VERY HIGH ENERGY OBSERVATIONS OF THE BINARIES V 404 CYG AND 4U 0115+634 DURING GIANT X-RAY OUTBURSTS

THE VERITAS COLLABORATION; A.C. ROVERO

ASTROPHYSICAL JOURNAL

IOP PUBLISHING LTD

Lugar: Londres; Año: 2016 vol. 831 p. 113 - 113

0004-637X

Transient X-ray binaries produce major outbursts in which the X-ray flux can increase over the quiescent level by factors as large as 10 7 . The low-mass X-ray binary V 404 Cyg and the high-mass system 4U 0115+634underwent such major outbursts in 2015 June and October, respectively. We present here observations at energies above hundreds of GeV with the VERITAS observatory taken during some of the brightest X-ray activity ever observed from these systems. No gamma-ray emission has been detected by VERITAS in 2.5 hr of observations of the microquasar V 404 Cyg from 2015, June 20?21. The upper flux limits derived from these observations on the gamma-ray flux above 200 GeV of F < 4.4  x 10^-12 cm^−2 s^−1 correspond to a tiny fraction (about 10^−6 ) of theEddington luminosity of the system, in stark contrast to that seen in the X-ray band. No gamma-rays have been detected during observations of 4U 0115+634 in the period of major X-ray activity in 2015 October. The flux upper limit derived from our observations is F < 2.1x10^-12 cm^−2 s^−1 for gamma-rays above 300 GeV, setting an upper limit on the ratio of gamma-ray to X-ray luminosity of less than 4%.