# A Peculiarly Short-duration Gamma-Ray Burst from Massive Star Core Collapse

B.-B. Zhang1,2,3,∗ E-mail<EMAIL_ADDRESS>Z.-K. Liu1,2,∗ Z.-K. Peng1,2,∗
Y. Li4 H.-J. Lü5 J. Yang1,2 Y.-S. Yang1,2 Y.-H. Yang1,2 Y.-Z. Meng1,2 J.-H.
Zou1,2,6 H.-Y. Ye1 X.-G. Wang5 J.-R. Mao7 X.-H. Zhao7 J.-M. Bai7 A. J. Castro-
Tirado8,9 Y.-D. Hu8,10 Z.-G. Dai11,1,2 E.-W. Liang5 B. Zhang3 E-mail:
<EMAIL_ADDRESS>
###### Abstract

Gamma-ray bursts (GRBs) have been phenomenologically classified into long and
short populations based on the observed bimodal distribution of duration[1].
Multi-wavelength and multi-messenger observations in recent years have
revealed that in general long GRBs originate from massive star core collapse
events[2], whereas short GRBs originate from binary neutron star mergers[3].
It has been known that the duration criterion is sometimes unreliable, and
multi-wavelength criteria are needed to identify the physical origin of a
particular GRB[4]. Some apparently long GRBs have been suggested to have a
neutron star merger origin[5], whereas some apparently short GRBs have been
attributed to genuinely long GRBs[6] whose short, bright emission is slightly
above the detector’s sensitivity threshold. Here we report the comprehensive
analysis of the multi-wavelength data of a bright short GRB 200826A.
Characterized by a sharp pulse, this burst shows a duration of 1 second and no
evidence of an underlying longer-duration event. Its other observational
properties such as its spectral behaviors, total energy, and host galaxy
offset, are, however, inconsistent with those of other short GRBs believed to
originate from binary neutron star mergers. Rather, these properties resemble
those of long GRBs. This burst confirms the existence of short duration GRBs
with stellar core-collapse origin[4], and presents some challenges to the
existing models.

School of Astronomy and Space Science, Nanjing University, Nanjing 210093,
China

Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University),
Ministry of Education, China

Department of Physics and Astronomy, University of Nevada, Las Vegas, NV
89154, USA

Kavli institute for astronomy and astrophysics, Peking University, Beijing.
100871, P.R.China

Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical
Science and Technology, Guangxi University, Nanning 530004 China;

College of Physics, Hebei Normal University, Shijiazhuang 050024, China

Yunnan Observatories, Chinese Academy of Sciences, 650216, Kunming, China

Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía
s/n, E-18008, Granada, Spain

Departamento de Ingeniería de Sistemas y Automática, Escuela de Ingenierías,
Universidad de Málaga, Dr. Pedro Ortiz Ramos, 29071 Málaga, Spain

Universidad de Granada, Facultad de Ciencias Campus Fuentenueva s/n E-18071
Granada, Spain

Department of astronomy, School of Physical Science, University of Science and
Technology of China, Hefei 230026, Anhui, China ∗Co-First Authors. These
authors contributed equally:Bin-Bin Zhang, Zi-Ke Liu and Zong-Kai Peng

GRB 200826A triggered the $Fermi$ Gamma-ray Burst Monitor (GBM)[7] at 04:29:52
Universal Time on 26 August 2020. [8]. Follow-up observations detected the
optical counterpart [9] and identified its host galaxy and redshift at
$z=0.7481$. (ref. [10]). The 10-800 keV-band light curve, as detected by GBM,
is shown in Figure 1a. The duration in the same energy range of the burst is
measured as $T_{90}\sim 0.96_{-0.07}^{+0.06}$ s (see Table 1 for a collection
of all the measured parameters of GRB 200826A, and see the details of data
analysis in Methods), which is consistent with the value of $T_{90}=1.14\pm
0.13$ in 50-300 keV energy range[11]. The lightcurve pulse profile is quite
sharp. There is no signal above the background both before and after the
burst, hinting that the short duration is genuine. We further measure the
“amplitude parameter” of this burst[12], which is defined as the ratio between
the peak flux and the average background flux of the GRB lightcurve (see
Methods). We obtain $f=7.58\pm 1.23$ for GRB 200826A. To make a long GRB a
“tip-of-iceberg” short GRB, the effective $f$ value is typically $<1.5$ (ref.
[12]). The disguised short GRB 090426 with a massive star origin[6] had
$f=1.48\pm 0.11$, which is consistent with having underlying long-duration
emission not observed above the detection level, although its origin remains
an open question due to its low hydrogen column density and weak Ly-$\alpha$
lines[13]. The enormous $f$ value of GRB 200826A, therefore, suggests that it
is a genuinely short GRB and cannot be the tip-of-iceberg of a long GRB
(Figure.1b).

Figure 1c shows the standard long/short classification diagram in the duration
- hardness ratio domain, where the hardness ratio is defined as observed
counts ratio between the 50-300 keV band and the 10-50 keV band within the
$T_{90}$ duration (see Methods). One can see that GRB 200826A falls into the
distribution of short GRBs, even though near the softer end of the hardness
ratio distribution (with a value 0.803). We perform the spectral analysis of
the GRB (see Fermi data analysis in Methods and Extended Data Figure 1). Its
time-averaged photon index is $\alpha=-0.68\pm 0.05$ , spectral peak energy is
$E_{\rm p}\simeq 120.29_{-3.67}^{+3.93}$ keV and they show the same
evolutionary pattern as light curves (Extended Data Figure 2 and Extended Data
Figure 3). These are not too different from other short GRBs. Without the
redshift and host galaxy information, this burst would be classified as a
member in the short GRB population.

The conclusion changes when the redshift information is considered. With the
measured redshift, we derive an isotropic energy of $E_{\rm\gamma,iso}\simeq
7.09\pm 0.28\times 10^{51}$ erg based on the measured fluence of $4.85\pm
0.19\times 10^{-6}{\rm erg\hskip 3.0ptcm^{-2}}$. This energy value is
relatively large compared with other short GRBs but is typical in the long GRB
population. Such a discrepancy is more obvious when GRB 200826A is plotted in
the $E_{\rm\gamma,iso}-E_{\rm p,z}$ relation for both long and short GRBs
(where $E_{\rm p,z}=E_{\rm p}(1+z)$ is the spectral peak energy in the burst
rest frame). Observations showed that there is an empirical relation between
the two, the so-called Amati-relation[14] (see Methods). Long and short GRBs
are known to follow the same trend but form distinct tracks[4]. We find that
GRB 200826A squarely lies on the long GRB track rather than on the short GRB
track (Figure 2a).

The peculiarity of GRB 200826A is also reflected in its value of the
$\epsilon\equiv E_{\rm\gamma,iso,52}/E_{\rm p,z,2}^{5/3}$ parameter[15], where
$E_{\rm\gamma,iso,52}$ is the isotropic gamma-ray energy in units of $10^{52}$
erg, and $E_{\rm p,z,2}$ is $E_{p,z}$ in units of 100 keV. Following the
convention of ref. [16], hereafter we define Type I and Type II GRBs as those
with a compact star merger origin and a massive star core collapse origin,
respectively. It has been shown that GRBs can be more distinctly classified
into their respective physical categories (Type I and Type II) in the
$\epsilon-T_{90}$ space, with a separation line of $\epsilon\sim 0.03$ (ref.
[15]). We obtain $\epsilon=0.33$ for GRB 200826A and find that it
statistically falls into the Type II rather than the Type I category (Figure
2b, see Methods).

Long GRBs usually show “spectral lags”, with the softer emission peaking later
than the harder emission[17]. Short GRBs, on the other hand, typically show
negligible lags[18]. The (10-20) keV to (250-300) keV spectral lag of GRB
200826A is 0.157$\pm 0.051$ s (See Methods and Extended Data Figure 4). This
is at odds for short GRBs but is typical for long GRBs.

Binary neutron star mergers are the leading engines for short-duration GRBs.
These mergers usually happen after a long delay since star formation so that
the SFR and sSFR of their host galaxies are usually not high. The host galaxy
and the position (see Methods) of the GRB inside the galaxy often gives the
clue regarding the progenitor of a GRB[19, 20, 21]. We performed a follow-up
observation of GRB 200826A using Gran Telescopio Canarias (GTC) and identified
the host galaxy (Figure 3a) which was found to lie at redshift $0.7481\pm
0.0003$ (ref. [11]). We measured its half-light radius as 0.77", corresponding
to 5.7 kpc, in our GTC $r$-band image. The optical afterglow of GRB 200826A
has a 0.35" offset from the center of the host galaxy, which corresponds to
2.56 kpc. The normalized offset is $r=R_{\rm off}/R_{50}=0.45$. We estimate
the cumulative light fraction $F_{\rm light}=0.79$. We also calculated the
star formation rate (SFR; see Methods) to be $>1.44\ M_{\odot}{\rm yr^{-1}}$
and specific star formation rate (sSFR) to be $>0.35\ {\rm Gyr^{-1}}$. Our
results are consistent with those in ref.[11]. Due to kicks at the births of
neutron stars from SNe explosions, these mergers also usually occur at a site
far away from star formation regions, often in the outskirts or even outside
their host galaxies[22, 20]. In Figure 3b-e, we place GRB 200826A in the $r$,
$F_{\rm light}$, SFR, and sSFR distributions for long (Type II) and short
(Type I) GRBs[23]. Statistically speaking, one can see that the host galaxy
properties of GRB 200826A are more consistent with belonging to typical Type
II GRBs rather than Type I.

To make our conclusion more quantitative, we apply the method of ref.[21] that
classifies GRBs into physical categories using multi-wavelength observational
data (see Methods). The logarithmic odds $\log O{\rm(II:I)_{host}}$ using host
galaxy information only is 2.5, indeed consistent with a Type II origin. When
the prompt emission information is considered, if one ignores the $f$
information, the overall indicator $\mathcal{O}$, which is defined as the
logarithmic probability ratio between Type II and Type I GRBs plus 0.7 and
assigns Type II/I GRBs as positive/negative values, is 1.5. This places GRB
200826A into the Type II (massive star core-collapse) GRB category (Figure
3f). When the very large $f$ value ($f=7.58\pm 1.23$, a character of Type I
GRBs) is added as one of the criteria, the method of ref.[21] gives
$\mathcal{O}=-3.1$, still placing GRB 200826A in the Type I category. This
again indicates the uniqueness of GRB 200826A, which points towards a
genuinely short burst with other properties consistent with being a massive-
star-core-collapse event, so that the GRB classification scheme requires
further refinements.

Finally, a Type II origin of GRB 200826 is further supported by the follow-up
observations by Gemini-North 8-meter telescope[11](see Extended Data Figure
5), which revealed a possible optical bump at $25.45\pm 0.15$ mag in $i$-band.
This bump could be consistent with a supernova, a signature of a massive star
core collapse event.

The fact that GRB 200826A is genuinely short but its many properties are
inconsistent with being of a neutron star merger origin raises a great
challenge in identifying its progenitor star. For an accretion-powered GRB
engine, the shortest timescale to power a relativistic jet is defined by the
free-fall time scale of the star, which reads[24] $t_{\rm
ff}\sim(\frac{3\pi}{32\rm G\bar{\rho}})^{1/2}\sim 210~{}\rm
s~{}(\frac{\bar{\rho}}{100~{}\rm g~{}\rm cm^{-3}})^{-1/2}$, where $\bar{\rho}$
is the mean density of the accreted matter. One can see that for the typical
density $\bar{\rho}\sim 100~{}\rm g~{}\rm cm^{-3}$ of a massive star, the
duration should be long. If the $\sim 1$ s duration of GRB 200826A is indeed
the total duration of the central engine, one immediately poses a lower limit
on the density of the accreted materials, i.e. $\bar{\rho}>4.4\times 10^{6}\
{\rm g\ cm^{-3}}\ T_{90}^{-2}$. This would directly rule out any massive star
progenitor with an accretion-powered engine.

There are two ways to get around this argument. The first possibility is
indeed to introduce a compact object rather than a massive star at the central
engine. One category of model invokes a white dwarf (WD) engine. Since single
WDs cannot make GRBs, one may consider various binary mergers invoking WDs[25,
26], e.g., WD-WD mergers, WD-NS mergers, and WD-BH mergers. One challenge of
this scenario is whether such GRBs can still occur in star-forming regions in
the host galaxies. Moreover, the detection of a possible supernova bump[11]
would disfavor this model. The other possible channel is that GRB 200826A’s
progenitor was a massive star, but it collapsed some time before the GRB was
made. Such a scenario, known as “supranova”, has been discussed in the
literature[27]. The GRB is produced by the implosion of a supermassive neutron
star, probably triggered by the spindown or fallback accretion of the neutron
star. As the GRB jet is launched, there are no more low-density materials
surrounding the source, so that the duration is short. The source is still in
the star forming region. One challenge of this model is to account for the
relatively shallow decay in the X-ray afterglow, which seems to require
additional energy injection during the afterglow phase (Methods). The possible
supernova bump[11] also requires that the delay time from massive star core
collapse to GRB jet launching cannot be much longer than a day.

The second possibility is that the true duration of the central engine is
long. However, during the majority of the active central engine time, the
$\gamma$-ray emission is below the detection threshold of GRB detectors. Such
a “tip-of-iceberg” interpretation may apply to other short-duration, low-$f$
Type II GRBs such as GRB 090426 (refs. [6, 12]). However, the large $f$ value
of GRB 200826A makes this simple interpretation unlikely. One may consider two
modified scenarios. One is that the total central engine timescale $\Delta
t_{\rm eng}$ is long. However, the majority of the time is used for the jet to
penetrate through the stellar envelope in a timescale $\Delta t_{\rm jet}\sim
10$s. The observed GRB duration is $\Delta t_{\rm GRB}=\Delta t_{\rm
eng}-\Delta t_{\rm jet}$, which could be as short as $\sim$ 1 s (ref. [28]).
Within this interpretation, one needs to introduce a coincidence between
$\Delta t_{\rm eng}$ and $\Delta t_{\rm jet}$. This possibility has been
emphasized also in ref. [11]. Alternatively, one may envisage that the engine
indeed lasted a duration much longer than 1 s, but during the majority of the
time the jet may carry heavy baryon loading and is mildly relativistic or non-
relativistic so that no bright $\gamma$-rays could be produced during this
period of time. One possibility is that the engine is a new-born magnetar
which initially injects baryon-loaded, neutrino-driven wind[29]. The advantage
of this model is that the X-ray plateau observed in this burst (Methods,
Extended Data Figure 6) may be interpreted as the spindown of such a new-born
magnetar. A short duration for this scenario may be achieved by invoking
differential-rotation-induced magnetic bubbles as the mechanism of producing
GRB prompt emission[30]. With a relatively high initial seed magnetic field
strength $B\sim B_{14}\times 10^{14}$ G, the total duration may be estimated
as $t\sim 1B_{14}^{-1}\Omega_{4}^{-1}~{}\rm s$, where $B_{14}=B/10^{14}$ G and
$\Omega_{4}$ is the characteristic differential angular rotation speed in
units of $10^{4}~{}\rm s^{-1}$ (ref. [30]).

Table 1: Properties of GRB 200826A Duration [$T_{90}$] (10-800 keV) | $0.96_{-0.07}^{+0.06}~{}\rm s$
---|---
$f$-parameters [$f$] | $7.58\pm 1.23$
Hardness Ratio (50-300/10-50 keV) | $0.803$
Spectral photon index [$\alpha$] | $-0.68\pm 0.05$
Spectral peak energy [$E_{\rm p}$] |  $120.29_{-3.67}^{+3.93}$ $\rm keV$
Isotropic energy [$E_{\rm\gamma,iso}$] |  $7.09\pm 0.28\times 10^{51}$ $\rm erg$
Total fluence |  $4.85\pm 0.19\times 10^{-6}$ $\rm erg$ $\rm cm^{-2}$
$\epsilon$ \- parameter | $0.33$
Time lag(10-20 keV $\sim$ 250-300 keV) | $0.157{\pm 0.051}~{}\rm s$
Redshift [$z$] | $0.7481\pm 0.0003$
Offset [$R_{\rm off}$] |  $2.6$ $\rm kpc$
Half light radius [$R_{50}$] |  $5.8$ $\rm kpc$
Normalized offset [$R_{\rm off}$/$R_{50}$] | $0.45$
Cumulatve light fraction [$F_{\rm light}$] | $0.79$
$\mathcal{O}$ calculated with(out)$f$ |  $-3.1$(1.5)
log $O$(II:I)prompt calculated with(out) $f$ |  $-5.2$(-0.57)
log $O$(II:I)host | $2.5$
Peak flux [$F_{\rm p}$] |  $9.11_{-1.17}^{+1.47}\times 10^{-6}$ $\rm erg$ $\rm cm^{-2}\rm s^{-1}$
Peak luminosity [$L_{\rm\gamma,p,iso}$] |  $1.41_{-0.21}^{+0.23}\times 10^{52}$ $\rm erg$ $\rm s^{-1}$
Star formation rate [SFR] | >$1.44\ \rm M_{\odot}~{}yr^{-1}$
Stellar mass [$M_{*}$] |  $\rm 4.1\pm 2.9\times 10^{9}\ M_{\odot}$.
Specific star formation rate [sSFR] | > $\rm 0.35\ Gyr^{-1}$
Optical transient location | $\rm 00h27m08.5s\quad+34^{\circ}01^{\prime}38^{\prime\prime}.3$
SN-association | likely
Figure 1: Temporal properties of GRB 200826A. a, $T_{\rm 90}$ calculation :
The upper panel shows the light curve and background flux level with blue and
purple solid lines, respectively. The accumulated counts as a function of time
are shown with the black curve in the lower panel. The orange horizontal
dashed (solid) lines are drawn at 5$\%$(0$\%$) and 95$\%$ (100$\%$) of the
total accumulated counts. The $T_{90}$ interval is marked by the green
vertical dashed lines. b, $T_{90}-f(f_{\rm eff})$ plot. The $f$ parameter
measures the ratio between peak flux average background flux of a GRB
(Methods). $f_{\rm eff}$ is the effective $f$ parameter when a long GRB
becomes a disguised short GRB by arbitrarily lowering its flux level. GRB
200826A is highlighted by the orange solid star. Blue circles correspond to
$f$ parameter values of short GRBs, black circles mark $f_{\rm eff}$ values of
long GRBs and green vertical line is the division line at $2~{}\rm s$. c, GRB
200826A on the $T_{\rm 90}$-HR diagram : Blue and black points represent the
short and long GRBs, respectively; orange solid star marks GRB 200826A; and
the black dashed line separate long and short GRBs at $2~{}\rm s$. All error
bars represent 1-$\sigma$ uncertainties.  Figure 2: GRB 200826A in energy-
related correlations. a, $E_{\rm p,z}-E_{\rm\gamma,iso}$ correlation diagram
for short and long GRBs. Dashed borderlines show the 3$\rm\sigma$ regions for
each correlation. b, One and two dimensional distributions of GRB samples in
the $\epsilon-T_{90}$ space. $\epsilon$ is defined as $\epsilon$ =
$E_{\rm\gamma,iso,52}/E_{\rm p,z,2}^{5/3}$, where
$E_{\rm\gamma,iso,52}=E_{\rm\gamma,iso}/10^{52}~{}\rm erg$ and $E_{\rm
p,z,2}=E_{\rm p,z}/100~{}\rm keV$ ( See Methods. Blue and black solid circles
represent the Type I and Type II GRB candidates, respectively. The special
case of the short-duration GRB 090426 with a high-$\rm\epsilon$ is marked by
an open black circle. Green diamonds denote the nearby low-luminosity long
GRBs. Probability contours of Type I and II GRB clusters are also displayed in
the grey regions (see Methods). The dashed horizontal line is the $\epsilon$ =
0.03 division line. The Gaussian fits to the distributions of $\epsilon$ and
$T_{90}$ of the two populations are plotted as black curves. Contours with
2-$\sigma$ and 3-$\sigma$ level are marked in grey. One can see that GRB
200826A (marked as a orange star) comfortably falls in the Type II region in
both plots. All error bars represent 1-$\sigma$ uncertainties.  Figure 3: Host
properties of GRB 200826A. a, The Gran Telescopio CANARIAS (GTC) Image of GRB
200826A’s host galaxy. b, Normalized offset ($r=R_{\rm off}/R_{50}$)
distributions of our samples. c,  Distributions of cumulative light fraction
$F_{\rm light}$. d, Star formation rate (SFR) of the host galaxies. e,
specific star formation rate (sSFR; the ratio between SFR and stellar mass of
host galaxy $M_{*}$) of the host galaxies. f, Posterior odds O (II:I) of
prompt emission and host properties. GRB 200826A is marked with a orange star.
In b, c, d, and e, the black and blue histograms are for Type I and Type II
GRBs, respectively. The vertical dashed orange lines represent the location of
GRB 200826A.

## References

  * [1] Kouveliotou, C. et al.  Identification of Two Classes of Gamma-Ray Bursts.  ApJ 413, L101-104 (1993).
  * [2] Woosley, S. E. & Bloom, J. S.  The Supernova Gamma-Ray Burst Connection.  ARA&A 44, 507–556 (2006).
  * [3] Abbott, B. P. et al.  Multi-messenger Observations of a Binary Neutron Star Merger.  ApJ 848, L12-70 (2017).
  * [4] Zhang, B. et al.  Discerning the Physical Origins of Cosmological Gamma-ray Bursts Based on Multiple Observational Criteria: The Cases of z = 6.7 GRB 080913, z = 8.2 GRB 090423, and Some Short/Hard GRBs.  ApJ 703, 1696–1724 (2009).
  * [5] Gehrels, N. et al.  A new $\gamma$-ray burst classification scheme from GRB060614.  Nature 444, 1044–1046 (2006).
  * [6] Levesque, E. M. et al.  GRB090426: the environment of a rest-frame 0.35-s gamma-ray burst at a redshift of 2.609.  MNRAS 401, 963–972 (2010).
  * [7] Meegan, C. et al.  The Fermi Gamma-ray Burst Monitor.  ApJ 702, 791–804 (2009).
  * [8] Sagues Carracedo, A., Dunwoody, R., Meegan, C. & Fermi GBM Team.  GRB 200826A: Fermi GBM observation.  GRB Coordinates Network 28287, 1 (2020).  URL https://gcn.gsfc.nasa.gov/gcn3/28287.gcn3.
  * [9] Ahumada, T. et al.  GRB200826A: Zwicky Transient Facility Identifies Optical Afterglow Candidate of a Fermi Short GRB (Trigger 620108997).  GRB Coordinates Network 28295, 1 (2020).  URL https://gcn.gsfc.nasa.gov/gcn3/28295.gcn3.
  * [10] Rothberg, B., Kuhn, O., Veillet, C. & Allanson, S.  GRB200826A.  GRB Coordinates Network 28319, 1 (2020).  URL https://gcn.gsfc.nasa.gov/gcn3/28319.gcn3.
  * [11] Ahumada, T., et al.  Discovery and confirmation of the shortest Gamma Ray Burst with a collapsar.  Nature Astronomy, submitted (2020).
  * [12] Lü, H.-J., Zhang, B., Liang, E.-W., Zhang, B.-B. & Sakamoto, T.  The ‘amplitude’ parameter of gamma-ray bursts and its implications for GRB classification.  MNRAS 442, 1922–1929 (2014).
  * [13] Antonelli, L. A. et al.  GRB 090426: the farthest short gamma-ray burst?.  A&A 507, L45-48 (2009).
  * [14] Amati, L. et al.  Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts.  A&A 390, 81–89 (2002).
  * [15] Lü, H.-J., Liang, E.-W., Zhang, B.-B. & Zhang, B.  A New Classification Method for Gamma-ray Bursts.  ApJ 725, 1965–1970 (2010).
  * [16] Zhang, B.  A burst of new ideas.  Nature 444, 1010–1011 (2006).
  * [17] Norris, J. P. et al.  Attributes of Pulses in Long Bright Gamma-Ray Bursts.  ApJ 459, 393-412 (1996).
  * [18] Yi,T.-F., Liang, E.-W., Qin, Y.-P. & Lu, R.-J.  On the spectral lags of the short gamma-ray bursts.  MNRAS 367, 1751–1756 (2006).
  * [19] Fruchter, A. S. et al.  Long $\gamma$-ray bursts and core-collapse supernovae have different environments.  Nature 441, 463–468 (2006).
  * [20] Berger, E.  Short-Duration Gamma-Ray Bursts.  ARA&A 52, 43–105 (2014).
  * [21] Li, Y., Zhang, B. & Yuan, Q.  A Comparative Study of Long and Short GRBs. II. A Multiwavelength Method to Distinguish Type II (Massive Star) and Type I (Compact Star) GRBs.  ApJ 897, 154-164 (2020).
  * [22] Gehrels, N. et al.  A short $\gamma$-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225.  Nature 437, 851–854 (2005).
  * [23] Li, Y., Zhang, B, Lü, H.-J.,  A Comparative Study of Long and Short GRBs. I. Overlapping Properties.  ApJS 227, 7–38(2016).
  * [24] Zhang, B.  Physics of Gamma-ray Burst,418–443 Cambridge University Press, (2018).
  * [25] Belczynski, K., Bulik, T. & Rudak, B.  Study of Gamma-Ray Burst Binary Progenitors.  ApJ 571, 394–412 (2002).
  * [26] Middleditch, J.  A White Dwarf Merger Paradigm for Supernovae and Gamma-Ray Bursts.  ApJ 601, L167–170 (2004).
  * [27] Vietri, M.& Stella, L.  A Gamma-Ray Burst Model with Small Baryon Contamination.  ApJ 507, 2031-2056 (2011).
  * [28] Bromberg, O., Nakar, E., Piran, T.,  An Observational Imprint of the Collapsar Model of Long Gamma-Ray Bursts,  ApJ 749, 110-114(2012).
  * [29] Metzger, N. et al.  The protomagnetar model for gamma-ray bursts.  MNRAS 413, L45–48 (1998).
  * [30] Kluźniak, W. and Ruderman, M..  The Central Engine of Gamma-Ray Bursters.  ApJ 505, L113-117 (1998).
  * [31] B.B.Z acknowledges support by the National Key Research and Development Programs of China (2018YFA0404204), the National Natural Science Foundation of China (Grant Nos. 11833003, U2038105), and the Program for Innovative Talents, Entrepreneur in Jiangsu. Y.Z.M. is supported by the National Postdoctoral Program for Innovative Talents (grant No. BX20200164). This work was supported in part by the Natural Science Foundation of China (Grant Nos U1831135[XHZ], 11922301[HJL], 12041306[YL]), the Guangxi Science Foundation (2017GXNSFFA198008[HJL]), and the Bagui Young Scholars Program[HJL]. We also acknowledge the use of public data from the Fermi Science Support Center (FSSC).
  * [32] [Author Contributions] BBZ and HJL initiated the study. BBZ and BZ coordinated the scientific investigations of the event. BBZ, ZKL, ZKP, YL, HJL, JY, YSY, YHY, YZM and JHZ processed and analyzed the data. AJCT and YDH carried out the GTC optical observations. JRM, XHZ and JMB carried out the Lijiang 2.4m optical observations. XGW and EWL carried out the LCOGT observations. BZ and ZGD contributed to the theoretical interpretations to the event. BBZ and BZ wrote the paper with the contributions from all coauthors.
  * [33] [Competing Interests] The authors declare that they have no competing financial interests.
  * [34] [Correspondence] Correspondence and requests for materials should be addressed to B.-B. Zhang<EMAIL_ADDRESS>and B. Zhang<EMAIL_ADDRESS>

## Methods

###  0.1 Fermi data analysis.

We extracted photon events collected by the Sodium Iodide (NaI) scintillation
detector nb whose boresight has the smallest angle relative to the burst
location and conduct the following temporal and spectral analysis as described
in ref.[35].

Light curve analysis: We subtracted the background from the light curves using
the approach described in ref.[36]. From the sharp peak from
$T_{0}+0.09\rm\hskip 4.0pts$ to $T_{0}+1.05\rm\hskip 4.0pts$ (Figure 1a), we
obtained its $T_{90}$ $\sim$ $0.96~{}\rm s$, representing the time between the
epochs when 5$\%$ and 95$\%$ of the total fluence are collected.

Spectral analysis: We conducted both the time-integrated and time-resolved
spectral analysis during the $T_{90}$ time interval. The time-dependent
spectral analysis is performed by slicing $T_{90}$ into 19 segments with a bin
size of 0.05 seconds. Each segment is guaranteed to have at least 20 net
photon counts. The background spectra were obtained by modeling the data in
the signal-free time interval using the baseline method[36]. We used
$GBM\hskip 3.0ptResponse\hskip 3.0ptGenerator$ (obtained from
https://fermi.gsfc.nasa.gov/ssc/data/analysis/rmfit/gbmrsp-2.0.10.tar.bz2) to
generate the detector response matrices (DRMs). Finally, we adopted
$McSpecFit$[36] to fit both preprocessed spectra and plotted the best fitting
results. The mainstream spectra fitting models, such as the single power law
(PL), cutoff power law (CPL), band function (Band), black body (BB), and their
hybrids were utilized to fit both spectra. By assessing the fitting
performance via the combination of Profile-Gaussian likelihood (PGSTAT/dof)
and Bayesian information criterion (BIC)[37], the CPL model was favored for
best representing the properties of both time-integrated and time-resolved
spectra. The CPL model is described in the form of

$N(E)=A\Big{(}\frac{E}{100~{}\rm keV}\Big{)}^{\alpha}{\rm
exp}\Big{[}\frac{-(\alpha+2)E}{E_{\rm p}}\Big{]},$ (1)

where $\alpha$ is the photon spectral index, $A$ is the normalized
coefficient, and $E_{\rm p}$ is the peak energy of the spectrum. We fitted
time-dependent spectra in 19 aforementioned time intervals using the CPL
model. Fitting results demonstrate an intensity tracking pattern of spectral
evolution, which is shown in Extended Data Figure 3. Time-integrated fitting
results are exhibited in Extended Data Figure 2. We list the results in
Extended Data Figure 1.

###  0.2 Amplitude parameter.

The $f$ parameter is defined by $f\equiv\frac{F_{\rm p}}{F_{\rm b}}$, where
$F_{\rm p}$ and $F_{\rm b}$ correspond to the peak flux and average background
flux of a GRB, respectively. In order to distinguish intrinsically short GRBs
from disguised short GRBs due to the tip-of-iceberg effect, an effective
amplitude parameter $f_{\rm eff}=\frac{F_{\rm p^{\prime}}}{F_{\rm b}}$ for
long GRBs is defined, which is the $f$ value of “pseudo GRBs” whose flux is
re-scaled from the initial flux so that $T_{90}$ above the background flux is
shorter than 2 s(ref.[12]) . By comparing the $f-T_{90}$ diagram of short GRBs
and the $f_{\rm eff}-T_{90}$ diagram of long GRBs, one can see that most of
intrinsically short GRBs possess relatively large $f$ than $f_{\rm eff}$ of
long GRBs. A disguised short GRB would have $f<1.5$. GRB 200826A has
$f=7.58\pm 1.23$, which is much greater than the range for a disguised GRB. As
a result, this GRB is a genuinely short GRB.

###  0.3 Hardness ratio.

Long GRBs are usually softer than short GRBs, suggesting that they have a
lower hardness ratio (HR). We define HR as the photon counts ratio between
50-300 keV and 10-50 keV energy bands. The GRB sample (from ref.[38]) and GRB
200826A are marked in Figure 1c by points and star, respectively.

###  0.4 Amati relation.

The correlation between the GRB isotropic energy $E_{\rm\gamma,iso}$ and the
rest-frame peak energy $E_{\rm p,z}=(1+z)E_{\rm p}$ was discovered by Amati et
al.[39], and can be written as ${\rm log~{}}E_{\rm p,z}=a+b{\rm
log~{}}E_{\rm\gamma,iso}$. Typically, long and short GRBs follow different
tracks in the $E_{\rm p,z}-E_{\rm\gamma,iso}$ diagram. We over-plotted the
$E_{\rm p,z}-E_{\rm\gamma,iso}$ diagram (Figure 2a) using GRB samples with
known redshift from refs. [4] and [39]. Utilising the MCMC method, the optimal
fitting parameters and $3\sigma$ uncertainties are constrained. We obtain
$a=-25.01^{+3.91}_{-4.25}$ and $b=0.52^{+0.08}_{-0.07}$ for long GRBs, and
$a=-19.92^{+9.18}_{-7.60}$ and $b=0.45^{+0.15}_{-0.18}$ for short GRBs. The
luminosity distance is calculated with a cosmological parameter set of
$H_{0}=67.7$ km s-1 Mpc-1, $\Omega_{\rm m}=0.307$ and
$\Omega_{\rm\Lambda}=0.693$ throughout this paper.

###  0.5 $\epsilon$ parameter.

With the parameter $\epsilon$ = $E_{\rm\gamma,iso,52}/E_{\rm p,z,2}^{5/3}$,
where $E_{\rm\gamma,iso,52}=E_{\rm\gamma,iso}/10^{52}~{}\rm erg$ and $E_{\rm
p,z,2}=E_{\rm p,z}/100~{}\rm keV$, a classification method [15] was proposed
to clearly divide GRBs with high $\epsilon$ ($\epsilon>0.03$) and low
$\epsilon$ ($\epsilon<0.03$) at the separation line $\epsilon$ $\sim$ 0.03.
The two classes nicely match Type II and Type I GRBs, respectively. We
computed $\epsilon$ of GRB 200826A and plotted it on the ${\rm log~{}}T_{\rm
90,z}$ \- ${\rm log~{}}\epsilon$ panel (Figure 2b). The classification of a
GRB can be quantitatively evaluated by a GRB distribution probability, which
is defined as the number of GRBs falling inside an area of $d{\rm
log~{}}\epsilon$ by $d{\rm log~{}}T_{90,z}$ centered at ${\rm
log~{}}\epsilon$, ${\rm log~{}}T_{90,z}$ divided by the total number of the
GRB samples. We assume this probability of each GRB in our sample $p_{i}$ has
a normalized log-normal probability distribution $p$(${\rm
log~{}}\epsilon,\delta{\rm log~{}}\epsilon$). We calculated the integral of
$p_{i}$ over the range [${\rm log~{}}\epsilon-{d\rm log~{}}\epsilon/2,{\rm
log~{}}\epsilon+{d\rm log~{}}\epsilon/2$] and summed over the entire GRB
sample to obtain the probability $P({\rm log~{}}\epsilon,{\rm log~{}}T_{\rm
90,z})$. Dark grey ($P({\rm log~{}}\epsilon,{\rm log~{}}T_{\rm 90,z})>0.1$)
and light grey ($P({\rm log~{}}\epsilon,{\rm log~{}}T_{\rm 90,z})>0.003$ at
3$\sigma$ confidence level) regions in Figure 2b mark probability contours of
GRB clustering, respectively. GRB 200826A falls into the high $\epsilon$
region with $P({\rm log~{}}\epsilon,{\rm log~{}}T_{\rm 90,z})>0.003$, which
suggests that it is of a Type II origin.

###  0.6 Spectral lag.

For most GRBs, time lags among different bands, also known as spectral lags,
exist in the light curves[40] due to the fact that photons with different
energies arrive the observer at different times. We used the cross-correlation
function (CCF)[41, 42] to calculate the time lags of light curves among
different energy bands between $T_{0}$ \+ $0.5\rm\hskip 4.0pts$ and $T_{0}$ \+
$1.6\rm\hskip 4.0pts$ following the method described in ref.[43]. We obtain
non-zero spectral lags for GRB 200826A, which are shown in Extended Data
Figure 4.

###  0.7 X-ray Observations.

The Swift/XRT has performed a follow-up observation for a duration of $\sim
6\times 10^{4}~{}\rm s$ after the GBM trigger. The observation (obtained from
https://www.swift.ac.uk/xrt_live_cat/00021028/) provided an intrinsic column
density of $6.0\times 10^{20}$ cm-2. The X-ray light curve is shown in the
Extended Data Figure 6. We fitted the light curve using a broken power-law
model and obtained the decay slopes as $\sim-1.5$ and $\sim-0.4$, and a break
at $\sim 10^{5}$ s.

###  0.8 Optical transient.

An optical counterpart of GRB 200826A was located by Zwicky Transient Facility
(ZTF) and followed up by several ground and space facilities. We summarized
our own observations performed by the Gran Telescopio CANARIAS (GTC), Lijiang
2.4-meter telescope, and LCOGT network observatory in Extended Data Table 2.
Other sets of optical observations can be found in ref.[11]

###  0.9 Host galaxy observations and properties.

We observed the host galaxy with the Gran Telescopio CANARIAS (GTC) in
$r$-band, LCOGT network observatory in $I$-band. Those results allow us to
perform the following analyses on the host galaxy properties.

  * •

$R_{\rm 50}$. The ZTF position is used to estimate the half-light radius
$R_{\rm 50}$. We used the PyRAF/$ellipse$ routine to generate elliptical
intensity isophotes, constructed the total flux profile as a function of semi-
major axis, and calculated the half-light radius, $R_{\rm 50}$, through
interpolation. We obtained $R_{\rm 50}$ $\sim$ 0.77", corresponding to 5.7 kpc
at redshift 0.7481. With a 0.35" offset mentioned by Ahumada, T. et al., the
normalized offset $R_{\rm off}/R_{50}=0.45$ is a typical value for LGRBs.

  * •

$F_{\rm light}$. We estimated the cumulative light fraction $F_{\rm light}$
with our GTC image. In the intensity versus total flux profile, we
interpolated the mean intensity at the position of GRB 200826A to estimate the
total flux of the fainter regions and $F_{\rm light}=F_{\rm fainter}/F_{\rm
total}=0.79$. The offsets and $F_{\rm light}$ of GRB 200826A are typical for
LGRBs. Comparatively, GRB 090426A has a smaller offset and larger $F_{\rm
light}$ than GRB 200826A.

  * •

SFR. We calculated the star formation rate (SFR) with near-UV (NUV)
observations (and emission lines). Since the rest frame wavelength of the
$g$-band is in NUV, we are able to approximate the SFR with the Galactic
extinction corrected $g$-band magnitude from DESI, as 8.7 $M_{\odot}$ yr-1
based on Kennicutt 1998[44]. The [OII]$\lambda$3727 emission line flux present
in the GTC spectrum[11] is $3.81\times 10^{-17}$ erg s-1 cm-2, which
corresponds to a luminosity $1.03\times 10^{41}$ erg s-1. The SFR estimated
with ref. [44] using the [OII] emission line is $(1.4\pm 0.4)\times
10^{-41}L_{\rm[OII]}>{1.44}\ M_{\odot}$ yr-1. Emission lines represent star
formation rate with age 0-10 Myr, which is more related to the origin of
LGRBs, hence serves as a better indicator of distinguishing LGRBs and SGRBs.
We thus used the SFR estimated with [OII] emission line to obtain the Odds
O(II:I) (see below).

  * •

Stellar mass. To estimate the stellar mass, we used the Code Investigating
GALaxy Emission (CIGALE) [45] to perform Spectral Energy Distribution (SED)
fitting in $g$, $r$, $i$ and $z$ bands after considering the Galactic
extinction [46]. Several models, including the sfhdelayed star formation
history model with an initial SFR of 0.1 $M_{\odot}$ yr-1, bc03 stellar
population model[47] with a Chabrier initial mass function, dustatt_calzleit
dust attenuation model with the UV bump centroid to be 217.5 nm, dale2014 dust
emission model[48] with various AGN fractions, were taken into account. Our
results yield a stellar mass of $M_{*}=(4.1\pm 2.9)\times 10^{9}M_{\odot}$.
Correspondingly, the specific star formation rate (sSFR) can be calculated as
$>{0.35}$ Gyr-1.

###  0.10 $\mathcal{O}$.

A naive Bayesian scheme has been proposed by Li et al., 2020[21] to determine
the physical categories of GRBs based on multi-wavelength properties. The
parameter $\mathcal{O}={\rm log\ O(II:I)}+0.7$ is the logarithmic Type II to
Type I probability ratio corrected for the potential correlation between
parameters. We calculate $\mathcal{O}$ based on the parameter sets from the
prompt emission and host galaxy properties. A GRB is classified as a Type II
or Type I if $\mathcal{O}$ is positive or negative. Without inclusion of the
$f$ parameter, we obtain a positive value $\mathcal{O}$ = 1.5, placing GRB
200826A in the Type II rather than Type I GRB category. With the method
proposed in ref.[21], we further separated the parameter sets into groups of
prompt emission and host galaxy to compute log O(II:I)prompt and log
O(II:I)host, respectively, and place GRB 200826A on the distribution diagram
(Figure 3f).

###  0.11 Data Availability

Processed data are presented in the tables and figures in the paper. Source
and optical observational data are available upon reasonable requests to the
corresponding authors. The Fermi/GBM data are publicly available at
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/.

###  0.12 Code Availability

Upon reasonable request, the code (mostly in python) used to produce the
results and figures will be provided.

* [35] Zhang, B.-B. et al.  A Comprehensive Analysis of Fermi Gamma-ray Burst Data. I. Spectral Components and the Possible Physical Origins of LAT/GBM GRBs.  ApJ 730, 141-173 (2011).
* [36] Zhang, B. B. et al.  Transition from fireball to Poynting-flux-dominated outflow in the three-episode GRB 160625B.  Nature Astronomy 2, 69–75 (2018).
* [37] Zhang, B. B. et al.  A peculiar low-luminosity short gamma-ray burst from a double neutron star merger progenitor.  Nature Communications 9, 447-452 (2018).
* [38] Goldstein,A. et al.  An Ordinary Short Gamma-Ray Burst with Extraordinary Implications:Fermi-GBM Detection of GRB 170817A.  ApJ 848, L14-27 (2017).
* [39] Amati, L. et al.  Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts.  A&A 390, 81–89 (2002).
* [40] Wei, J.-J., Zhang, B.-B., Shao, L., Wu, X.-F. & Mészáros, P.  A New Test of Lorentz Invariance Violation: The Spectral Lag Transition of GRB 160625B.  ApJ 834, L13-18 (2017).
* [41] Norris, J. P., Marani, G. F. & Bonnell, J. T.  Connection between Energy-dependent Lags and Peak Luminosity in Gamma-Ray Bursts.  ApJ 534, 248–257 (2000).
* [42] Ukwatta, T. N. et al.  Spectral Lags and the Lag-Luminosity Relation: An Investigation with Swift BAT Gamma-ray Bursts.  ApJ 711, 1073–1086 (2010).
* [43] Zhang, B.-B. et al.  Unusual Central Engine Activity in the Double Burst GRB 110709B.  ApJ 748, 132-140 (2012).
* [44] Kennicutt, J., Robert C.  The Global Schmidt Law in Star-forming Galaxies.  ApJ 498, 541–552 (1998).
* [45] S. Noll, D. Burgarella, E. Giovannoli, V. Buat, D. Marcillac, and J. C. Muñoz-Mateos.  Analysis of galaxy spectral energy distributions from far-UV to far-IR with CIGALE: studying a SINGS test sample.  A&A 507, 1793–1813 (2009).
* [46] Edward F. Schlafly and Douglas P. Finkbeiner.  Measuring Reddening with Sloan Digital Sky Survey Stellar Spectra and Recalibrating SFD.  ApJ 737, 103-115 (2011).
* [47] G. Bruzual and S. Charlot.  Stellar population synthesis at the resolution of 2003.  MNRAS 344, 1000–1028 (2003).
* [48] Daniel A. Dale, George Helou, Georgios E. Magdis, Lee Armus, Tanio Díaz-Santos, and Yong Shi.  A Two-parameter Model for the Infrared/Submillimeter/Radio Spectral Energy Distributions of Galaxies and Active Galactic Nuclei.  ApJ 784, 83-93 (2004).

Figure 1: Time-integrated and time-resolved spectral fitting results of GRB
200826A with cutoff power law model

Figure 2: Spectral fitting results within the interval of $T_{0}+0.50~{}\rm s$
$\sim$ $T_{0}+1.60~{}\rm s$. a, observed photon count rate spectra and fitted
model of nb(red), n7(blue) and b1(sky blue) detectors. b, de-convolved
spectra(black points) and best-fit power law model(red line). c, corner plots
and histograms show one and two-dimensional posterior probability
distributions of cutoff power-law model parameters at 1-$\sigma$ (purple
contours), 2-$\sigma$(yellow contours) and 3-$\sigma$(green contours)
confidence levels. Red error bars and crosses represent best-fit values with
1-$\sigma$ uncertainties.

Figure 3: Spectral evolution of GRB 200826A. Panels a and b show evolution of
the photon index ($\alpha$) and spectral peak energy ($E_{\rm p}$) of the cut-
off powelaw model, respectively. Panels c and d show the NaI and BGO light
curves. All error bars represent 1-$\sigma$ uncertainties. Figure 4: Spectral
lag calculation. a, light curves in different energy bands(10-20 keV $\sim$
300-500 keV) which are used to calculate lags. b, energy dependent spectral
lag between the lowest energy(10-20 keV) band and any higher energy band. All
error bars represent 1-$\sigma$ uncertainties. Figure 5: Observations of
optical counterpart and host galaxy of GRB 200826A. Figure 6: X-ray afterglow
of GRB 200826A. Black points represents the observed X-ray flux in 0.3-10 keV.
The solid orange line shows the broken power-law fitting with slopes $\alpha$
= -1.41${}_{-0.12}^{+0.24}$, $\beta$= -0.43${}_{-0.22}^{+0.17}$ and a break at
$t_{\rm b}$ = $1.51^{+0.31}_{-0.30}\times 10^{5}$ s. The dashed blue line
represents the magnetar spin-down energy injection model parameterized by
$L_{\rm sd}=L_{0}(1+t/\tau)^{-2}$ with $L_{0}=10^{44.9}~{}\rm erg~{}s^{-1}$
and $\tau=1.0\times 10^{6}~{}\rm s$. All error bars represent 1-$\sigma$
uncertainties. The upper limit is at the 3-$\sigma$ level.