Very-high-energy observations of the binaries V 404 Cyg and 4U 0115+634 during giant X-ray outbursts

TL;DR

This study used VERITAS to observe two X-ray binary systems during their major outbursts, but found no gamma-ray emission, setting stringent upper limits and indicating a disconnect between X-ray and gamma-ray activity.

Contribution

First-time high-energy observations of V 404 Cyg and 4U 0115+634 during giant outbursts, establishing upper limits on gamma-ray emission during intense X-ray activity.

Findings

No gamma-ray emission detected from V 404 Cyg

No gamma-ray emission detected from 4U 0115+634

Gamma-ray to X-ray luminosity ratio less than 4%

Abstract

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+634 underwent such major outbursts in June and October 2015, 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 hours 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 $< 4.4\times 10^{-12}$ cm$^{-2}$ s$^{-1}$ correspond to a tiny fraction (about $10^{-6}$) of the Eddington 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 October 2015. The flux upper limit derived from our observations is F $< 2.1\times 10^{-12}$ 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%.