Swift Observations of GRB 150724B

J.P. Osborne (U. Leicester) and N.P.M. Kuin (MSSL/UCL) for the Swift team

1. Introduction

Bissaldi et al. (GCN Circ. 18065) reported the GRB detection. At 18:45:36.71 on 2015-07-24, Fermi-LAT detected high-energy emission from GRB 150724B, which was also detected by Fermi-GBM (trigger 459456340/150724782). Table 1 contains the best reported positions from Swift.

Kuin (GCN Circ. 18075) reported the detection with UVOT of an optical afterglow. Watson et al. (GCN Circ. 18078) reported the position from RATIR 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

XRT results are not available.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (Kuin GCN Circ. 18075) of the field of GRB 150724B 88.6ks after the LAT trigger (Bissaldi et al., GCN Circ. 18065), which was also observed at high energy by Fermi-GBM (Veres et al., GCN Circ. 18066) and Konus-Wind (Golenetskii et al. GCN Circ. 18074). Five UVOT exposures were obtained of which the first two show a weak detection at the XRT position (Osborne et al., GCN Circ. 18069). 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 0.06 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
23h27m33.88s +03°49'06.2" 2.3" XRT-final UKSSDC
23h27m33.71s +03°49'05.2" 4.1" XRT Osborne et al. GCN Circ. 18101

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Butler et al. 18072 RATIR Optical Observations RATIR detection
Optical Ivanov et al. 18077 MASTER upper limit MASTER upper limits
Optical Watson et al. 18078 Continued RATIR Optical Observations RATIR detection
Gamma-ray Bissaldi et al. 18065 Fermi-LAT detection Fermi LAT Emax=2 GeV
Gamma-ray Veres et al. 18066 Fermi GBM detection Fermi GBM Epeak=729±35 keV
T90=38 seconds
Fluence=3.55±0.04x10-5erg cm-2
(92nd percentile for long GRBs)
Gamma-ray Golenetskii et al. 18074 Konus-Wind observation Konus-Wind Epeak=663 (-69,+82) keV
Fluence=3.63(-0.29,+0.69)x10-5erg 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 88619 88799 178 19.64± 0.12
white 88802 90608 1765 20.99± 0.14
white 90614 90748 60 >19.53
white 94379 94558 176 >20.82
white 94562 96236 1648 >22.01

Table 3. UVOT observations reported by Kuin (GCN Circ. 18075). 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.

August 2, 2015