Gamma-ray Burst 220219B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #31619
Anna Y. Q. Ho (UCB), Daniel Perley (LJMU), Igor Andreoni (JSI), Mansi Kasliwal (Caltech), Dmitry Svinkin (Ioffe Institute), Aaron Tohuvavohu (U. Toronto), Harsh Kumar (IITB) on behalf of the Zwicky Transient Facility (ZTF) collaboration:
We report the discovery of ZTF22aabjpxh (AT2022cva), a rapidly fading red transient located at
```
16:03:39.35  +31:14:04.72 (J2000)
240.9139656,  31.2346451 (J2000)
```
first detected as part of the ZTF partnership survey.
The source was discovered on JD 2459630.0001 at r=16.59 ± 0.01 mag, which was 0.98 days after the last non-detection (r>19.34). The transient position is 2'' from a galaxy in SDSS with a photometric redshift of 0.25 ± 0.05. Due to the rapid rise of almost 3 magnitudes in one day, and the possible high luminosity, we triggered follow-up photometry with the Spectral Energy Distribution Machine (SEDM) on the 60-inch telescope at Palomar Observatory (P60). SEDM photometry obtained the night after discovery established a rapid fade rate of 2.25 magnitudes in 0.88 days as well as red colors (g-r=0.41 ± 0.08, corrected for Galactic extinction). The transient was also detected as part of the ZTF public survey that night, and the rapid fade triggered the ZTFReST pipeline (Andreoni & Coughlin et al. 2021).
Konus-Wind detected a long-duration GRB at UT 2022-02-19 09:27:25.350 that is consistent with the position of AT2022cva. The GRB burst time is 2.5 hours before the first ZTF detection. The GRB was also weakly detected by BAT. The ZTF optical position was outside the coded BAT FoV at the time of the burst, and the observed counts are consistent with a relatively soft burst coming from outside the BAT field of view. The Konus-BAT IPN annulus is consistent with the position of the ZTF transient, and the association is highly likely.
We conclude that AT2022cva is the likely afterglow to the Konus-Wind GRB. Given the possible low redshift (z~0.3) continued photometric and spectroscopic observations are encouraged.
Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, IN2P3, University of Warwick, Ruhr University Bochum and Northwestern University. Operations are conducted by COO, IPAC, and UW.
GCN Circular #31624
D. Svinkin, D. Frederiks, A. Ridnaia, A. Lysenko, and T. Cline on behalf of the Konus-Wind team, and
S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, and A. Tohuvavohu on behalf of the Swift-BAT team, report:
The long-duration GRB 220219B has been detected by Konus-Wind (KW) and Swift (BAT), so far, at about 34045 s UT (09:27:25). The burst was outside the coded field of view of the BAT.
We have triangulated it to a Konus-BAT annulus centered at RA(2000)=314.433 deg (20h 57m 44s) Dec(2000)=-14.987 deg (-14d 59' 15"), whose radius is 83.789 +/- 1.439 deg (3 sigma).
This localization may be improved.
The position of the optical transient ZTF22aabjpxh/AT2022cva (Ho et al., GCN Circ. 31619) is inside the annulus at 0.46 deg from its center line. The OT position is consistent with the Swift-BAT detection, Fermi-GBM non-detection, and the KW ecliptic latitude response.
The positional and temporal coincidence of this burst with the OT supports the conclusion that the OT is the GRB counterpart.
A triangulation map is posted at http://www.ioffe.ru/LEA/GRBs/GRB220219_T34045/IPN
The time history and spectrum will be given in forthcoming GCN Circulars.
GCN Circular #31634
P. A. Evans (U. Leicester) reports on behalf of the Swift team:
Swift has initiated a ToO observation of the Konus-Wind GRB 220219B. Automated analysis of the XRT data will be presented online at https://www.swift.ac.uk/ToO_GRBs/00021481
Any uncatalogued X-ray sources detected in this analysis will be reported on this website and via GCN COUNTERPART notices. These are not necessarily related to the Konus-Wind event. Any X-ray source considered to be a probable afterglow candidate will be reported via a GCN Circular after manual consideration.
Details of the XRT automated analysis methods are detailed in Evans et al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).
This circular is an official product of the Swift-XRT team.
GCN Circular #31643
S. Belkin (IKI, HSE), A. Pozanenko (IKI), N. Pankov (HSE), E. Klunko (ISTP) report on behalf of GRB IKI FuN:
We observed ZTF22aabjpxh/AT2022cva discovered by ZTF (Ho et al., GCN 31619) with AZT-33IK telescope of Mondy observatory on 2022-02-21 and 2022-02-22. ZTF22aabjpxh/AT2022cva was observed also in optic (Kim et al., GCN 31628; Hu et al., GCN 31639) and in X-ray by NICER (Pasham et al., GCN 31635). ZTF22aabjpxh/AT2022cva is considered the likely afterglow of GRB 220219B (Ho et al., GCN 31619; Svinkin et al., GCN 31624).
We clearly detect the SDSS galaxy at z=0.293 (Fremling et al., GCN 31629). Due to non-optimal seeing we could not discriminate the afterglow from the galaxy. We use aperture photometry and preliminary photometry of a source (afterglow+host galaxy) is following
```
Date       UT start  JD         Exp.    Filter  OT    Err.  UL(3sigma)
                     (mid, days)  (s)

2022-02-21 19:30:32  2.43966  30*120   R    19.19    0.03    22.8
2022-02-22 19:31:06  3.43936  29*120   R    19.34    0.03    22.8
```
The photometry is based on nearby SDSS DR12 stars
```
id                  r      R(Lupton) V(Lupton)
J160337.10+311328.5 16.987 16.79     17.19
J160343.10+311452.9 18.700 18.51     18.90
```
Assuming the host galaxy has R = 19.7 we subtract a host galaxy flux from a flux of the source (afterglow+host galaxy) and estimated power law index of a light curve of the afterglow as -1.2, which is typical for a GRB afterglow.
GCN Circular #31644
A.P. Beardmore (U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), D.N. Burrows (PSU), J. D. Gropp (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of GRB 220219B (Svinkin et al., GCN Circ. 31624), collecting 3.5 ks of Photon Counting (PC) mode data between T0+322.5 ks and T0+369.3 ks.
One uncatalogued X-ray source has been detected at a position consistent with the GRB optical afterglow candidate ZTF22aabjpxh/AT2022cva (Ho et al., GCN Circ. 31619). The X-ray source is below the RASS limit and shows no definitive signs of fading. Details of this source are given below:
Source 1:
```
  RA (J2000.0):  240.91414  =  16:03:39.39
  Dec (J2000.0): +31.23404  =  +31:14:02.5
  Error: 2.8 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: 0.0250 [+0.0032, -0.0031] ct s^-1   
  Flux: (1.06 [+0.14, -0.13])e-12 erg cm^-2 s^-1 (observed, 0.3-10 keV)
```
The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/ToO_GRBs/00021481.
This circular is an official product of the Swift-XRT team.
GCN Circular #31646
A. Tsvetkova, D. Frederiks, A. Lysenko, A. Ridnaia, D. Svinkin, M. Ulanov, and T. Cline on behalf of the Konus-Wind team, report:
The long-duration GRB 220219B (IPN triangulation: Svinkin et al., GCN 31624), consistent with the optical transient ZTF22aabjpxh/AT2022cva reported by ZTF (Ho et al., GCN 31619), triggered Konus-Wind at T0=34045.350 s UT (09:27:25.350).
The burst light curve consists of two broad pulses: the first one lasts from ~T0-34 s to ~T0+13 s peaking at ~T0, the second pulse lasts from ~T0+18 s to ~T0+45 s peaking at ~T0+20 s, The emission is seen up to ~1 MeV.
As observed by Konus-Wind, the burst had a fluence of the first pulse 1.21(-0.15,+0.16)x10^-5 erg/cm^2, a fluence of the whole burst 1.76(-0.14,+0.12)x10^-5 erg/cm^2, and a 64-ms peak energy flux, measured from T0-0.064 s, of 1.76(-0.27,+0.52)x10^-6 erg/cm^2/s (all in the 20 keV - 10 MeV energy range).
Since the main fraction of the first pulse was detected before the trigger, its spectral analysis was performed using the KW 3-channel light curve data.
Modelling its spectrum (measured from T0-33.723 s to T0+13.380 s) by the exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep), yields alpha = -1.86(-0.09,+4.36) and Ep = 39(-24,+27) keV.
Fitting the most intense part of the first pulse (measured from T0-1.340 s to T0+1.604 s) with by the exponential cutoff model yields alpha = -1.75(-0.20,+4.25) and Ep = 61(-46,+24) keV.
The spectrum of the second emission pulse (from T0+16.640 s to T0+49.408 s) is well described by a simple power law (PL) with the photon PL index of 2.60(-0.10,+0.10) and chi2/dof = 69.5/61.
Assuming the redshift z = 0.293 (Fremling et al., GCN 31629) and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315, and Omega_Lambda = 0.685 (Planck Collaboration, 2014), we estimate the following burst rest-frame parameters: the isotropic energy release in the first pulse is Eiso = 2.75(-0.34,+0.36)x10^51 erg, the isotropic peak luminosity is Liso = 5.17(-0.79,+1.53)x10^50 erg/s; the rest-frame peak energy of the first pulse is Epiz = 50(-31,+35) keV; and the rest-frame peak energy of the 'peak' spectrum is Eppz = 79(-59,+31) keV.
With these values, GRB 220219B is within 68% prediction bands for for both 'Amati' and 'Yonetoku' relations for the sample of >300 long KW GRBs with known redshifts (Tsvetkova et al., 2021, ApJ, 908, 83), see http://www.ioffe.ru/LEA/GRBs/GRB220219_T34045/GRB220219B_rest_frame.pdf
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB220219_T34045/
All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.

GCN Circular #31647
ZTF22aaajecp/AT2022cmc: DECam Photometry
James Freeburn (Swinburne, OzGrav), Jielai Zhang (Swinburne, OzGrav), Igor Andreoni (University of Maryland, NASA GFSC), Armin Rest (STScI, Johns Hopkins), Dougal Dobie, Anais Möller, Simon Goode, Jeff Cooke (Swinburne, OzGrav), Katie Auchettl (University of Melbourne, OzGrav), Frank Valdes (NOAO), Charlie Kilpatrick (Northwestern University), David Jones (UCSC), Tim Abbott (NOAO), Bruce Gendre (University of Western Australia, OzGrav), Amy Lien (University of Tampa), Lee Spitler (Macquarie University)
As part of the Kilonova and Transients Program (KNTraP, NOIRLab Proposal ID 2022A-679480, PI: Zhang) observing run, we conducted follow-up observations of ZTF22aaajecp/AT2022cmc (I. Andreoni et al. GCNC #31590) with the Dark Energy Camera (DECam) on the CTIO 4m Victor Blanco Telescope between February 15 and 22, approximately 3-10 days after the initial ZTF detection. We took nightly observations each with a combination of g, r, i and z band exposures. We present preliminary photometry (calibrated with the Pan-STARRS photometric catalogs using Gaia star selection) from these exposures.
We would also like to acknowledge the CTIO staff, especially Kathy Vivas, Jacqueline Seron, Alberto Alvarez and Claudio Aguilera, who kept our observations running smoothly.

                                                                                    
filter,UTC,MJD,magnitude,error                                                            
g,2022-02-15 08:43:36.768,59625.36362,20.567,0.21                                         
g,2022-02-16 08:59:18.958,59626.37452,>20.88,n/a                                          
g,2022-02-17 08:51:49.783,59627.36933,>20.80,n/a                                          
g,2022-02-18 08:52:29.856,59628.36979,21.296,0.24
r,2022-02-15 08:44:36.384,59625.36431,20.587,0.13                                         
r,2022-02-16 09:00:19.432,59626.37522,21.024,0.34                                         
r,2022-02-17 08:53:48.480,59627.37070,21.522,0.33                                         
                                                                                          
i,2022-02-15 08:45:35.136,59625.36499,20.133,0.20                                         
i,2022-02-16 09:01:18.624,59626.37591,21.243,0.28                                         
i,2022-02-17 08:54:47.892,59627.37139,21.364,0.38                                         
i,2022-02-18 08:55:27.840,59628.37185,21.212,0.22                                         
i,2022-02-19 09:03:22.618,59629.37735,21.277,0.19                                         
i,2022-02-20 09:03:44.446,59630.37760,21.630,0.23                                         
i,2022-02-21 08:51:57.024,59631.36941,21.648,0.25                                         
i,2022-02-22 08:51:46.489,59632.36929,21.625,0.27
i,2022-02-15 08:45:35.136,59625.36499,20.133,0.20                                         
i,2022-02-16 09:01:18.624,59626.37591,21.243,0.28                                         
i,2022-02-17 08:54:47.892,59627.37139,21.364,0.38                                         
i,2022-02-18 08:55:27.840,59628.37185,21.212,0.22                                         
i,2022-02-19 09:03:22.618,59629.37735,21.277,0.19                                         
i,2022-02-20 09:03:44.446,59630.37760,21.630,0.23                                         
i,2022-02-21 08:51:57.024,59631.36941,21.648,0.25                                         
i,2022-02-22 08:51:46.489,59632.36929,21.625,0.27                                         
                                                                                          
z,2022-02-16 09:02:18.240,59626.37660,21.094,0.22                                         
z,2022-02-17 08:55:46.848,59627.37207,21.234,0.28                                         
z,2022-02-18 08:58:06.816,59628.37369,21.292,0.23                                         
z,2022-02-19 09:06:22.930,59629.37943,21.354,0.21                                         
z,2022-02-20 09:07:01.470,59630.37988,21.263,0.26                                         
z,2022-02-21 08:55:14.880,59631.37170,21.812,0.36                                         
z,2022-02-22 08:55:05.176,59632.37157,22.183,0.45

GCN Circular #31652
N. Pankov (HSE), A. Pozanenko (IKI), E. Klunko (ISTP), S. Belkin (IKI), report on behalf of GRB IKI FuN:
We observed the optical transient of ZTF22aaajecp/AT2022cmc (Andreoni et al., GCN 31590) with AZT-33 telescope of Mondy observatory starting on Feb. 24 (UT) 18:51:38. We detect the transient (Andreoni et al., GCN 31590; Kumar et al., GCN 31597; Pankov et al., GCNs 31593, 31625; Perley, GCN 31594; Lipunov et al., GCN 31600; Kann et al., GCN 31626; Yao et al., GCN 3164; Freeburn et al., GCN 31647) at the redshift of z=1.193 (Tanvir et al., GCN 31602; Lundquist et al., GCN 31612).
Preliminary photometry of the transient is following

                                                                                     
Date       UT start   MJD     Filter Exp.    OT    Err.  UL(3sigma)                       
                       (mid, days)    (s)                                                 
2022-02-24 18:51:38   59634.817 R    45*120  21.10  0.07  23.1

The photometry is based on the nearby SDSS-DR12 stars

                                                                                     
R.A. Dec R(Lupton transformations)                                                        
203.7398307 33.1702037 17.794                                                             
203.7684775 33.1572431 17.347

GCN Circular #31654
I.A. Smith (U. Hawaii), D.A. Perley (LJMU), and N.R. Tanvir (U. of Leicester) report:
We have been observing the VLA location of the transient ZTF22aaajecp/AT2022cmc (Andreoni et al. GCN Circ. 31590; Perley et al. GCN Circ. 31592) using the SCUBA-2 sub-millimeter continuum camera on the James Clerk Maxwell Telescope. The source was detected with a 850 micron flux density of 4.9 mJy/beam at a mid-point of UT 2022-02-21.510.
We thank Jasmin Silva, Alexis-Ann Acohido, Harold Pena, and the JCMT staff for the prompt support of these observations that were taken under project M22AP030.
GCN Circular #31665
D. Dobie (Swinburne/OzGrav), A. O'Brien (UW Milwaukee), A. Zic (Macquarie U./CSIRO), D.L. Kaplan (UW Milwaukee), D.A. Perley (LJMU), A.Y.Q Ho (UC Berkeley)
We carried out observations of ZTF22aaajecp/AT2022cmc (Andreoni et al. GCN 31590) with the Australia Telescope Compact Array (ATCA) on 2022-02-26 (~15.25 days post-discovery).
The array was in the EW367 array configuration and we excluded CA06 from our processing to obtain more uniform u-v coverage. We used 3C286 for gain, bandpass and flux scale calibration to ensure consistency with observations from Northern radio facilities.
We detect a compact radio source at the location of the transient and measure a flux density of ~1.6 mJy at 33.5 GHz.
Further observations are planned.
Thank you to CSIRO staff for supporting these observations.
GCN Circular #31667
Itai Sfaradi (HUJI), Lauren Rhodes (Oxford), David Williams (JBCA/Manchester), Joe Bright (Berkeley), Assaf Horesh (HUJI), Rob Fender (Oxford), Dave Green (Cambridge), David Titterington (Cambridge)
We report the detection of radio emission from the fast and red optical transient AT2022cmc (ZTF22aaajecp; Andreoni et al. 2020, GCN 31590).
We observed the field of AT 2022cmc with the Arcminute Microkelvin Imager Large Array (AMI-LA) for 4 hours on the 2022-02-26, with a central frequency of 15.5 GHz. A point source is detected in the AMI-LA image at RA = 13:34:43.28 Dec = +33:13:02.43, consistent with the reported position of AT2022cmc. The source is detected at a flux density of ~0.5 mJy.
We plan to perform follow-up observations with the AMI-LA, and encourage further high cadence multi frequency radio observations with other facilities.
We thank the staff of MRAO for scheduling and performing this observation.
GCN Circular #31683
S. Belkin (IKI, HSE), A. Pozanenko (IKI), N. Pankov (HSE), E. Klunko (ISTP), I. Reva (FAI), V. Kim (FAI) report on behalf of GRB IKI FuN:
We are continuing observations of ZTF22aabjpxh/AT2022cva discovered by ZTF (Ho et al., GCN 31619) with AZT-33IK telescope of Mondy observatory and Zeiss-1000 on 2022-02-21 between 2022-02-23 and 2022-03-04, i.e. upt 13.4 days after GRB 220219B trigger. ZTF22aabjpxh/AT2022cva was observed in optic (Kim et al., GCN 31628; Hu et al., GCN 31639) and in X-ray by NICER (Pasham et al., GCN 31635) and XRT/Swift (Beardmore et al., GCN 31644). ZTF22aabjpxh/AT2022cva is considered the likely afterglow of GRB 220219B (Ho et al., GCN 31619; Svinkin et al., GCN 31624; Tsvetkova et al., GCN 31646).
We clearly detect the SDSS galaxy at z=0.293 (Fremling et al., GCN 31629). Due to non-optimal seeing we could not discriminate the afterglow from the galaxy. We use aperture photometry of a source (afterglow + host galaxy). Based on our preliminary photometry we plot a light curve of the source, see upper panel of the Figure in
http://grb.rssi.ru/GRB220219B/GRB220219B_AT2022cva_LC.png
To estimate power law index of the afterglow we fit our photometric data by a single power law + constant host galaxy flux. The host galaxy fit is R = 19.68+/-0.05 and PL index alpha = -1.75+/-0.4. The light curve of the afterglow after subtraction of a flux of the host is presented at bottom panel of the Figure in
http://grb.rssi.ru/GRB220219B/GRB220219B_AT2022cva_LC.png
Power Law index is broadly compatible with the index of -1.6 obtained in XRT observations up to ~8 days, see XRT light curve at
https://www.swift.ac.uk/xrt_live_cat/00021481/
We may conclude the source ZTF22aabjpxh/AT2022cva is the optical afterglow of GRB 220219B.
Since the redshift of GRB 220219B (ZTF22aabjpxh/AT2022cva) is z=0.293 (Fremling et al., GCN 31629) we are searching for a supernova signature. Based on the Figure referenced above we still don't see any conclusive evidence for the existence of SN.
We urge further multicolour observations to confirm/search for SN associated with GRB 220219B.
GCN Circular #31720
L. Rhodes (Oxford/MPIfR), R. Fender (Oxford/UCT) and D.R.A. Williams (JBCA) report:
We observed ZTF22aabjpxh/AT2022cva (Ho et al., GCN 31619) with the Arcminute Microkelvin Imager - Large Array at 15.5GHz. The observation was made on 27th February 2022 at 04:42UT and lasted 4 hours. We detect a point source at the position of ZTF22aabjpxh with a flux density of 490+/-30uJy. The noise in the field is 34uJy. Subsequent observations show that the source is fading.
We thank the staff at the MRAO for scheduling these observations.
GCN Circular #31736
S. Belkin (IKI, HSE), A. Pozanenko (IKI), E. Klunko (ISTP), A. Moskvitin (SAO RAS), R. Ya. Inasaridze (AbAO), N. Pankov (HSE), V. Kim (HSE, FAI), I. Reva (FAI), on behalf of GRB IKI FuN, and A. Rossi (INAF-OAS) report:
We are continuing observations of ZTF22aabjpxh/AT2022cva discovered by ZTF (Ho et al., GCN 31619) with AZT-33IK telescope of Mondy observatory, AS-32 telescope of Abastumani observatory, and Zeiss-1000 telescope of SAO RAS between 2022-02-24 and 2022-03-09. ZTF22aabjpxh/AT2022cva was observed in optic (Kim et al., GCN 31628; Hu et al., GCN 31639; Belkin et al., GCNs 31643, 31683) and in X-ray by NICER (Pasham et al., GCN 31635) and XRT/Swift (Beardmore et al., GCN 31644). ZTF22aabjpxh/AT2022cva is considered an afterglow of GRB 220219B (Ho et al., GCN 31619; Svinkin et al., GCN 31624; Tsvetkova et al., GCN 31646; Belkin et al., GCN 31683).
We clearly detect the SDSS galaxy at z=0.293 (Fremling et al., GCN 31629). Due to non-optimal seeing we could not discriminate the afterglow from the galaxy. We use aperture photometry of a source (afterglow + host galaxy). Based on our preliminary photometry we plot a light curve of the source, see an upper panel of the Figure in
http://grb.rssi.ru/GRB220219B/GRB220219B_AT2022cva_LC.png
To estimate power law index of the afterglow we fit our photometric data by a single power law + constant host galaxy flux between start of our observations and up to 13.4 days after GRB 220219B trigger. The host galaxy fit is R = 19.68+/-0.05 and PL index alpha = -1.75+/-0.4 (Belkin et al., GCN 31683). The light curve of the afterglow after subtraction of a flux of the host is presented at the bottom panel of the Figure in
http://grb.rssi.ru/GRB220219B/GRB220219B_AT2022cva_LC.png
Starting on ~15.5 days the light curve is rising above afterglow approximation by the single power law and we consider it as a possible evidence of rising supernova.
We urge further spectroscopic and multicolour observations to confirm for SN associated with GRB 220219B.
GCN Circular #31739
A. Rossi, E. Pian, F. Cusano, E. Maiorano, E. Palazzi (INAF-OAS) on behalf of the CIBO collaboration, and S. Belkin (IKI, HSE) report :
We report the results of the spectroscopic follow-up observations of ZTF22aabjpxh/AT2022cva, the afterglow of GRB 220219B (Ho et al., GCN 31619; Kim et al., GCN 31628; Hu et al., GCN 31639; Belkin et al., GCNs 31643, 31683; Pasham et al., GCN 31635; Beardmore et al., GCN 31644; Tsvetkova et al., GCN 31646) at z=0.293 (Fremling et al., GCN 31629). The optical spectra were obtained with the Multi-Object Double Spectrographs (MODS) instrument mounted on the 2x8.4-m LBT telescope (Mt. Graham, AZ, USA) at ~11 UT on 2021-03-07, ~16 days (i.e. ~12 rest-frame days) after the burst trigger. The spectra covers the wavelength range 3200-10000 AA, and we obtained a total of 6 exposures of 900s. The resulting spectrum has been corrected for Galactic extinction (A_V = 0.025 mag).
The slit was place in order to cover both the host galaxy and the GRB position. In addition to the host spectra, a second trace is well visible in the 2D image of the spectrum although superimposed to the one of the host galaxy. Its low S/N extracted spectrum shows features similar to those exhibited by XRF-associated SN2006aj at a similar phase (Mazzali et al. 2006, Nature, 442, 1018). This confirms the bump observed by Belkin et al. (GCN 31736) as due to the emerging contribution of the SN.
We acknowledge the excellent support from the LBTO, in particular B. Rothberg and J. Williams, and from the LBT-INAF, particularly E. Marini in obtaining these observations.

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