Optical confirmation of a faint brightening from the black hole candidate X-ray binary IGR J17091-3624 with XB-NEWS

ATel #17065; Katerina Fialova, Payaswini Saikia, David M. Russell, Kevin Alabarta, D. M. Bramich, Sandeep Rout (NYU Abu Dhabi), M. Cristina Baglio (INAF-OAB), and Fraser Lewis (Faulkes Telescope Project & The Schools’ Observatory, LJMU)
on 6 Mar 2025; 08:04 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)

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IGR J17091-3624 is a transient Galactic black hole candidate X-ray binary originally detected by INTEGRAL in April 2003 (ATel #149). The source has exhibited multiple outbursts - in 1994, 2001, 2003, 2007, 2011, 2016 and 2022. Recently, the source has entered a new outburst phase, initially detected by INTEGRAL in February 2025, around MJD 60713 (ATel #17034) and subsequently confirmed by NICER observations (ATel #17038). Here, we report recent optical observations of the ongoing outburst with the Las Cumbres Observatory (LCO) network of telescopes.

We have been observing the source since 2008 using the LCO 2-m and 1-m robotic telescopes (ATel #15284), as part of a campaign monitoring ~50 low-mass X-ray binaries (Lewis et al. 2008). All LCO photometric analysis and data calibration were performed using the "X-ray Binary New Early Warning System" pipeline (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020 for details).

The last optical detections of IGR J17091 taken before this new outburst began, were from October 27, 2024 (MJD 60610), before a gap due to Sun constraint. At that time, the source was in quiescence with magnitudes, r'=20.76+/-0.11, i'=19.50+/-0.07, and z_s=18.61+/-0.04. The last V-band detection was on September 30, 2022 (MJD 59852), with magnitude 20.55+/-0.09. However, note that the reported magnitude in quiescence could be contaminated by a nearby source located only 1 arcsec away.

We resumed monitoring after the Sun constraint. The first observation (in r'-band) was on February 10, 2025 (MJD 60716), before the new outburst was announced. Following observations restarted on February 13 (MJD 60719) for i' and z_s, and on February 18 (MJD 60724) for V. The first optical magnitudes we obtained during this period were much brighter than in quiescence, with V=20.75+/-0.15, r'=19.44+/-0.03, i'=18.82+/-0.07, z_s=18.34+/-0.07. The newest optical magnitudes from March 5 (MJD 60739) are as follows: V = 21.15+/-0.10, r'=19.96+/-0.04, i'=18.75+/-0.02, and z_s=18.09+/-0.02. The newer magnitudes show that the source is neither rising, nor decaying currently, but just fluctuating while maintaining a roughly flat behavior.

From our long-term optical monitoring, we see that previous outbursts of the source also exhibit a generally flat profile with some fluctuations. In 2011, although the initial rise phase was not fully observed (leaving open the possibility that it may have briefly reached brighter magnitudes), we recorded a peak in the r' band at 19.08+/-0.02 (MJD 55677). The 2016 outburst reached r'=19.34+/-0.06 and i'=18.35+/-0.04 (MJD 57514). More recently, the 2022 outburst achieved notably brighter levels, with i'=17.22+/-0.02 (MJD 59740) and r'=18.15+/-0.04 (MJD 59743). The current outburst is 0.26 mag fainter in the r' band and 0.19 mag fainter in i' band compared to the outburst in 2016 and by 1.45 mag in r' band and 1.32 mag in i' band compared to the outburst in 2022. If we have already reached the peak in this current outburst, it is a faint outburst.

We also acquired higher cadence (minute-timescale, 11 observations over an ~50-minute span) observations of IGR J17091 in the i'-band on February 24 (MJD 60730). The optical magnitude of the source fluctuated from about 18.78 to 18.60, a range of ~0.18 mag. We calculated the optical fractional RMS deviation in the flux to be 3.96+/-0.85 (following the methods described by Vaughan et al. 2003), suggesting significant optical variability in minute-timescale.

We will continue monitoring the source at optical wavelengths with LCO. Further multi-wavelength observations are encouraged to study the variability in the source.

Optical light curve of IGR J17091