Radio flaring from MAXI J1810-222 during possible X-ray state transition

ATel #16129; Katie Savard, Francesco Carotenuto (Oxford), Thomas Russell (INAF-IASF Palermo), Sara Motta (INAF-Brera), Rob Fender (Oxford), Patrick Woudt (U. Cape Town), James Miller-Jones (Curtin), on behalf of the ThunderKAT collaboration
on 12 Jul 2023; 10:09 UT
Credential Certification: Francesco Carotenuto (francesco.carotenuto@cea.fr)

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The candidate black hole X-ray binary MAXI J1810-222 has been in outburst since November 2018 (ATel #12254), transitioning several times between hard and soft X-ray states during this period (Russell et al. 2022). This source was last reported in the hard-state (ATel #16026). We report here on renewed flaring activity of the source, and evidence for a possible spectral transition to the soft state. This behavior (radio flaring and X-ray spectral softening) is commonly associated with discrete ejection events from black hole X-ray binaries.

We observed MAXI J1810-222 with the MeerKAT radio telescope as part of the ThunderKAT Large Survey Programme (Fender et al. 2017), with weekly monitoring beginning from the 30th April 2023 (ATel #16025). Observations were carried out at a central frequency of 1.28 GHz, with a total bandwidth of 860 MHz. We used PKS J1939-6342 for flux and bandpass calibration, and J1833-2103 for complex gain calibration.

MAXI J1810-222 was observed 8 times between 2023-04-30 and 2023-06-17, with 15 minutes of on-source time for each observation. The source was detected at flux densities of between ~300-100 μJy at 1.28GHz. However, our most recent observations on 2023-06-24 and 2023-06-30 showed a significant increase in radio luminosity. On 2023-06-24 MAXI J1810-222 was observed for 15 minutes between 23:30:31 and 23:45:27 UTC (MJD 60119.985 ± 0.005). Fitting the radio emission coincident with the known source position to a point source in the image plane, we measure a flux density of 1.62 ± 0.04 mJy. Roughly one week later, on 2023-06-30, MAXI J1810-22 was again observed for 15 minutes between 01:10:18 and 01:25:25 UTC (MJD 60125.054 ± 0.005), exhibiting a flux density of 2.01 ± 0.03 mJy. These are the most luminous radio detections of this source to-date.

Shortly after this flare in radio, Swift/XRT observed MAXI J1810-222 in WT mode between 2023-07-09 10:55:30 and 11:07:10 UT with a total exposure time of 700s. Fitting the 0.6-10keV X-ray spectrum with a simple absorbed power-law results in a photon index of 4.2 ± 0.5 and equivalent hydrogen column density of 1.4 ± 0.3 × 10²² cm², indicating a much softer spectrum with respect to previous observations (ATel #16026). Alone, an absorbed disk blackbody fits the data poorly and statistics were not significant enough to fit for both power-law and disk blackbody. The derived 1-10 keV unabsorbed flux is 4.7 (+1.7, -1.0) × 10^-10 erg cm^-2 s^-1.

Further radio and X-ray monitoring is planned. Multiwavelength follow-up is encouraged.

ThunderKAT targets X-ray binaries, Cataclysmic Variables, Supernovae and Gamma-Ray Bursts. As part of this programme, we perform weekly monitoring observations of all bright, active, southern hemisphere X-ray binaries in the radio band. For further information on this programme please contact Rob Fender and/or Patrick Woudt.

References:
Fender et al. 2017, arXiv:1711.04132
Russell et al. 2022, MNRAS, 513, 6196, arXiv:2205.05721