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The Swift Detection of a Large Flare from the RS CVn Binary SZ Psc

ATel #6940; S. A. Drake (CRESST/USRA/GSFC), R. A. Osten (STScI), H. Krimm (CRESST/USRA/GSFC), M. De Pasquale (UCL), N. Gehrels (NASA/GSFC), S. Barthelmy (NASA/GSFC)
on 17 Jan 2015; 02:47 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

Subjects: X-ray, Binary, Star, Variables

Referred to by ATel #: 7983

The Swift team reports the detection and preliminary analysis of a large flare from the 3.966-day period RS CVn binary star system SZ Psc (first reported in D'Elia et al. 2015, GCN Circ. 17303) which triggered the Swift Burst Alert Telescope (BAT) hard X-ray detector at T0 = 09:08:42 on 15 January 2015. Using the data set from T0-239 to T0+963 sec (where T0 is the BAT trigger time) from the recent telemetry downlink, we report further analysis of this BAT trigger. The mask-weighted light curve shows that the source entered the BAT field of view about 100 seconds before the start of the trigger interval and that the count rate appeared to be falling from an earlier peak during this time. During and after the 320-sec trigger interval, the rate was variable, with multiple peaks and an average rate of around 0.02 ct/s/cm2 (15-50 keV) or ~100 mCrab. The partial coding was 100%. Based on results from the BAT transient monitor, the source remained significantly above background until at least 14:25 UT on 15 January 2015. The time-averaged BAT spectrum from T0+0.0 to T0+320.0s is well fitted by either a simple power-law model or a thermal bremmstrahlung model. The power law index of the best fit to the time-averaged spectrum for this model is 3.16 +/- 0.50, while the temperature of the best-fit bremmstrahlung model is 16.2 (+8.0, -5.3) keV. The fluence in the 15-150 keV band is 4.3 +/- 1.1 x 10^-7 erg/cm^2. All the quoted errors are at the 90% confidence level. Searches of archival data from the BAT monitor shows that the source has not previously been detected in the monitor.

The Swift X-Ray Telescope (XRT) started observing SZ Psc at T0(BAT)+380.5s. The soft X-ray (0.3-10 keV) rate was then ~90 ct/s, corresponding to 3.7 X 10^-9 erg/cm^2/s. The initial and all subsequent XRT observations during the first day were made in Windowed Timing mode, and thus, despite the fairly bright V magnitude of 7.44 should not be affected by optical loading. The XRT count rate increased to ~100 ct/s by the end of the initial observation at T0(BAT)+2200s, and then over the next 8 hours declined rapidly, dropping to ~30 ct/s at T0+30000s. After a a gap of ~16000s, further XRT observations from the period from T0+58000s to T0+92000s decayed much more slowly from a count rate of ~7 to 3 ct/s. At the end of these observations the soft X-ray flux of SZ Psc of 6.1 X 10^-11 erg/cm^2/s was within a factor of 2 of the flux observed in previous observations of this source, e.g., the ROSAT All-Sky Survey Bright Source Catalog quotes a count rate equivalent to a flux of 4.3 X 10^-11 erg/cm^/s (0.1 - 2.4 keV), and the XMM-Newton Slew Survey detection of 23.7 ct/s implies a flux of 3.2 X 10^-11 erg/cm^2/s (0.2-12 keV). Preliminary spectral analysis of the XRT data in the 0.5-10 keV band for the first XRT observation (the soft X-ray rise phase) using a 1T APEC fit yields a temperatures of 21.2 (+0.9,-0.8) keV in agreement with the initial BAT measurement. The inferred metallicity is somewhat subsolar (Z = 0.7).

The Swift Ultraviolet/Optical Telescope (UVOT) observations of the flare peak and early decay phases of this optically bright source were saturated as noted in the GCN referred to above.

The peak XRT flux of 4.1 X 10^-09 corresponds to an X-ray luminosity of 4.6 X 10^33 erg/s at the 97 pc distance of this system (van Leeuwen 2007, A&A, 474, 653) which is ~10% the combined systemic bolometric luminosity of 4.3 X 10^34 erg/s. Flares with X-ray luminosities > 1.0 X 10^33 erg/s have been detected from only a few active binaries previously, e.g., the flare of UX Ari on 2014-07-15 reported by Krimm et al, ATel #6319. The flare of SZ Psc is the most luminous flare in X-rays ever seen from any active late-type star to our knowledge. It is the third such large flare seen from this binary system, with 2 somewhat weaker ones previously detected by the MAXI/GSC (see Negoro et al., ATel #3737).

Swift/BAT transient monitor light curve for SZ Psc