolute magnitude of $M\simeq-$20 – $-$ 19 mag, which corresponds to an
apparent magnitude of $m\simeq$ 9.5 – 10.5 mag, for a distance of $D\simeq$ 8
Mpc (Table 1; e.g., Firth et al. 2015). There is a quick magnitude drop-off of
$\Delta m\approx$ 1 mag in the first 30 days (e.g., Firth et al. 2015).
Peculiar, lower luminosity, quicker drop-off, type Ia SN have been documented,
with typical peak absolute magnitudes of $M\simeq-$17 mag (SN2002cx prototype;
Jha et al. 2006), the most extreme of which was found to have a peak absolute
magnitude of and $M\simeq-$14 mag (e.g., Foley et al. 2009), corresponding to
apparent magnitudes of $m\simeq$ 12.5 and 15.5 mag, respectively.

Type II SN, expected in a system such as Leoncino Dwarf with signs of ongoing
star formation, are the result of the rapid collapse, and subsequent violent
explosion, of a massive (M${}_{\star}\simeq$ 8 – 50 M⊙) star (e.g., Smartt et
al. 2009). Typically, type II SN show a peak absolute magnitude of
$M\simeq-$17 mag, which, for a distance of $D\simeq$ 8 Mpc (Table 1),
translates into an apparent magnitude of $m\simeq$ 12.5 mag (e.g., Kasen &
Woosley 2009). For approximately 100 days after the initial explosion, the
magnitude of a type II SN is maintained within $\Delta m\approx$ 1 mag of the
peak magnitude, after which the magnitude drops; 5 months after the explosion
the absolute magnitude is still $M\approx-$15 mag (e.g., Kasen & Woosley
2009).

Calcium-rich SN are rare, peculiar, low-luminosity SN which show strong
calcium lines, approximately two months after their peak brightness (SN2005E
prototype; e.g., Perets et al. 2010). They are more prevalent in early-type
hosts in dense environments and/or showing signs of a recent merger, and are
often found offset from the galaxy center, sometimes as far as 150 kpc (e.g.,
Foley 2015). The origin of such SN remains controversial. The lack of a post-
explosion source rules out massive star progenitors and globular clusters, as
well as compact dwarf galaxy hosts, unless they are ultra-faint dwarf
satellites (e.g., Lyman et al. 2016). The large offsets suggest kicked, high-
velocity systems, such as old, merging white dwarf – neutron star binaries or
the helium detonation in double white dwarf systems interacting with a
supermassive black hole (e.g., Foley 2015; Lyman et al. 2016; Lunnan et al.
2017). Calcium-rich SN are generally less luminous then other SN and are
faster fading, dropping off by $\Delta m\approx$ 1 mag within the first month
(e.g., Perets et al. 2010). A typical peak absolute magnitude of $M\simeq-$15
mag corresponds to an apparent magnitude of $m\simeq$ 14.5 mag (for a distance
of $D\approx$ 8 Mpc; Table 1; e.g., Foley et al. 2015).

The transient source is much fainter ($m\approx$ 21.0 mag; Sect. 2.1.2) than
would be expected from a hypernova/SN observed at peak, at this distance (Fig.
2; left-hand column; row 4 – 5). If the transient is a hypernova/SN, its
apparent magnitude ($m\approx$ 21.0 mag; Sect. 2.1.2) suggests that it was
detected quite some time after its initial explosion. A transient arising from
a core-collapse SN, associated with regions of massive star formation (e.g.,
Smartt et al. 2009), is inconsistent with the transient/main source offset
($d\approx$ 0.3 kpc, for $D\simeq$ 8 Mpc; Table 1), the isolation and the lack
of a host HII region (Fig. 2 and 3). On the other hand, a calcium-rich SN
origin for the transient (e.g., Perets et al 2010; Foley et al. 2015; Lyman et
al. 2016; Lunnan et al. 2017) is compatible with the (possible) interaction of
the host with companion galaxies (Sect. 2.2), the isolation, the lack of a
post-explosion source, the lack of a host HII region (Fig. 2 and 3) and the
transient/main source offset ($d\approx$ 0.3 kpc, for $D\simeq$ 8 Mpc; Table
1). Notwithstanding, because the hypernova/SN was observed during a very late
stage, a (net) magnitude variation of the transient of $\Delta m\approx$ 1 – 2
mag over approximately 60 years should have produced a detectable source in
the HST images (Fig. 3; top; ACS limiting
magnitude626262http://www.stsci.edu/hst/acs/documents/handbooks/current/c05_imaging3.html
for 1000 s integration is $m_{\rm lim}\approx$ 26 – 27.5 mag). In addition,
the role of the hypernova/SN in the changing morphology/brightness centroid
shift/flux variability (Sect. 2.1.2, 2.1.3, 2.1.4 and 2.3.1) remains unclear.
Hence, although this scenario can not be entirely discarded, it does pose
challenges to explain several of the observables.

Stellar Merger: Stellar mergers generally produce events with complex,
irregular, multi-peak light curves (e.g., Smith et al. 2016). Within 1 – 2
months, light curves may show magnitude variations as large as $\Delta
m\approx$ 5 mag (e.g., Smith et al. 2016). Peak absolute magnitudes range from
$M\simeq-$10 – $-$5, depending on the mass of the progenitor stars (e.g.,
Kochanek, Adams & Belczynski 2014), which translates into apparent magnitudes
of $m\simeq$ 19.5 – 24.5 mag, for a distance of $D\simeq$ 8 Mpc (Table 1). It
is, therefore, possible that the transient source ($m\approx$ 21.0 mag; Sect.
2.1.2) was a stellar merger, observed at, or near, peak magnitude (Fig. 2;
left-hand column; row 4 – 5). In addition, (net) fading of the stellar merger
peak event by $\Delta m\approx$ 5 mag (over a period of approximately 40
years) may possibly produce an apparent brightness centroid shift/morphology
variation in the main source (Sect. 2.1.2 and 2.1.3). Because stellar merger
events can be quite heterogenous (e.g., Smith et al. 2016), this scenario is
not inconsistent with the isolation, the lack of a host HII region (Fig. 2 and
3) and the transient/main source offset ($d\approx$ 0.3 kpc, for $D\simeq$ 8
Mpc; Table 1). However, this scenario can not readily account for the peak
flux variability in the main source (Sect. 2.1.4 and 2.3.1). Hence, as a
stellar merger potentially explains most of the observables, it remains as a
possible scenario for the observed phenomenon.

Nova: Novae, possible progenitors of type Ia SN, consist of the cataclysmic
thermonuclear explosion of an accreting white dwarf in a binary system, and
may be recurrent. Absolute magnitudes of a nova are $M\simeq-$10 – $-$5 mag at
peak, which, for a distance of $D\simeq$ 8 Mpc (Table 1), translates into an
apparent magnitude of $m\simeq$ 19.5 – 24.5 mag (e.g., Tang et al. 2014). Nova
generally show a light curve with a flatter decline after peak magnitude than
a type II SN; a fast nova will typically take less than 25 days to decay from
peak by $\Delta m\approx$ 2 mag, while a slow nova will take over 80 days
(e.g., Hachisu & Kato 2015). Because the magnitude of the transient source is
$m\approx$ 21.0 mag (Sect. 2.1.2), this signals that the transient could have
been a nova observed at, or near, peak magnitude (Fig. 2; left-hand column;
row 4 – 5). As some novae can be found in galaxy outskirts (e.g., Shafter et
al 2014), this scenario is not inconsistent with the isolation, the lack of a
host HII region (Fig. 2 and 3) and the transient/main source offset
($d\approx$ 0.3 kpc, for $D\simeq$ 8 Mpc; Table 1). (Net) fading of the nova
by $\Delta m\approx$ 4 mag after peak over a period of approximately 40 years
can possibly provide an explanation for the brightness centroid
shift/morphology variation in the main source (Sect. 2.1.2 and 2.1.3), but,
like the stellar merger scenario, it can not easily account for the peak flux
variability in the main source (Sect. 2.1.4 and 2.3.1). Therefore, this
scenario remains as a possible explanation, as it describes most of the
observables.

LBV636363luminous blue variable Star: LBVs are evolved supergiant/hypergiant
stars, some of the most variable, luminous and massive (20 – 100 M⊙) stars
observed. These sources belong to the S Doradus instability strip of the
Hertzsprung-Russell diagram. The high mass loss rates and high luminosities
result in short lifetimes (e.g., Groh et al. 2014) for the progenitor (few
Myr) and LBV phases ($<$ Myr). The traditional view of LBV stars is that they
correspond to a transitional phase of the most massive single stars (e.g.,
Humphreys & Davidson 1994). More recently, and largely based on population
comparison and on the isolated environments of some LBV stars, it has been
suggested that LBV stars are the product of binary evolution, i.e., they are
evolved massive blue stragglers (e.g., Smith & Tombleson 2015; Smith 2016).
LBV stars are rare, and only several tens of these sources are known in the
Milky Way (e.g., Nazé, Rauw, & Hutsemékers 2012) and Local Group galaxies
(e.g., Massey 2010).

LBV stars show unpredictable, and sometimes dramatic, variations in both their
brightness and their spectra. LBV stars can be quiescent or dormant for
decades or centuries, during which they are generally of spectral type B.
Variations of $\Delta m\approx$ 0.1 – 0.2 mag can occur on timescales of a day
to several weeks/months (e.g., Sterken 2003). Intermittent variations of
$\Delta m\lesssim$ 2 mag, where the bolometric luminosity remains constant but
the spectral type varies from early B supergiant to late B/early A, can occur
on timescales of years to a decade (e.g., Sterken 2003). Very rare ’giant
eruptions’, sometimes mimicking SN events (’SN imposters’; e.g., Kochanek,
Szczygieł & Stanek 2012), can occur on timescales of several tens of years,
and can produce magnitude variations of $\Delta m\approx$ 3 mag, accompanied
by large mass loss rates (with the possible production of nebulae), and an
increase in the bolometric luminosity (e.g., Sterken 2003).

Among XMPs, there is at least one documented case of a giant eruption
associated with a LBV star: DDO 68 (Pustilnik et al. 2008; Izotov & Thuan
2009; Bomans & Weis 2011; Pustilnik et al. 2017). The eruption appears to have
lasted for 2 – 6 years; between 2005 and 2010 the LBV star brightened by
$\Delta m\approx$ 3.1 mag, while between 2010 and 2015 the LBV star dimmed by
$\Delta m\approx$ 3.7 mag, with a maximum absolute magnitude of $M\approx$
$-$10.5 mag (Pustilnik et al. 2017). Archival (POSS) data for DDO 68 appear to
suggest possible previous eruptions in 1955 and 1999 (Bomans & Weis 2011;
Pustilnik et al. 2017).

Utilizing the full range of peak absolute magnitudes observed in LBV stars
($M\simeq$ $-$13 – $-$9 mag; e.g., Smith et al. 2011b) provides a range of
peak apparent magnitudes of $m\simeq$ 16.5 – 20.5 mag, for a distance of
$D\simeq$ 8 Mpc (Table 1). Hence, if the transient source ($m\approx$ 21.0
mag; Sect. 2.1.2) is a LBV star, it must have been observed a short time after
its peak (Fig. 2; left-hand column; row 4 – 5); quiescent LBV stars can have
absolute magnitudes as low as $M\simeq$ $-$6 mag (e.g., Smith et al. 2011b),
or $m\simeq$ 23.5 mag (for a distance of $D\simeq$ 8 Mpc; Table 1). A LBV star
provides an adequate explanation for the transient timescale, the isolation
(e.g., Smith & Tombleson 2015; Smith 2016), the lack of a host HII region
(Fig. 2 and 3; see below, however) and the transient/main source offset
($d\approx$ 0.3 kpc, for $D\simeq$ 8 Mpc; Table 1; see below, however). In
addition, (net) fading of the LBV peak event by $\Delta m\lesssim$ 3 mag (over
a period of approximately 40 years) may possibly produce an apparent
brightness centroid shift/morphology variation in the main source (Sect. 2.1.2
and 2.1.3). However, this scenario has its caveats. Firstly, it is difficult
to interpret the flux variability of the main source (Sect. 2.1.4 and 2.3.1)
within such a context. Secondly, although a fraction of LBV stars are isolated
(e.g., Smith & Tombleson 2015; Smith 2016), in the only XMP with a documented
LBV star, the LBV star appears embedded within an HII region (DDO 68;
Pustilnik et al. 2017); no clear HII region at the location of the transient
source is discernible in the HST images (Fig. 3; top), although it should be
detectable given the typical lifetime of an HII region (few Myr; e.g., Alvarez
et al. 2006). Thirdly, a (net) magnitude variation of $\Delta m\lesssim$3 mag
of the transient over a period of approximately 60 years would result in a
quiescent LBV source ($m\lesssim$ 23.5 mag) that should have been detectable
in the HST images (Fig. 3; top). Lastly, as LBV stars brighten, they become
redder (e.g., Sterken 2003), which appears to contradict the POSS data (Table
2; Fig. 2; row 2 – 5). Consequently, as the LBV scenario explains some of the
observables challenged by other scenarios (e.g., transient timeline), it
remains a contender for the observed phenomenon.

Gravitational Lensing: The transient source and/or change in
morphology/brightness centroid shift/flux variation may be due to a lensing
effect (Sect. 2.1.2, 2.1.3, 2.1.4 and 2.3.1). Figure 5 contains plots of the
gravitational lensing equations in Appendix B for a range of distances and
lens masses.

Figure 5: Top Left: Distance observer-lens as a function of the Einstein
radius, considering a perfectly aligned system and a point-mass lens, for five
different lensing masses, and also considering a SIS density profile (grey
solid line) with $\sigma_{\rm v}$ = 100 km s-1. The vertical black solid line
marks the characteristic Einstein radius $\theta_{\rm e}\approx$ 1 arcsec. Top
Right: Distance observer-lens as a function of the Einstein time, for five
different lensing masses. A transverse velocity of $v_{\rm T}$ = 500 km s-1
has been assumed. The three vertical black solid lines mark the characteristic
timescales $t$ = 1 and $t$ = 40 – 60 yr. For the two top figures, distances
are marked as follows: 1 AU (black dotted line), 50 AU (black dashed line;
distance to the edge of the Solar System), 0.1 – 4 kpc (black solid line;
distance interval for the lens given the timescale $t$ = 1 – 40 yr), 25 kpc
(black long dashed line; distance to the edge of the Milky Way stellar disk),
150 kpc (black dot-dashed line; distance to the edge of the Milky Way halo)
and 4 Mpc (black long dot-dashed line; half-way distance observer-lensed
source). Bottom Left: Amplification factor as a function of the angular
separation lens-lensed source, in units of the Einstein radius. Bottom Right:
Astrometric shift as a function of the angular separation lens-lensed source
in units of the Einstein radius, for a lensing mass of Mlens = 5 $\times$ 105
M⊙ at a distance observer-lens of $D_{\rm ol}$ = 0.1 kpc (green solid line),
Mlens = 105 M⊙ at a distance observer-lens of $D_{\rm ol}$ = 0.1 kpc (magenta
solid line), Mlens = 5 $\times$ 105 M⊙ at a distance observer-lens of $D_{\rm
ol}$ = 4 kpc (purple solid line), and Mlens = 104 M⊙ at a distance observer-
lens of $D_{\rm ol}$ = 0.1 kpc (blue solid line). The horizontal black dashed
lines mark the observed proper motion/central brightness shift during 40 years
($\delta_{\rm c}$ = 0.3 arcsec and 1 arcsec).

Assuming that the transient source is the result of strong lensing of a
compact source (e.g., stellar cluster) within Leoncino Dwarf (Fig. 3; top),
Equation [B5] (Fig. 5; top right) can be used to constrain the distance
observer-lens ($D_{\rm ol}$) based on the observed timescale ($t_{\rm e}$) of
the event. An upper limit of $t\equiv t_{\rm e}\approx$ 40 yr has been
adopted, based on the timeline of the POSS images and the disappearance of the
transient source, and a lower limit of $t\equiv t_{\rm e}\approx$ 1 yr has
been adopted, a reasonable observational window for imaging such an event
(Fig. 2; row 4 – 5; Sect. 2.2 and 2.3). Assuming an Einstein radius of
$\theta_{\rm e}\approx$ 1 arcsec (approximate distance between the transient
source and the edge of the main source; Fig. 2; row 4 – 5), and a transverse
velocity of $v_{\rm T}$ = 500 km s-1, Equation [B5] then provides a distance
observer-lens of $D_{\rm ol}\approx$ 0.1 – 4 kpc (Fig. 5; top right); the lens
must be within the Milky Way halo. Figure 5 and Equation [B1] show that, for
this distance and Einstein radius, the lens mass must be M${}_{\rm
lens}\approx$ 104 – 5 $\times$ 105 M⊙. Smaller lens masses at similar lens
distances would imply observational windows of a month or less, while smaller
lens distances would require even larger lens masses for the same timescale
(Fig. 5; top left).

If more extreme values for the Einstein radius ($\theta_{\rm e}\approx$ 10
arcsec; larger than the size of Leoncino Dwarf) and transverse velocity
($v_{\rm T}$ = 200 km s-1) are adopted, than $D_{\rm ol}\approx$ 0.004 – 0.2
kpc (Eq. [B5]) and M${}_{\rm lens}\approx$ 5 $\times$ 104 – 2 $\times$ 106 M⊙
(Eq. [B1]) are obtained; these values are still within the realm of a massive
lens within the Milky Way halo.

Gravitational lensing is achromatic, (de)amplifying (Eq. [B3]) the photometric
signal by the same amount regardless of the band (Fig. 5; bottom left).
However, the fact that the transient source is not detectable in the red POSS
I image (Fig. 2; left-hand column; row 2 – 3) can simply reflect the bluer
(rather than redder) SED646464spectral energy distribution of the lensed
source. Even with amplification, it may be insufficient, in the red band, to
push the image of the lensed source above the average limiting magnitude of
$m_{\rm lim}\approx$ 20.0 mag of the red POSS I image (Fig. 2; left-hand
column; row 2 – 3).

Instances of (micro)lensing by small masses within the Milky Way halo are not
uncommon (e.g., Alcock et al. 2001; van der Marel 2004), but these generally
never exceed a few solar masses. The constrained lens mass alone (M${}_{\rm
lens}\approx$ 104 – 5 $\times$ 105 M⊙) excludes many Milky Way sources, such
as individual stars, planets and brown dwarfs. Milky Way objects of M${}_{\rm
obj}\approx$ 104 – 106 M⊙ may, however, be stellar clusters. However, the
absolute magnitudes (see also Fading Stellar Cluster) are such that they
should be visible at kpc distances; for example, a globular cluster at a
distance of $D\approx$ 4 kpc, would have an apparent magnitude of $m\approx$ 6
mag. Hence, stellar clusters can be discarded.

Given that the object appears to be dark, the lens may be a failed, low-mass,
dark matter halo, i.e., a dark matter halo unable to retain the necessary
baryons to evolve into a galaxy (e.g. Sawala et al. 2015). In this case, a
NFW656565Navarro-Frenk-White (Navarro, Frenk & White 1997), SIS (e.g., Fort &
Mellier 1994) or other density profile (e.g., Muñoz, Kochanek & Keeton 2001)
should be used to estimate the total lens mass. These profiles require a total
lens mass much larger than that provided by the point-mass approximation
(e.g., Kravtsov 2010). In this case, the required total lens mass is too large
to be considered a viable possibility. Figure 5 and Equation [B6] show that,
for a SIS666666singular isothermal sphere density profile (e.g., Fort &
Mellier 1994) to produce an Einstein radius of $\theta_{\rm e}\approx$ 1
arcsec at $D_{\rm ol}\approx$ 4 kpc, requires a velocity dispersion of
$\sigma_{\rm v}\approx$ 190 km s-1. From the relation between the virial mass
(Mvir) and the velocity dispersion, M${}_{\rm vir}\propto\sigma_{\rm v}^{3}$
(e.g., Munari et al. 2013), it follows that a virial mass of M${}_{\rm
vir}\approx$ 1013 M⊙ is obtained; this corresponds to a halo mass of a Milky
Way-type galaxy. The application of the NFW profile provides similiar,
unrealistic results.

From this evidence, its follows that the lens must be dark, compact and
massive (M${}_{\rm lens}\approx$ 104 – 5 $\times$ 105 M⊙). These
characteristics fall within the realm of IMBHs, in which case the lens point-
mass approximation is appropriate. Within this scenario, relative motion of
the lens, over a period of approximately 40 years, could induce flux
variability (Sect. 2.1.4 and 2.3.1) and morphological distortions (Sect.
2.1.2; Fig. 2; row 2 – 5) via weak lensing of the more extended parts of/other
sources within Leoncino Dwarf. The brightness centroid shift ($\approx$1
arcsec; Sect. 2.1.3) provides an additional constraint if it is interpreted as
a lensing astrometric shift (Eq. [B4]) over a period of approximately 40
years. Figure 5 (bottom right) shows that astrometric shifts of $\delta_{\rm
c}\approx$ 1 arcsec can be produced for $u\approx$ 1 – 2 (angular separation
lens-lensed source in units of the Einstein radius) by a M${}_{\rm
lens}\approx$ 105 M⊙ lens mass at a distance of $D_{\rm ol}\approx$ 0.1 kpc.
Smaller astrometric shifts can be produced for smaller distances and/or
smaller lens masses (Fig. 5; bottom right).

In Section 2.4.1, a plausible explanation was provided for the behavior of the
WISE data: astrometric/photometric shifts representing one weak infrared
source associated with Leoncino Dwarf. In the very unlikely event that the
WISE data represents true short-term variability, then a multi-component lens
may be necessary. Clumpy lenses (subhalos hosted within larger halos) and
secondary intergalactic low-mass field lenses have been invoked to explain
anomalies in flux ratios, time delays, image separations and image distortions
in quasar-galaxy lens systems (e.g., Dalal & Kochanek 2002, Inoue & Chiba
2005, Chen, Kravtsov & Keeton 2003, Metcalf 2005a; Metcalf 2005b; Miranda &
Macciò 2007, Zackrisson & Riehm 2010; Erickcek & Law 2011). However, clumpy
lenses and secondary lenses within the Milky Way halo would constitute highly
contrived scenarios.

The presence of a significant number of rogue IMBHs in galaxies in general,
and in the the Milky Way bulge and halo, in particular, is theoretically
predicted. First pointed out by Madau & Rees (2001), a population of relic
IMBHs naturally results from the (black hole) gravitational recoil and merging
of galaxy building blocks within a hierarchical galaxy formation scenario.
Subsequent numerical simulations (e.g., O’Leary & Loeb 2009; Micic, Holley-
Bockelmann & Sigurdsson 2011; Rashkov & Madau 2014) predict that tens to
several thousand (depending on the model) IMBHs (M${}_{\rm IMBH}\approx$ 103 –
106 M⊙) should exist in the Milky Way bulge and halo. Approximately half of
these relic IMBHs are ’naked’ (devoid of their halo due to tidal forces), with
a more centrally concentrated (within several tens of kpc of the Galactic
center) distribution. There are also predictions that such objects should be
associated with old, compact ($\lesssim$1 pc), high velocity dispersion
stellar cusps, but only a handful should be detectable in the visible (O’Leary
& Loeb 2012; Rashov & Madau 2014). Results further suggest that these IMBHs
may be found as (meso)lensing candidates (Micic, Holley-Bockelmann &
Sigurdsson 2011; Rashkov & Madau 2014; Chapline & Frampton 2016), and may be
candidates for primordial black holes (Carr, Kühnel, & Sandstad 2016). Oka et
al. (2016, 2017) recently reported the first observational evidence for an
IMBH (M${}_{\rm IMBH}\approx$ 105 M⊙) in the Milky Way using CO observations
of a molecular cloud; they find a broad CO velocity width, which allows to
constrain the IMBH mass, as well as a compact gas clump near the CO emission
center and a point-like continuum source (see Ravi, Vedantham & Phinney 2017,
however). Tsuboi et al. (2017) report on the large velocity width of the
ionized gas and compact size of the infrared IRS13E complex using
ALMA676767Atacama Large Millimeter Array, which can potentially be interpreted
as an ionized gas flow in Keplerian orbit around a M${}_{\rm IMBH}\approx$ 104
M⊙ IMBH. Recent work by Vedantham et al. (2017) suggest that the symmetric
achromatic long-term variability in the light curves of active galaxies arises
from the movement of luminal/subluminal jet features across caustics created
by 103 – 106 M⊙ subhalo condensates or black holes within intervening
galaxies.

70 – 2000 IMBHs within the Milky Way bulge and halo (Rashov & Madau 2014)
correspond to a number density of $N\approx$ 5 $\times$ 10-6 – 10-4 kpc-3. If
these IMBHs are randomly distributed within the Milky Way bulge and halo, the
probability of finding one such IMBH towards a galaxy 10 arcsec in size is
approximately 1 in 107; the statistical probability assigned by the current
models of IMBH formation to this scenario is very low, and they would require
a significant refurbishing if such a transient is produced by an IMBH.

Although the present data do not allow to firmly establish a precise
interpretation for the phenomena, a lensing event appears to be the only
scenario capable of simultaneously explaining the multiple changes observed in
Leoncino Dwarf over a period of approximately 40 years. This includes
describing the transient source as a strong lensing event, the changes in
morphology and flux of the main source as time-varying weak lensing signals
from the extended galaxy/compact sources within the galaxy, and the brightness
centroid shift of the main source as an astrometric shift induced by a weak
lensing effect. The observed parameters (timescale, flux variation, offset,
size and brightness centroid shift) allow to constrain the lens system to a
compact, dark lens of mass M${}_{\rm lens}\approx$ 104 – 5 $\times$ 105 M⊙,
likely an IMBH, at a distance observer-lens of $D_{\rm ol}\approx$ 0.1 – 4
kpc. However, although lensing appears to explain all of the time-dependent
observables (Sect. 2.1.2, 2.1.3, 2.1.4, 2.3.1 and 2.4), and notwithstanding
the lensing scenario consistency with the predicted properties of relic IMBHs
(mass range, distribution, compactness, potential lensing effects), the
likelihood of such an event being caused by an IMBH in the Milky Way halo is
still highly unlikely, and, therefore, discardable.

##  4 Conclusions

Leoncino Dwarf is a unique source; it is an ultra-faint, extremely metal-poor
dwarf galaxy, one of the current low-metallicity record-holders in the local
Universe. Adding to its uniqueness, a blue transient source, to the North of
the main source, appears to be present in images from 1955. The main source
appears also to show a change in morphology, a brightness centroid shift, and
peak flux variability ($\Delta m\simeq$ 0.7 mag) over a period of
approximately 40 years. Variability of massive stars resolved in the HST
images could, in principle, explain the brightness centroid shift, morphology
change and (long- and short-term) peak flux variability of the main source,
but this scenario requires an independent explanation for the transient. The
passing of a Solar System object, a fading stellar cluster, dust enshroudment
by feedback processes, and variable accretion processes onto a compact object
have been investigated, and excluded, as possible explanations for the
transient phenomena. A lensing event could provide an interesting alternative,
simultaneous explanation for all the observables. The timescale for the
transient and other variable observables requires a dark, compact, massive
(M${}_{\rm lens}\approx$ 104 – 5 $\times$ 105 M⊙) lens, likely a IMBH, within
the Milky Way halo ($D_{\rm ol}\approx$ 0.1 – 4 kpc). Strong lensing describes
the transient source, while weak lensing of the extended galaxy/compact
sources within the galaxy describes the morphological and peak flux changes,
as well as the observed brightness centroid shift (astrometric shift).
However, as this scenario is statistically highly unlikely according to
current theoretical predictions, it can also be discarded. The transient could
have been the result of a cataclysmic event such as a SN or hypernova, as long
as the event was caught in the later/late stages of the light curve. However,
in these cases, a (net) magnitude variation of $\Delta m\approx$ 1 – 2 mag of
the transient over an approximately 60-year period should have produced a
detectable quiescent source and/or the host HII region should have deen
detected in the SDSS and HST images. An episode related to a stellar
merger/nova or giant eruption of a LBV star, observed at, or near, peak
magnitude, may explain the transient source, and, possibly, the brightness
centroid shift/morphology change, but can not account for the remaining
observable, the peak flux variation of the main source, unless massive star
variability in the main source is evoked.

## Acknowledgments

The authors would like to thank Evencio Mediavilla for useful discussions on
gravitational lensing, Joana Ascenso for help on the PSF modeling, Santiago
González-Gaitán for pointing out calcium-rich SN, and Dave Monet for valuable
guidance regarding the treatment of POSS data. The authors would also like to
thank the anonymous reviewer for comments and suggestions that have greatly
improved this manuscript. M. E. F. gratefully acknowledges the financial
support of the ”Fundação para a Ciência e Tecnologia” (FCT – Portugal),
through the research grant SFRH/BPD/107801/2015. This work has been partly
funded by the Spanish Ministery of Economy and Competitiveness, project
Estallidos AYA2013-47742-C04-02-P, AYA2013-47742-C04-01-P and
AYA2016-79724-C04-2-P. This work has made use of the WISE, 2MASS, FIRST, NVSS,
VLSS, Herschel, Chandra and XMM-Newton databases, the NASA Jet Propulsion
Laboratory’s HORIZONS Web-Interface, the SDSS DR12, STScI digitized POSS data,
the NASA/IPAC Extragalactic Database (NED), VizieR and the Ned Wright’s
Cosmological Calculator. We have also consulted the IAU Central Bureau for
Astronomical Telegrams.

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##  Appendix A Transient Flux Error Estimation

The integrated signal of the transient, $S$, is approximately given by

$S\simeq n_{pix}\,f\ I_{max},$ (3)

where $I_{max}$, $n_{pix}$ and $f$ represent the maximum signal, the number of
pixels encompassed by the source, and a factor of the order unity,
respectively; the latter accounts for the variation of the signal in the
pixels surrounding the maximum. On the other hand, the noise on the integrated
signal, $\Delta S$, can be expressed as

$\Delta S\simeq\sqrt{n_{pix}}\,\Delta I,$ (4)

where $\Delta I$ represents the error in the intensity in a single pixel.
Equation ([A1]) assumes that the noise in the different pixels contributing to
$S$ is similar and independent. Combining the two expressions above, the
relative error of the integrated flux is

$\frac{\Delta S}{S}\simeq\frac{1}{f\,\sqrt{n_{pix}}}\,\frac{\Delta
I}{I_{max}}.$ (5)

If the transient has a FWHM of approximately 4 arcsec, and is detected with a
pixel size of $\sim$1 arcsec (Sect. 2.1.1), then $n_{pix}\simeq 16$. A value
of $\Delta I/I_{max}\simeq 1/5$ is measured (Sect. 2.1.2) which, together with
Equation ([A3]), leads to $\Delta S/S$ values between 0.05 and 0.10 when $f$
varies between 1 and 0.5, which is the range of signals within the FWHM of the
source. Because the above estimate is only approximate, a conservative limit
of

$\frac{\Delta S}{S}\simeq 0.2$ (6)

has been adopted (Sect. 2.1.2).

##  Appendix B Gravitational Lensing Equations

The lensing of a source requires a set of specific circumstances in order to
occur, namely, the relative alignment between the observer, the lens and the
lensed source. Strong lensing effects, such as high magnifications, strong
image distortions and mutiple images, are produced when the lens is well-
aligned with the line-of-sight. If the lensed source is extended, strong
lensing will produce multiple arcs and rings, while if the lensed source is
point-like, such as a quasar, it will produce point-like images. Weak lensing
occurs when the lens is farther from the line-of-sight. In this case, a single
image is produced, subject to mild magnification and distortion.

When the lensed source, center of the lens and the observer are perfectly
aligned, the lensed source appears to the observer as a ring. The radius of
the ring, the Einstein radius ($\theta_{\rm e}$), provides a characteristic
length-scale for the lensing phenomenon. The lensing equation (e.g., Hawkins
1996; Gould 2000) provides a relation between the Einstein radius, the lens
mass (Mlens), and the relative angular distances ($R$). For a point-mass lens

$\theta_{\rm e}=\Big{(}\frac{4\,G\,{\rm
M_{lens}}}{c^{2}\,R}\Big{)}^{\frac{1}{2}},$ (7)

where $G$ is the gravitational constant, $c$ is the speed of light, and

$R=\frac{D_{\rm ol}\,D_{\rm os}}{D_{\rm ls}},$ (8)

where $D_{\rm ol}$ is the distance observer-lens, $D_{\rm os}$ is the distance
observer-lensed source and $D_{\rm ls}$ is the distance lens-lensed source.
For low-redshift sources, $D_{\rm ls}\simeq D_{\rm os}-D_{\rm ol}$. If the
lensed source is misaligned from the observer-lens axis by some angle $\beta$,
which may be a function of time ($t$), the total photometric amplification
($A$) of the lensed source due to a point-mass lens is

$A=\frac{u^{2}+2}{u\sqrt{u^{2}+4}},$ (9)

where $u\equiv u(t)=\frac{\beta}{\theta_{\rm e}}$ is the angular separation
lens-lensed source in Einstein radius units. The center of brightness is also
shifted ($\delta_{\rm c}$) relative to the true position of the lensed source
by the point-mass lens

$\delta_{\rm c}=\theta_{\rm e}\frac{u}{u^{2}+2}.$ (10)

For a lens moving across the line-of-sight with a transverse velocity ($v_{\rm
T}$), the timescale ($t_{\rm e}$) for the variation (Einstein time), i.e., the
time to cross the Einstein radius, is given by

$t_{\rm e}=\frac{D_{\rm ol}\,\theta_{\rm e}}{v_{\rm T}}.$ (11)

If, instead of a point mass, a SIS (e.g., Fort & Mellier 1994) density profile
is considered for the lens, a profile commonly explored in lensing studies,
then the lensing equation becomes

$\theta_{\rm e}=\frac{4\,\pi\,\sigma_{\rm
v}^{2}}{c^{2}}\,\frac{D_{ls}}{D_{os}},$ (12)

where $\sigma_{\rm v}$ is the velocity dispersion. The lens Equation ([B1])
still holds for an extended lens, but, in this case, the lens mass corresponds
only to the mass contained within the Einstein radius and not the total lens
mass.