Swift Observations of GRB 140320B

V. D'Elia (SSDC and INAF-OAR) and S.R. Oates (UCL-MSSL) for the Swift team

1. Introduction

Mereghetti et al. (GCN Circ. 16004) reported the GRB detection. A gamma ray burst lasting about 100 s has been detected by IBAS in the IBIS/ISGRI data at 09:26:00 UT of March 20. Table 1 contains the best reported positions from Swift, and the latest XRT position can be viewed at http://www.swift.ac.uk/xrt_positions.

Guidorzi et al. (GCN Circ. 16003) reported the position from FTN for the optical afterglow of this GRB. 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 D'Elia et al. (GCN Circ. 29371).

We have analysed 6.0 ks of XRT data for the INTEGRAL-detected burst GRB 140320B (Pagani et al. GCN Circ. 16013), from 12.0 ks to 29.4 ks after the INTEGRAL trigger. The data are entirely in Photon Counting (PC) mode. We cannot determine at the present time whether the source is fading.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.4 (+0.4, -0.3). The best-fitting absorption column is 7.2 (+7.1, -4.8) x 1020 cm-2, consistent with the Galactic value of 2.4 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.8 x 10-11 (3.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.2 (+7.1, -4.8) x 1020 cm-2
Galactic foreground: 2.4 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 2.4 (+0.4, -0.3)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020370.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (Oates and Starling GCN Circ. 16019) of the field of GRB 140320B 11975 s after the INTEGRAL trigger (Mereghetti et al., GCN Circ. 16004). No optical afterglow consistent with the XRT position (Pagani et al., GCN Circ. 16013) and the FTN optical position (Guidorzi et al., GCN Circ. 16003) 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 0.02 mag. in the direction of the GRB (Schlegel et al. 1998).

Figure 1. The BAT light curve is not available.

XRT light curve

Figure 2. The XRT light curve. Any data from a crosshatched region are not included in the fit.

RA (J2000) Dec (J2000) Error Note Reference
09h42m14.66s +60°16'08.3" 3.6" XRT-final UKSSDC
09h42m14.66s +60°16'08.3" 3.6" XRT-refined D'Elia et al. GCN Circ. 29371

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Guidorzi et al. 16003 FTN optical afterglow candidate FTN detection
Optical Elenin et al. 16005 ISON-NM optical observations ISON-NM detection
Optical Xin et al. 16006 Xinglong TNT upper limit TNT upper limits
Optical Perley and Cenko 16024 P60 detection of a very red afterglow Palomar 60-inch detection
Gamma-ray Mereghetti et al. 16004 A long GRB detected by INTEGRAL INTEGRAL Fluence=5x10-7erg cm-2

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 11975 29408 5864 >23.1

Table 3. UVOT observation reported by Oates and Starling (GCN Circ. 16019). The start and stop time of the exposure are given in seconds since the BAT trigger. The preliminary 3-σ upper limit is given. No correction has been made for extinction in the Milky Way.

January 30, 2021