Potential optical outburst precursor in the black hole system GRO J0422+32

ATel #17441; Kevin Alabarta (INAF-OAB), Yi-Jung Yang, David M. Russell, Sandeep K. Rout, Mokhine Motsoaledi, Dimitrios Kantzas, D. M. Bramich, Dalya Akl (NYU Abu Dhabi), Katerina Fialova (Peking University), M. Cristina Baglio (INAF-OAB), Payaswini Saikia (Yale University) and Fraser Lewis (Faulkes Telescope Project & The Schools' Observatory, LJMU)
on 11 Oct 2025; 09:58 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

Tweet

GRO J0422+32 is a black hole low-mass X-ray binary discovered on August 5, 1992 (IAUC # 5580 ). The source exhibited a prolonged optical decline following the outburst and two mini-outbursts (Callanan et al. 1995). Since 1994, after the end of the last mini-outburst, the system has most likely remained in quiescence. In September 2020, the source started to show a slow rise in the optical brightness in several bands (ATel #16831). This optical rise, unlike other systems like V404 Cyg (ATel #7761) or Cen X-4 (a Baglio et al. 2022), did not lead to a new outburst or activity of the source (ATel #16831).

Here, we report on the detection of recent low-level optical activity from GRO J0422+32 with the Las Cumbres Observatory (LCO) network of telescopes. The system kept rising after the activity reported in ATel #16831, reaching peak magnitudes of g'~22.1, r~21.0, i'~20.2, z_s~20.0 in October 2024. After that, the source faded again. From July 29, 2025 (MJD 60885.60), when the system was at magnitudes g'=23.08+/-0.18, r'=22.02+/-0.08, i'=21.03+/-0.05 and z_s=20.38+/-0.05, the system started to brighten at 0.02 mag/day in the g' and r' bands, and 0.01 mag/day in the i' band. In addition, after September 29, 2025 (MJD 60947.43), the source also showed some flickering around average magnitudes of g'=21.73+/-0.21, r'=20.72+/-0.17, i'=20.10+/-0.19 and z_s =19.66+/-0.18, where the errors represent the standard deviation around the mean. The latest magnitudes were obtained with LCO on October 05 (MJD 60953.22; g'= 21.94+/-0.14) and October 06, 2025 (MJD 60954.06; r'=20.79+/-0.22, i'=19.85+/-0.12).

To check for possible X-ray emission, we also observed GRO J0422+32 with Swift/XRT on October 02 (MJD 60950), October 04 (MJD 60952), and October 05 (MJD 60953), obtaining four PC-mode snapshots. The total exposure time was 4.86 ks. We processed the data with xrtpipeline and measured the count rate at the optical position using XIMAGE/sosta on the 0.3-10 keV sky and exposure maps (source half-box 18 pixels, background annulus 40-80 pixels, exposure-weighted). No source is detected in any snapshot. The resulting 3-sigma count-rate upper limits are 1.78e-2, 2.31e-2, 9.93e-3, and 4.78e-3 counts/s on MJD 60950.83, 60950.90, 60952.50 and 60953.34, respectively.

Swift/UVOT observations were performed with exposures of 70-570 s per filter. The source was not detected in any filter, with all measurements below the 3-sigma significance threshold. The derived 3-sigma upper limits (Vega / AB) are V > 20.08 / 20.07, UVW1 > 18.69 / 20.20, UVM2 > 20.18 / 21.87, and UVW2 > 20.26 / 21.99 mag across the different epochs (V, UVM2, UVW2 on MJD 60953.35; UVW1 on MJD 60950.91). These consistent non-detections indicate that the source was fainter than the UVOT sensitivity limits during these epochs.

The slow rise detected in optical frequencies could indicate an upcoming outburst/activity of the source, similar to e.g. V404 Cyg (ATel #7761), Cen X-4 (ATel #14254, #14302) and GX 339-4 (ATel #16096, #16260), although GRO J0433+32 did reach similar magnitudes in October 2024. We encourage multi-wavelength observations over the coming months to monitor the system's evolution.

The LCO observations of GRO J0422+32 are performed as part of an ongoing monitoring program of ~50 low-mass X-ray binaries (Lewis et al. 2008). All the obtained images are analysed with the real-time data analysis pipeline "X-ray Binary New Early Warning System" developed in-house (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020, Alabarta et al., in prep, and ATel #13451 for details).

LCO optical light curves of GRO J0422+32