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INTEGRAL hard X-ray observation of the nova GK Per during its 2015 outburst

ATel #7244; M, Tuerler, C. Ferrigno, D. Eckert (ISDC, Uni. of Geneva, CH), K. Watanabe (FGCU, USA), E. Kuulkers (ESA/ESAC, Spain)
on 18 Mar 2015; 15:00 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

Subjects: X-ray, Gamma Ray, Binary, Nova, Transient, Variables

Referred to by ATel #: 7404

INTEGRAL observed serendipitously the 2015 outburst of the nova GK Per (ATel #7217) in hard X-rays. The transient source was in its field of view during an observation of NGC 1275 in the Perseus cluster covering the complete satellite revolution 1518, from 2015-03-14 at 02:38 UT to 2015-03-16 06:59 UT. The intermediate polar cataclysmic variable GK Per (Nova Per 1901) is detected for the first time by INTEGRAL with high significance.

The average fluxes for GK Per in the usual INTEGRAL bands are:
JEM-X 3-10 keV: 19±1 mCrab,
JEM-X 10-20 keV: 47±3 mCrab,
IBIS 20-40 keV: 37.8±0.5 mCrab,
IBIS 40-80 keV: 20.3±0.6 mCrab;
only statistical uncertainties at 1σ confidence level are reported.

A spectral analysis yields a smoothly curved combined JEM-X and ISGRI spectrum in the energy range from 3 to 80 keV. A good fit of the data is obtained with a cut-off powerlaw model, which has a photon index of Γ=0.65±0.06, a high-energy cutoff at 15.5±0.8 keV, and a normalization at 1 keV of 1.94±0.21 x 10-2 photons/cm2/s/keV. This yields a reduced χ2=0.98 for 18 d.o.f., using 3% of systematic uncertainties. The model fluxes in the 3-20 keV and the 20-60 keV energy bands are of 5.8×10-10 ergs/cm2/s and of 4.1×10-10 ergs/cm2/s, respectively.

The time coincidence of this nova with the reported gamma-ray outburst from the Perseus region by AGILE (ATel #7239) suggests a possible link. The reported position of the AGILE source is between NGC 1275 (0.7° away) and GK Per (2.5° away). A contamination of the persistent emission from NGC 1275 by a predominantly low-energy (~100-300 MeV) emission from GK Per - affected by lower angular resolution - might have been possible. Indeed, classical novae have been recently found by Fermi to be gamma-ray emitters (Fermi coll., 2014, Science 345, 554).