# Unprecedentedly bright X-ray flaring in Cygnus X-1 observed by INTEGRAL

## TL;DR

This paper reports the first observation of extremely bright, short-duration X-ray flares in Cygnus X-1, revealing unprecedented energetic events likely caused by jet activity or interactions with stellar wind.

## Contribution

It presents the first detection of such intense flaring in Cygnus X-1 despite extensive prior monitoring, highlighting a new extreme behavior in this well-studied black hole binary.

## Key findings

- Flares reached peak luminosity of 1.1-2.6×10^38 erg/s within seconds.
- Flares observed across all INTEGRAL instruments with no significant spectral change.
- No corresponding radio flux increase detected during flares.

## Abstract

We study three extraordinarily bright X-ray flares originating from Cyg X-1 seen on 2023 July 10 detected with INTEGRAL. The flares had a duration on the order of only ten minutes each, and within seconds reached a 1-100 keV peak luminosity of $1.1-2.6\times10^{38}$ erg/s. The associated INTEGRAL/IBIS count rate was about ${\sim}$10x higher than usual for the hard state. To our knowledge, this is the first time that such strong flaring has been seen in Cyg X-1, despite the more than 21 years of INTEGRAL monitoring, with almost ${\sim}$20 Ms of exposure, and the similarly deep monitoring with RXTE/PCA that lasted from 1997 to 2012. The flares were seen in all three X-ray and $\gamma$-ray instruments of INTEGRAL. Radio monitoring by the AMI Large Array with observations 6 h before and 40 h after the X-ray flares did not detect a corresponding increase in radio flux. The shape of the X-ray spectrum shows only marginal change during the flares, i.e., photon index and cut-off energy are largely preserved. The overall flaring behavior points toward a sudden and brief release of energy, either due to the ejection of material in an unstable jet or due to the interaction of the jet with the ambient clumpy stellar wind.