Swift, Chandra and NuSTAR observations of GK Per during the 2015 outburst
ATel #7404; Polina Zemko (University of Padua, Italy), Marina Orio (INAF Padova, Italy and University of Wisconsin, Madison, USA) and Philip A. Evans (University of Leicester, UK)
on 16 Apr 2015; 16:39 UT
Credential Certification: Polina Zemko (polina.zemko@studenti.unipd.it)
Subjects: X-ray, Binary, Cataclysmic Variable, Nova, Variables
We report the preliminary results of multi-mission observations of the old nova GK Per during its current dwarf nova outburst (see also ATEL #7217, #7244, #7248, #7276).
Swift observed GK Per twice a day for two weeks on March 12-25 2015 and once a day from March 26 until April 9 2015, when the object reached its maximum optical luminosity. The Swift-XRT count rate varied in the range 0.8-2.1 cnts/s in different observations. All the X-ray spectra in the 0.3-10 keV range showed two completely distinct components: a very soft blackbody-like one, and a hard component that is well fitted with a high temperature thermal plasma, absorbed by two partially covering absorbers. While the total X-ray flux was approximately constant during the observations, about 1-2e-10 ergs/s/cm^2, the very soft component became more luminous with time, reaching 0.6 cnts/s below 1 keV. The Swift-XRT data reveal a clear modulation with a period of 351.354(8) s (consistent with the white dwarf spin period) and an amplitude of 2 cnts/s in the hard X-rays. This modulation is not present below 1.5 keV. The pulse profile has a possible secondary maximum at phase 0.5.
Coordinated NuSTAR and Chandra ACIS-S/HETG grating observations were done on April 4 2015, close to optical maximum. The NuSTAR average count rate during the 42,348 s exposure was 3.634 +/- 0.009 cnts/s in the 3-79 keV range and the time integrated spectrum is best fitted with a thermal plasma at about 13 keV, absorbed by two partially covering absorbers, and a Gaussian representing the 6.4 keV iron reflection line. Analysis of the time resolved NuSTAR spectra is still underway. The unabsorbed X-ray flux in the same energy range was 7.3e-10 erg/s/cm^2. The NuSTAR light curve shows large amplitude oscillations between 3 and 17 cnts/s with the 351.35 s period. The HETG grating spectrum exposure, lasting for 34,950 s, revealed copious continuum flux in the hard portion of the spectrum, but in addition to the 6.4 keV line, emission lines due to high ionization stages of iron and silicon, hydrogen-like and helium-like emission lines of magnesium, neon and oxygen were detected. The time-averaged count
rate was 0.100+/-0.001 cnts/s in the MEG summed first orders and 0.122+/-0.001 cnts/s in the HEG summed first orders, about half than in Chandra ACIS-S+HETG observations done near maximum in the 2002 outburst. The observed spectrum was harder than in 2002.