IGR J17091-3624 returns back to quiescence : Optical monitoring with LCO
ATel #17166; Payaswini Saikia, David M. Russell, Sandeep Rout, Kevin Alabarta, D. M. Bramich, Katerina Fialova (NYU Abu Dhabi), M. Cristina Baglio (INAF-OAB), and Fraser Lewis (Faulkes Telescope Project and The Schools Observatory, LJMU)
on 30 Apr 2025; 20:13 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)
Subjects: Optical, Black Hole, Transient
IGR J17091-3624 is a black hole X-ray binary that has exhibited multiple outbursts, previously observed in 1994, 2001, 2003, 2007, 2011, 2016, and 2022. The source has recently entered a new outburst phase since 2025 February, first detected with INTEGRAL (ATel #17034), and subsequently confirmed with NICER (ATel #17038) and Las Cumbres Observatory (LCO, ATel #17065). Coordinated IXPE and VLT observations during hard state reported a highly significant (>5 sigma) X-ray and optical polarization degree from the source (ATel #17093, Ewing et al. 2025).
We present results from our optical monitoring campaign of IGR J17091 during its 2025 outburst and its subsequent decline back to quiescence. Observations have been carried out for IGR J17091 since 2011 using the LCO 2-m and 1-m robotic telescopes (see ATel #15284, ATel #17065). During the 2025 outburst, the source was first detected in the optical bands on MJD 60719 (2025 Feb 13, ATel #17065). At that time, we measured magnitudes of V=20.75+/-0.15, r'=19.44+/-0.03, i'=18.82+/-0.07, z_s=18.34+/-0.07, confirming the X-ray rise (ATel #17034, ATel #17038).
Following this, the source exhibited a gradual brightening across all observed optical bands, displaying a double-peaked profile. The first peak occurred on MJD 60740 (2025 March 8), reaching maximum brightness of this outburst, with measured magnitudes of V=20.36+/-0.05, r'=19.32+/-0.018, i'=18.24+/-0.02, and z_s=17.60+/-0.02. This was followed by a slight decline, after which the source brightened again, reaching a secondary peak around MJD 60766 (April 1). Subsequently, the system entered a steady decline phase across all optical bands. Our most recent observations, obtained on MJD 60793 (April 28), yielded magnitudes of r'=20.69+/-0.12, i'=19.38+/-0.06, and z_s=18.83+/-0.07. Based on the r'-band light curve, we estimate that the source likely returned to quiescence between MJD 60785 (April 20, r'=20.18+/-0.08) and MJD 60789 (April 24, r'=20.73+/-0.11). These recent values are consistent with a return to quiescence. For comparison, prior to the onset of the 2025 outburst, the most recent quiescent measurements, taken on MJD 60610 (2024 October 27), were r'=20.76+/-0.11, i'=19.50+/-0.07, and z_s=18.61+/-0.04. It should be noted, however, that all quiescent optical magnitudes may be subject to contamination from a nearby source located approximately 1 arcsecond away from the target.
We will continue monitoring the source 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).
Optical lightcurve : IGR J17091-3624 : 2025 Outburst
