[vsnet-grb-info 15305] Swift observations of a flare from UX Ari

[GCN Circulars]

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  <hans.a.krimm at nasa.gov>

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.