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ATEL # 2466; P. A. Evans (U. Leicester), W. Renz (GEOS/AAVSO), H. A. Krimm (CRESST/GSFC/USRA), J. Bortle, A. Henden, E. van Ballegoij, R. Pickard (AAVSO)
on 8 Mar 2010; 18:30 UT
Password Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)
Subjects: Optical, X-ray, Cataclysmic Variables
Data from the AAVSO (www.aavso.org) show that GK Per maintained a quiescent visual magnitude of ~13.0-13.2 until 2010 Mar 7. It was observed at this level at 20:58 UT on 2010 Mar 6, although there have been some detections at ~12.8 from as early as 20:06 on 2010 Mar 5. Observations taken at 00:43 UT on 2010 Mar 7 revealed a visual magnitude of 12.6 and all subsequent observations confirm the brightening. The latest light curve of GK Per can be seen at http://bit.ly/9PpMpq
GK Per is regularly monitored by the Swift-BAT transient monitor. A rise in hard X-ray (15-50 keV) flux was seen just before the optical rise noted above. In a combined contiguous pair of Swift observations starting at 12:43:44 UT 2010-Mar-06, with total exposure 576 seconds, BAT recorded a count rate of 0.025 ± 0.004 ct/s/cm2, or approximately 110 mCrab. The rate dropped somewhat after this point and the average daily rate for 2010-Mar-07 is 0.006 ± 0.001 ct/s/cm2 (27 mCrab). The most recent BAT monitor data available is for an observation beginning 00:06:48 UT on 2010-Mar-08. GK Per is normally not detected in daily BAT monitor observations, with a 1-sigma upper limit of 9 mCrab.
Pointed Observations with the Swift satellite began at 19:57 UT on 2010 Mar 7. The mean X-ray count-rate was 1.8 count s-1, confirming the outburst (the quiescent Swift-XRT count-rate was 0.1 count s -1, see Evans et al. 2009, 399, 1167).
Swift observations are ongoing and multiwavelength follow up is recommended