Swift Observations of GRB 130305A

1. Introduction

At 11:39:16, BAT triggered on GRB 130305A (trigger #550329) (Cummings and Palmer GCN Circ. 14257). No source was found on board. This burst was also detected by Fermi (trignum=384176354) and INTEGRAL (TRIGGER_NUM: 6794, Sub_Num: 0). Table 1 contains the best reported positions from Swift.

Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

Standard analysis products for this burst are available at http://gcn.gsfc.nasa.gov/swift_gnd_ana.html.

2. BAT Observations and Analysis

The BAT mask-weighted lightcurve does not show more detail than the raw lightcurve due to the extreme partial coding (2.5%). There is a single peak, approximately symmetrical, a total of about 20 seconds long. Note, however, the much longer duration mentioned at high energy by the Konus-Wind team (Golenetskii et al., GCN # 14262.

The following spectral results appear to cover about 90% of the total flux of the burst, based on the raw lightcurve excess over constant background. Again, the partial coding makes the uncertainty of the fit parameters large. A fit to a simple power law function has a photon index of 0.78 ± 0.20. This is harder than most other BAT-detected long GRBs. The fluence from 15-150 keV was (4.8 ± 1.8) x 10^-6 ergs/cm².

The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/550329/BA/.

3. XRT Observations and Analysis

We have analysed 16 ks of XRT data for the Swift/BAT-detected burst: GRB 130305A, from 31.0 ks to 210.2 ks after the Swift/BAT trigger. The data are entirely in Photon Counting (PC) mode. An X-ray source is detected within the Swift/BAT error circle.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.4 (+0.5, -0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 (+0.6, -0.5). The best-fitting absorption column is 3.7 (+2.9, -2.1) x 10²¹ cm^-2, in excess of the Galactic value of 4.8 x 10²⁰ cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.5 x 10^-11 (7.0 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.7 (+2.9, -2.1) x 10²¹ cm^-2
Galactic foreground: 4.8 x 10²⁰ cm^-2
Excess significance: 2.5 σ
Photon index: 1.9 (+0.6, -0.5)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020234.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 130305A 30975s after the BAT trigger discovered in ground analysis (Breeveld et al. GCN Circ. 14268). No optical afterglow consistent with the XRT position (Malesani et al., GCN Circ. 14263) 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 E_B-V of 0.05 mag. in the direction of the GRB (Schlegel et al. 1998).

Figure 1. The BAT light curve is not available.

Figure 2. The XRT light curve.

RA	Dec	Error	Note	Reference
07h46m59.43s	+52o01' 60.0"	3.8"	XRT	Malesani et al. GCN Circ. 14263
07h47m05.8s	+52o02' 11"	1.8"	BAT-refined	Cummings et al. GCN Circ. 14277

Table 1. Positions from the Swift instruments.

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Cenko	14265	P60 Observations	Palomar 60-inch
Optical	Butler et al.	14267	RATIR Observations	RATIR
Optical	Cucchiara et al.	14272	Gemini-North upper limit	Gemini	upper limits
Optical	Kumar and Pandey	14274	Optical Observations	Nainital
Optical	Stecklum et al.	14309	TLS Tautenburg observations	Tautenburg
Optical	Singer et al.	14313	PTF P48 optical upper limits	Palomar 48-inch	upper limits
Optical	Volnova et al.	14324	optical upper limit in Mondy observatory	Mondy	upper limits
Optical	Cucchiara et al.	14326	Gemini follow-up observations	Gemini
Gamma-ray	Guiriec et al.	14260	Fermi LAT Detection	Fermi LAT	detection
Gamma-ray	Yu and Xiong	14261	Fermi GBM observation	Fermi GBM	detection
Gamma-ray	Golenetskii et al.	14262	Konus-Wind observation	Konus-Wind	detection

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

Filter	Tstart(s)	Tstop(s)	Exp(s)	Mag
u	30975	59030	3890	>21.4

Table 3. UVOT Observations. The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.