TITLE: GCN CIRCULAR NUMBER: 16597 SUBJECT: Swift observations of a flare from UX Ari DATE: 14/07/16 20:44:36 GMT FROM: Hans Krimm at NASA-GSFC H. A. Krimm (CRESST/GSFC/USRA), S. A. Drake (CRESST/GSFC/USRA), L. M. Z. Hagen (PSU), P. Kuin (UCL/MSSL), R. Osten (STScI), C. Pagani (U Leicester), D. M. Palmer (LANL) The Swift team reports on its observations of a flare from the RS CVn star UX Ari. This source triggered the Swift/BAT onboard (Baumgartner, et al., GCN Circ. 16594) and observations were carried out by all Swift instruments. The trigger time was 21:14:11 UT on 15 July 2014. For the BAT, using the event data set from T-239 to T+963 sec from the recent telemetry downlink, the time-averaged spectrum from T+0 to T+320 sec (with some short data gaps) is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.94 +- 0.38. The fluence in the 15-150 keV band is 7.2 +- 1.4 x 10^-7 erg/cm2. All the quoted errors are at the 90% confidence level. The BAT mask weighted light curve shows a flat peak from T-40, when the source came into the BAT field of view, until T+963, at the end of the event data. The source was already in outburst when the observations started. Data from the Swift/BAT transient monitor show activity from the source beginning at 15 July 2014, 19:38:37 UT (96 minutes before the trigger) and continuing until 16 July 2014, 00:48:37 UT. There was no detection in an observation starting at starting at 01:58 UT on July 16. The peak rate in the monitor (15-50 keV) is 0.03 ± 0.004 ct/s/cm^2 (~135 mCrab). The XRT observations consist of a total of 6.0 ks of Windowed Timing mode data. The XRT light curve can be modeled with a broken power-law, with an initial shallow decay with index alpha1 = 0.06 (+/-0.02) followed by a steeper decay after a break at T0+4.3 ks with an index alpha1 = 0.92 (+/-0.01) The XRT spectrum can be fitted with an absorbed two APEC model, with temperatures kT1 = 1.66 (+0.04, -0.05) keV and kT2 = 7.84 (+/-0.20) keV and a best-fitting absorption column of 3.8 (+/-0.1) x 10^20 cm^-2. The observed (unabsorbed) 0.3-10 keV flux is 4.37 x 10^-9 (4.51 x 10^-9) erg cm^-2 s^-1. The temperatures are consistent with what is expected for a large flare from this type of source. The column density to UX Ari (based on EUVE and other soft X-ray observations) is typically a few times 10^18 cm^-2, so the column derived from the flare spectrum, if real, would imply a major associated coronal mass ejection. UX Ari is too bright for UVOT aperture photometry, and during the flare initially exceeded the brightness limits for using UVOT readout streak photometry (Page et al, 2013, MNRAS Vol. 436, p. 1684.). The lower limits to the flare brightness exceeded uvw2 < 8.80, uvm2 < 8.27, uvw1 < 8.86. On 2014-07-15 at 22:50 UT the uvm2 magnitude had faded within the observable range, with uvm2=8.46+/-0.13, while on 2014-07-16 at 1:43UT the magnitude was uvm2=9.64+/-0.24. We note that the recent flare was also detected by MAXI (Kawagoe et al., ATel #6315). This source has been detected several times earlier in the BAT as reported by Krimm et al. (ATel #5907), the most recent being in mid-February 2014. The only other on board trigger was on 7 January 2012, reaching a peak of 0.013 ± 0.002 ct/s/cm^2 (~60 mCrab). We note that this Notice has also been posted as ATel #6319.