Swift Observations of GRB 230307A

D.N. Burrows (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

Dalessi (GCN Circ. 33407) reported the GRB detection. At 15:44:06.67 UT on 07 March 2023, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 230307A (trigger 699896651 / 230307656). Table 1 contains the best reported positions from Swift.

Levan et al. (GCN Circ. 33439) reported the position for the optical afterglow of this GRB. Breeveld and Salvaggio (GCN Circ. 33471) reported the detection with UVOT of an optical afterglow. Levan et al. (GCN Circ. 33580) determined a redshift of 3.87 from JWST. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

2. BAT Observations and Analysis

BAT did not observe this burst.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 33465). Of the sources reported by Burrows et al. (GCN Circ. 33429), "Source 2" is fading with >3-sigma significance, and is therefore likely the GRB afterglow.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.1 (+0.6, -0.5).

4. UVOT Observations and Analysis

The Swift/UVOT began observations (Breeveld and Salvaggio GCN Circ. 33471) of the field of GRB 230307A 99.5 ks after the Fermi/GBM-detected trigger (Dalessi et al., GCN Circ. 33407). A fading source consistent with the enhanced XRT position (Burrows et al., GCN Circ. 33465) and consistent with the optical counterpart reported by Levan et al. (GCN Circ. 33439), Lipunov et al. (GCN Circ. 33441), Fulton et al. (GCN Circ. 33443), O'Connor et al. (GCN Circ. 33447), Im et al. (GCN Circ. 33449), Vanderspek et al. (GCN Circ. 33453) and Bom et al. (GCN Circ. 33459) is detected in the initial UVOT exposures. Table 3 gives preliminary magnitudes using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc., 1358, 373). No correction has been made for the expected extinction in the Milky Way corresponding to a reddening of EB-V of 0.084 mag. in the direction of the GRB (Schlegel et al. 1998).

Figure 1. The BAT light curve is not available.

RA (J2000) Dec (J2000) Error Note Reference
04h03m26.24s -75°22'43.8" 3.2" XRT Burrows et al. GCN Circ. 33465

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 33428 Fermi GRB 230307A: Global MASTER-Net
observations report 		MASTER
Optical Pérez-García et al. 33434 optical upper limit BOOTES upper limits
Optical Levan et al. 33439 Possible optical afterglow
Optical Fulton et al. 33443 ATLAS information on the ULTRACAM
optical afterglow candidate 		ATLAS detection
Optical O'Connor et al. 33447 Gemini-South Confirmation of the
Optical Afterglow		 Gemini detection
Optical Im et al. 33449 GECKO optical observations and
detections of the afterglow		 SSO detection
Optical Vanderspek et al. 33453 TESS Detection of the Optical Afterglow detection
Optical Bom et al. 33459 SOAR/Goodman detection of the possible
host galaxy		 Southern Astro.Research Telescope
Optical Gillanders et al. 33485 Continued Gemini-South observations
confirm rapid optical fading 		Gemini
Optical Levan et al. 33569 JWST observations consistent with the
presence of a kilonova		 JWST
Optical Levan et al. 33580 JWST NIRSpec observations, possible
higher redshift		 JWST redshift
Optical 33635 iTelescope optical upper limit (likely) iTelescope upper limits
Radio Anderson et al. 33475 ATCA radio detection ATCA detection
X-ray Xiao 33410 Solar Orbiter STIX observation Spectr.Tele. for Imag.X-rays light curve
X-ray Katoch et al. 33437 AstroSat LAXPC detection LAXPC detection
X-ray Sugita et al. 33448 MAXI/GSC upper limit MAXI upper limits
X-ray Liu et al. 33466 soft X-ray detection with LEIA Lobster Eye Imager detection
X-ray Grefenstette 33478 Far off-axis NuSTAR detection of the GRB NuSTAR detection
X-ray Escorial et al. 33558 Chandra late-time detection of the
X-ray afterglow		 Chandra detection
Gamma-ray Fermi 33405 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Xiong et al. 33406 GECAM detection of an extremely bright
burst		 GECAM
Gamma-ray Dalessi 33407 Fermi GBM detection of a very bright GRB Fermi GBM
Gamma-ray Dalessi et al. 33411 Fermi GBM Observation of a very bright
burst		 Fermi GBM Epeak=936±3 keV
T90=35 seconds
Fluence=2.951±0.004x10-3erg cm-2
(brighter than 100.0% of long GRBs)
Gamma-ray Casentini et al. 33412 AGILE/MCAL detection AGILE
Gamma-ray Kozyrev et al. 33413 IPN triangulation of GRB 230307A
(long/very bright)		 IPN
Gamma-ray Navaneeth et al. 33415 AstroSat CZTI detection CZTI
Gamma-ray Biltzinger et al. 33416 BALROG localization (Fermi Trigger
699896651/ GRB 230307656)		 BALROG
Gamma-ray Dafcikova et al. 33418 Detection by GRBAlpha GRBAlpha
Gamma-ray Ripa et al. 33424 VZLUSAT-2 detection VZLUSAT
Gamma-ray Kozyrev et al. 33425 Improved IPN localization for GRB
230307A		 IPN
Gamma-ray Svinkin et al. 33427 Konus-Wind detection Konus-Wind Epeak=1000 (-16,+15) keV
Duration=~196 seconds
Fluence=3.58(-0.08,+0.08)x10-3erg cm-2
Gamma-ray Li 33438 ASO-S/HXI detection Hard X-ray Imager
Gamma-ray Casentini et al. 33444 AGILE/MCAL analysis AGILE Epeak=1260 keV
Gamma-ray Kozyrev et al. 33461 Further improved IPN localization for
GRB 230307A		 IPN
Gamma-ray V. Savchenko 33472 INTEGRAL upper limit on Hard X-ray
afterglow		 INTEGRAL
Gamma-ray Dalessi 33551 Bad Time Intervals for Fermi GBM data Fermi GBM
Gamma-ray Svinkin et al. 33579 further analysis of the Konus-Wind
detection and rest-frame energetics		 Konus-Wind
Neutrino Collaboration 33430 Upper limits from a neutrino search
with IceCube		 IceCube upper limits
Other Burns et al. 33414 possibly the second highest GRB energy
fluence ever identified
Other Lipunov 33442 Erratum of the GCN Circ. 33441
Other Camisasca et al. 33577 short mininum variability timescale
compatible with a merger origin		 light curve
Other Bulla et al. 33578 good match with kilonova models

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 99525 110441 2265 21.28 ± 0.16
white 133031 208070 2931 22.09 ± 0.32
white 212899 420530 3418 >22.25
u 83621 88252 422 >20.44

Table 3. UVOT observations reported by Breeveld and Salvaggio (GCN Circ. 33471). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

May 9, 2023