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Swift observations of HD 305560

ATel #6526; A. Maselli (INAF-IASFPA), K. L. Page (U. Leicester), H. A. Krimm (CRESST/GSFC/USRA) and S. R. Oates (IAA-CSIC, MSSL-UCL)
on 2 Oct 2014; 19:52 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

Subjects: Ultra-Violet, X-ray, Gamma Ray, Star

At 04:44:43 on 2014 October 02, the Swift Burst Alert Telescope (BAT) triggered on what appears to be the Be star HD 305560 (GCN Circ. #16874).

Using the data set from T-239 to T+963 sec from the recent telemetry downlink, we report further analysis of the BAT data. The BAT ground-calculated position is RA, Dec = 161.562, -60.604 which is equivalent to

RA(J2000) = 10h 46m 15.0s
Dec(J2000) = -60d 36' 13.9"

with an uncertainty of 3.1 arcmin (radius, sys+stat, 90% containment). The partial coding was 53%.

The mask-weighted light curve shows that the source entered the field of view during a slew at around T-100 seconds and that the count rate was relatively flat through the duration of the event data. In the 15-50 keV range, the count rate during this period averaged 0.04 +/- 0.005 ct/s/cm^2. Examination of the BAT transient monitor images before the trigger shows no detection in earlier intervals going back to 2014 September 01, the most recent observation ending 13 minutes before the trigger.

Observations made after the burst response interval show the source fading rapidly in the BAT monitor 15-50 keV band. By 09:36:32.0 UT, the rate was down to 0.005 +/- 0.003 ct/s/cm^2 and by 10:49:28.0, the source had faded to a level consistent with background. This rapid fading is not consistent with the typical behaviour of a high-mass X-ray binary system in outburst.

The time-averaged spectrum from T+0.00 to T+320.00 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.42 +/- 0.39. The fluence in the 15-150 keV band is 1.0 +/- 0.2 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T+0.00 sec in the 15-150 keV band is 0.1 +/- 0.0 ph/cm2/sec. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/614193/BA/

The Swift X-Ray Telescope (XRT) started observing HD 305560 at T0+424 s (where T0 corresponds to the time at which HD 305560 entered the BAT field of view). Using data from the whole available dataset we found a bright, uncatalogued X-ray source located at RA, Dec 161.6173, -60.5644 which is equivalent to

RA(J2000) = 10h 46m 28.14s Dec(J2000) = -60d 33' 51.7"

with an uncertainty of 3.6 arcseconds (radius, 90% confidence). This location is 2.9 arcmin from the BAT ground-calculated position, within the BAT error circle, and is 2.5 arcsec from the SIMBAD position of the Be star HD 305560.

Initially, the soft X-ray 0.3-10 keV count rate was at the level of ~15 ct/s; then, after reaching a peak of 20 ct/s (corresponding to 1.8e-9 erg/cm^2/s, observed) at ~T0+540 s, it was followed by a moderate decay, reaching a count rate of ~6 ct/s by ~1.2 ks after the trigger. The second snapshot of XRT data started at T0+5.7 ks, with the soft X-ray emission showing a flaring phase, with the count rate rapidly rising from ~0.6 ct/s to ~18 ct/s at T0+6.4 ks and keeping at that count rate level until this snapshot concluded, at T0+6.9 ks. Data collected at T0+11.5 ks caught the source in an evident decaying phase with a count rate of ~7.6 ct/s down to 0.6 ct/s when the observation concluded, at T0+18.4 ks.

The XRT data collected between T0+424 s and 18.4 ks can be fitted with a hard power-law (Gamma = 0.81 +0.07/-0.12) plus an optically thin thermal component (kT = 0.45 +/- 0.15 keV), with a high absorbing column of (3.4 +0.7/-0.4)x10^22 cm^-2. Time-slicing the XRT spectra provides no strong evidence for changes in these temperatures during this interval.

Because of the brightness of the optical source (V~9.8 mag), the UV/Optical Telescope (UVOT) data are saturated in all 6 optical and UV filters and have therefore not been analysed in detail.