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

¹ Dr. Karl Remeis-Observatory, Friedrich-Alexander-Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
² Université Paris Cité, Université Paris-Saclay, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
³ APC, Université Paris Cité, CNRS, CEA, Rue Alice Domont & Léonie Duquet, 75013 Paris, France
⁴ NASA Goddard Space Flight Center, Astrophysics Science Division, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
⁵ CRESST and Center for Space Sciences and Technology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁶ Astrophysics Group, Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0US, UK
⁷ Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁸ University of Geneva, Department of Astronomy, Chemin d’Ecogia 16, 1290 Versoix Switzerland
⁹ , Washington University, MSC 1105-109-02, One Brookings Drive, St. Louis, MO 63130-4899, USA
¹⁰ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹¹ Julius-Maximilians-Universität Würzburg, Fakultät für Physik und Astronomie, Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Emil-Fischer-Str 31, 97074 Würzburg, Germany
¹² Departament d’Astronomia i Astrofisica, Universitat de València, C/ Dr. Moliner, 50, 46100 Burjassot, València, Spain
¹³ Observatori Astronòmic, Universitat de València, C/ Catedràtic José Beltrán, 46980 Paterna, València, Spain
¹⁴ Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA
¹⁵ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Villafranca del Castillo, 28692 Madrid, Spain

^⋆ Corresponding author.

Received: 20 December 2024
Accepted: 25 August 2025

Abstract

We study three extraordinarily bright X-ray flares originating from Cyg X-1 seen on July 10, 2023, 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 × 10³⁸ erg s⁻¹. The associated INTEGRAL/IBIS count rate was approximately ten times 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 ∼20 Ms of exposure – and the similarly deep monitoring with RXTE/PCA from 1997 to 2012. The flares were seen in all three X-ray and γ-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.