Gradual optical brightening in GRO J0422+32 during quiescence: precursor to an outburst?

ATel #16831; Katerina Fialova, David M. Russell, Kevin Alabarta, D. M. Bramich, Sandeep Rout and Payaswini Saikia (NYU Abu Dhabi), M. Cristina Baglio (INAF-OAB), and Fraser Lewis (Faulkes Telescope Project & The Schools’ Observatory, LJMU)
on 27 Sep 2024; 06:32 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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The black hole candidate low-mass X-ray binary, GRO J0422+32 (V518 Per; Nova Persei 1992) was first discovered on August 5, 1992, during an outburst detected by BATSE aboard the Compton Gamma-Ray Observatory (IAUC # 5580 ). The source exhibited a prolonged optical decline following the outburst, with at least two distinct mini-outbursts observed approximately 120 days apart (Callanan et al. 1995). Since early 1994 when the mini-outbursts ended, there have been no reports of further outbursts, so the system has likely remained in quiescence since then. Here, we report recent low-level optical activity, and long-term optical monitoring, of GRO J0422+32 with the Las Cumbres Observatory (LCO) network of telescopes.

Our observations, which have taken place on an approximately weekly basis since February 2nd, 2006 (MJD 53768) are part of a broader program monitoring approximately 50 low-mass X-ray binaries (Lewis et al. 2008). The majority of data were obtained using the 2m Faulkes Telescope North (FTN), located at Haleakala Observatory in Maui, Hawai'i, with exposures primarily in the SDSS i'-band filter. Since September 2020, GRO J0422+32 has been observed simultaneously in SDSS g', r', i', and z_s filters with the 4-color MuSCAT3 camera, with detections in all bands. Our regular monitoring has shown that, except for a few outliers, the i'-band magnitudes have remained between ~20.0 and ~21.5 from 2006 to 2024 (see light curves linked below), confirming that the source has likely stayed in quiescence for the past 20 years since we started monitoring the source.

Similar to the passive and active states observed in A0620-00 (Cantrell et al. 2010), GRO J0422+32 shows notable long-term variability. A slightly fainter, passive state was recorded from September 2006 to November 2010 (MJD ~ 54000-55500), with i’-band magnitudes around i’ = 20.6-21.0. This phase was followed by a more active state from March 2012 to September 2017 (MJD ~ 56000-58000), during which the source brightened slightly to mag i' = 20.0-20.6. Around January 2019 (MJD ~ 58500), the source began to fade again, with a gradual decline noted over several months.

Recently, GRO J0422+32 has exhibited a slow rise in brightness since September 2020 (MJD ~ 59140), with rates of approximately 0.296 mag/year in the g' band, 0.215 mag/year in the r' band, 0.124 mag/year in the i' band, and 0.116 mag/year in the z_s band. This gradual rise suggests increasing activity, and the rise is faster at shorter wavelengths, which could be due to the accretion disc temperature rising. There has also been recent flaring in addition to the slow brightening, with peak i'-band magnitudes reaching ~19.75. The most recent observations, from September 9th, 2024 (MJD 60562), show magnitudes around i' ~ 20.7, indicating ongoing variability. The rise rate, up to 0.3 mag/year in g' band, appears to be the steepest measured in black hole XRBs in quiescence as far as we know, with rates of 0.02 to 0.17 mag/year measured in some other systems (Russell et al. 2018).

The slow rise could be an indication of an upcoming outburst of the source, similar to e.g. V404 Cyg (ATel #7761), Swift J1357.2-0933 (Russell et al. 2018), Cen X-4 (ATel #14254, #14302) and GX 339-4 (ATel #16096, #16260). We will continue to observe the system with LCO.

LCO images are processed and reduced, and magnitudes are extracted and calibrated using a real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details). This material is based upon work supported by Tamkeen under the NYU Abu Dhabi Research Institute grant CASS (Center for Astrophysics and Space Science).

GRO J0422+32 optical LCO light curve