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Swift monitoring of GK Per during the 2015 outburst

ATel #7248; Polina Zemko (Padova University, Italy) and Marina Orio (INAF-Padova, Italy and University of Wisconsin, USA)
on 18 Mar 2015; 21:18 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Marina Orio (orio@astro.wisc.edu)

Subjects: X-ray, Binary, Cataclysmic Variable, Nova, Star, Transient, Variables

Referred to by ATel #: 7276, 7404

The old nova GK Per started a dwarf nova outburst on 2015 March 6.84 UT (VSNET-ALERT 18388, 18389) and according to the AAVSO database it is at V~11.5 (see also ATEL #7217). Pointed observations of GK Per with Swift were started on March 12 2015 01:12:59 GMT. The nova has been monitored twice a day since then, with 1000 s long exposures. The Swift X-ray Telescope (XRT) count rate in the range 0.3-10.0 keV varies in different exposures between 0.89 and 1.44 counts/s. The X-ray spectra clearly show several components: a distinct blackbody-like radiation (Tbb~50 eV) and highly absorbed thermal bremsstrahlung, which may peak at much higher energy than the XRT range limit of 10 keV. There is no clear correlation between the count rate variations below 1 keV (where the blackbody-like component dominates) and above this energy. A preliminary fit to the X-ray spectra indicates a flux of 1.4-1.7e-10 ergs/cm^2/s in the 0.3-10 keV range, which at a distance of 470 pc (McLaughlin 1960, p.585) corresponds to a luminosity of 4e33 erg/s. GK Per is also detected with Swift-BAT during the current outburst, see http://swift.gsfc.nasa.gov/results/transients/weak/GKPer