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

TL;DR

Three extraordinarily bright X-ray flares from Cygnus X-1 were detected by INTEGRAL on 2023 July 10, reaching peak 1–100 keV luminosities of $1.1-2.6\times10^{38}$ erg s$^{-1}$ and lasting about 400 s. The flares were observed by IBIS/ISGRI (with limited JEM-X data) and are shown to be intrinsic to Cyg X-1, not due to background sources, marking the brightest INTEGRAL events from the system in over two decades. Timing and spectral analyses reveal three distinct flare morphologies with minimal hard X-ray spectral evolution, suggesting a brief, energetic release rather than global accretion-state change; the inferred luminosities are accompanied by substantial rms variability. The authors discuss several physical scenarios, including jet ejection, jet restructuring, or wind-clump interactions in the donor wind, and compare to a past RXTE event, highlighting the crucial role of long-term, multi-instrument monitoring for constraining jet–disk–wind coupling in accreting black holes.

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 $γ$-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.

Figures (7)

• Figure 1: Variability of Cyg X-1 on 2023 July 10. (a) IBIS and JEM-X light curves of the flares during rev. 2661. Gray bands indicate the time ranges of the flares. (b) Variation of the hardness ratio, $\mathrm{HR}=(\mathrm{H}-\mathrm{S})/(\mathrm{H}+\mathrm{S})$, between the 30--60 keV and 60--120 keV IBIS light curves (c) Evolution of the total event rate in the SPI detector, which is dominated by Cyg X-1. (d) and (e) IBIS light curves for the two other sources in the field of view, Cyg X-3 and 3A 1954+319, which are unaffected by the flare of Cyg X-1. Lighter colors indicate the 30--60 keV rate, darker ones the 60--120 keV rate. (f) Count rates in those pixels of IBIS illuminated and non-illuminated by Cyg X-1. The changes in the base count rate around MJD 60135.27, MJD 60135.31, and MJD 60135.35 are due to repointing of INTEGRAL, resulting in a change in the number of (non-)illuminated pixels and of the vignetting of the source.
• Figure 2: Distribution of count rates in 60 s bins for all IBIS observations of Cyg X-1, separated by state as determined following Grinberg2013. The bin width of the rate histogram is $80\,\mathrm{cts}\,\mathrm{s}^{-1}$. The count rates reached by the flares discussed in this paper are indicated by the bracket. At no time before the flares have rates of $350\,\mathrm{cts}\,\mathrm{s}^{-1}$ or higher had been reached. The flares therefore represent by far the brightest observations ever seen with IBIS for Cyg X-1.
• Figure 3: Long-term IBIS and BAT light curve of Cyg X-1. The top panel shows the source behavior in the larger context, while the lower panels focus on days surrounding the flare. (a) Swift/BAT (gray) and IBIS count rates, colored by the photon index of the corresponding Science Window, determined with applying the bknpower model. The canonical state of Cyg X-1 derived from the MAXI light curves according to Grinberg2014 in shown in the color strip on the top edge of the panel. The time of the flaring episode is indicated by the teal, dotted, vertical line. (b) IBIS data shown as in (a), but focusing on a shorter time interval and including the MAXI rates, instead of Swift/BAT. (c) the radio flux density measured with AMI. (d) MAXI on-demand data (grey), again together with the IBIS data colored as above). The interval between start of the first and end of the third flare is shaded in light gray and the length of each pointing by the horizontal error bars. A slight increase in the MAXI flux is visible around the time of the flare.
• Figure 4: Hardness--intensity diagram of Cyg X-1 determined from MAXI data. The position of Cyg X-1 during the flare is indicated by the black star. Red, green, and blue dots indicate individual bins in the daily light curves of MAXI, classified as soft, intermediate or hard state Grinberg2013. The source behavior four days before and four days after the flaring episode is indicated by arrows pointing towards later observations and becoming lighter with increased separation from the flare.
• Figure 5: Top: JEM-X and IBIS light curves of the three flares in the 3--10 keV and 30--60 keV band with 10 s and 5 s time resolution, respectively. The $y$-axis for the IBIS data is shifted up for readability. Bottom: Hardness ratio light curve with a time resolution of 60 s. The hardness ratio is defined as $\mathrm{HR}=(\mathrm{H}-\mathrm{S})/(\mathrm{H}+\mathrm{S})$ for 30--60 keV and 60--120 keV. The gap starting around 1900 s is due to a repointing of INTEGRAL.