Newly discovered SGR 1935+2154: Swift observations

ATel #6294; J. R. Cummmings, S. D. Barthelmy (GSFC), M. M. Chester (PSU), and K. L. Page (U Leicester)
on 6 Jul 2014; 20:32 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jay R Cummings (jayc@milkyway.gsfc.nasa.gov)

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At 09:32:48 UT on 07/05/2014, the Swift Burst Alert Telescope (BAT) triggered and located SGR 1935+2154 (trigger=603488). The source was originally assigned a GRB designation, GRB 140705A (Stamatikos et al. GCN 16520, Lien et al. GCN 16522). The BAT partial-coding fraction was 94%. Swift slewed immediately to the source position. The BAT light curve showed a short double peak with a total duration of about 0.10 +- 0.01 sec. The peak count rate was ~4500 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 09:33:49.6 UT, 61.0 seconds after the BAT trigger (Osborne et al. GCN 16521). Using 886 s of XRT Photon Counting mode data and 1 UVOT images, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 293.73199, +21.89673 which is equivalent to:
RA (J2000): 19h 34m 55.68s
Dec (J2000): +21d 53' 48.2"
with an uncertainty of 2.3 arcsec (radius, 90% confidence).

The Swift/UVOT began settled observations of the field of GRB 140705A 65 s after the BAT trigger (Chester et al. GCN 16523). No optical afterglow consistent with the XRT position was detected in the initial UVOT exposures.

The preliminary 3-sigma upper limit using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart exposure is > 20.6 (white, 147 seconds starting at T+65 seconds) and a subsequent exposure is > 21.8 (white 432 seconds starting at T+582 seconds). The magnitudes are not corrected for the Galactic extinction due to the reddening of E(B-V) = 4.88 in the direction of the burst (Schlegel et al. 1998).

The time-averaged BAT spectrum from T-0.032 to T+0.068 sec fit by a simple power-law model shows the power law index of 3.25 +- 0.15 (chi squared 111.9 for 57 d.o.f.). The fluence in the 15-150 keV band was 3.8 +- 0.4 x 10^-8 erg/cm2. The 1-sec peak photon flux measured from T-0.48 sec in the 15-150 keV band was 0.9 +- 0.2 ph/cm2/sec.

A single blackbody fit to the time-averaged spectrum shows a blackbody temperature of 8.2 +- 1.0 keV (chi squared 73.10 for 57 d.o.f.). A thermal bremsstrahlung model fit shows a temperature of 28.2 +- 3.2 keV (chi squared 63.0 for 57 d.o.f.). A double blackbody fit shows the lower temperature of 4.5 +- 1.2 keV and the higher temperature of 11.9 +- 1.3 keV (chi squared 56.1 for 55 d.o.f.). All the quoted errors are at the 90% confidence level.

Swift-BAT has observed three additional short, soft events from SGR 1935+2154 (Cummings, GCN 16530). The first was at 285.8 seconds after the discovery event reported in Stamatikos et al. The second was at T+497.4 seconds. The third was at T+27157 seconds (BAT trigger # 603514; no automated response occurred since the brightness was less than the threshold set for retriggering on the known source).

At the time of the earlier two events, Swift had slewed to the source position, so the source was on axis. At the time of the later event, the source was 17% coded in BAT.

The two earlier events were each about 0.03 +- 0.01 seconds long, and the later event was about 0.07 +- 0.01 seconds long.

The fluences of the earlier events were about 1/3 and 1/5 of the fluence of the discovery event and were too low to get reliable spectral data. A simple power-law fit of the spectrum of the later event has a photon index of 2.8 +- 0.2. The fluence from 15-150 keV in 0.07 seconds was (6.1 +- 1.2) x 10^-8 ergs/cm^2.