Exotic optical variability in the black hole X-ray binary IGR J17091-3624

TL;DR

IGR J17091-3624 is a black hole X-ray binary exhibiting exotic X-ray heartbeat variability, and this work presents the first long-term optical monitoring of the source alongside quasi-simultaneous X-ray and infrared data. The authors find a strong optical/X-ray correlation with a power-law index of $0.40 \pm 0.04$, indicating that optical emission is disk-dominated, likely from an X-ray irradiated or viscously heated outer disk. Extinction is constrained to $A_V \,\approx\ 4.3$–$6.6$ mag, translating to $N_H \approx (1.3$–$1.9) \times 10^{22}\ \,\rm cm^{-2}$, and a distance estimate of $8$–$17$ kpc, consistent with BHXB population trends. High-cadence optical data reveal minute-scale variability that may be reprocessed X-ray modulations, suggesting a coupling between optical and X-ray emission and motivating future higher-time-resolution, simultaneous multiwavelength campaigns.

Abstract

IGR J17091-3624 is a distinctive black hole X-ray binary exhibiting exotic variability, including complex "heartbeat" oscillations in its X-ray light curves, similar to those observed in GRS 1915+105, a system renowned for its structured, rapid X-ray variability but heavily obscured at optical wavelengths. In contrast, IGR J17091-3624 is less obscured, making it a more accessible target for optical investigations. Due to its weak radio emission, optical and infrared data are essential to probe the jet and outer disc behavior of IGR J17091-3624. This study presents the first long-term optical monitoring of IGR J17091-3624, using data from the Las Cumbres Observatory (LCO) over its 2011, 2016, and 2022 outbursts. We combine these observations with quasi-simultaneous X-ray data from Swift/XRT, RXTE, and NICER, employing light curve and variability analysis, spectral energy distributions, color-magnitude diagrams, and optical/X-ray correlations to investigate optical emission mechanisms. We find that the optical and X-ray fluxes are significantly correlated, following a power-law relation with the index 0.40\pm0.04, suggesting that the optical emission in IGR J17091-3624 is dominated by an X-ray-irradiated accretion disk. Based on optical spectral slope constraints, we estimate the extinction toward IGR J17091-3624 A_V = 4.3 to 6.6 mag. The global optical/X-ray correlation suggests a distance estimate of 8-17 kpc, in line with previous findings. High-cadence optical observations show tentative evidence of optical oscillations that may arise from reprocessed X-ray modulations, although confirming this will require higher time-resolution optical data.

Figures (13)

• Figure 1: Small cut-outs from LCO $r^{\prime}$-band images of the field of IGR J17091, acquired on different nights under good seeing conditions. Left: Image acquired at SSO with the 2-m Faulkes telescope on 2021 April 19 (MJD 59323). The seeing is 0.95$^{\prime\prime}$. IGR J17091 is in quiescence and marked with a yellow circle (0.5$^{\prime\prime}$ diameter). Right: Image acquired at CTIO with one of the 1-m telescopes on 2022 May 27 (MJD 59726). The seeing is 1.23$^{\prime\prime}$. IGR J17091 is in outburst and marked with a black circle (0.5$^{\prime\prime}$ diameter). Both: The nearest resolved source to IGR J17091 is marked with a cyan circle (BS2). The green circles indicate Gaia DR3 sources in the field.
• Figure 2: Long-term optical lightcurve of IGR J17091 covering the 2011, 2016 and 2022 outbursts in $i^{\prime}$ (black squares), $r^{\prime}$ (red circles) and V-band (blue diamonds) with the LCO telescopes. For zoomed-in versions of the light-curve during outbursts, please see Fig. \ref{['fig:mlc']}.
• Figure 3: Zoom in of the optical and X-ray lightcurves from the last three outbursts of IGR J17091 in 2011 (top panel), 2016 (middle panel) and 2022 (bottom panel), respectively. Note that the X-ray count rates for different instruments/telescopes are shown in different units (specified in the legends) with Swift/XRT and NICER count rates given in cts/s, Swift/BAT in cts/cm2/s and RXTE in crab units.
• Figure 4: (Left panel) De-reddened optical spectral energy distributions of IGR J17091 during the three outbursts, assuming $N_{\hbox{\scriptsize H}}=(1.537\pm0.002)\times 10^{22}\, \rm cm^{-2}$ or $A_{V} = 5.36\pm0.22$ mag wang2. (Right panel) De-reddened optical/IR SED of IGR J17091 including avavilable data in the NIR and upper limits in the mid-IR ranges, using the same $N_{\hbox{\scriptsize H}}$ value. The quiescent SED combines the $K_s$-band measurement from Magellan/PANIC Torres2011, deblended from the nearby sources BS1 and BS2, with the mean quiescent $i'$ and $r'$-band magnitudes from our LCO observations.
• Figure 5: Color-magnitude diagrams (CMD) of IGR J17091 covering the 2016 (magenta circles) and 2022 (blue squares) outbursts. Observed magnitudes are plotted, while the top axis spectral index is shown assuming an extinction value of $A_{V}$ = 5.36. The black solid line represents a blackbody model with different temperatures, which approximates the emission of an X-ray irradiated outer accretion disk.