Swift/XRT and ATCA observations reveal MAXI J1810-222 has returned to the hard X-ray spectral state

ATel #16154; Alessio Marino (ICE/CSIC), Thomas D. Russell (INAF-IASF Palermo), Katie Savard, Francesco Carotenuto (Oxford), Melania Del Santo (INAF-IASF Palermo)
on 26 Jul 2023; 12:56 UT
Credential Certification: Alessio Marino (marino@ice.csic.es)

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MAXI J1810-222 is a candidate black hole X-ray binary discovered going into outburst in November 2018 (ATel #12254), and it has been X-ray active ever since. The source has so far displayed a puzzling phenomenology, including rapid back-and-forth transitions between the hard and soft X-ray states (Russell et al., 2022) and possible ultra-fast outflows (Del Santo et al., 2023).

MAXI J1810-222 was reported to be in a hard-intermediate state in early May 2023 (Atel #16026). Between 2023-06-22 and 2023-06-30, bright radio flaring activity was observed with MeerKAT (Atel #16129) and ASKAP (Atel #16136), with recorded radio flux densities of up to 1.6-2.3 mJy (1.28 GHz). A follow-up Swift/XRT observation performed on 2023-07-09 confirmed that the source had entered into a soft state (Atel #16129). It is likely that the radio flaring activity was a result of transient jet ejecta being launched from the system around the hard-to-soft X-ray spectral transition, often observed in X-ray binaries.

Swift/XRT observed MAXI J1810-222 again on 2023-07-23 for 808 s. According to a preliminary spectral analysis, the 0.5-10 keV spectrum is well described with an absorbed (NH=(0.64+/-0.12)x10²² cm^-2) power-law with a photon index Γ of 1.90+/-0.15 . The obtained spectral index is strongly different from the one obtained in early July (i.e., where Γ was measured to be about 4.3) and signals a significant spectral hardening. The measured 0.3-10 keV (unabsorbed) flux of (1.6+/-0.1)x10^-10 erg cm^-2 s^-1 is compatible with the flux measured from the system during previous hard states (Russell et al., 2022).

MAXI J1810-222 was observed in the radio band with the Australia Telescope Compact Array (ATCA) on 2023-07-21, between 12:52:30 and 17:23:40 (UTC). The telescope was in an extended 6 km configuration. Data were recorded simultaneously at 5.5GHz and 9GHz, with 2GHz of bandwidth at each central frequency. PKS B1934-638 was used for flux and bandpass calibrations, and B1817-254 was used for gain calibration. Data were analysed and imaged following standard procedures within the Common Astronomy Software Applications package (CASA, version 5.1.2; CASA Team et al. 2022). We used a Briggs robust parameter of 0 for imaging, balancing sensitivity and resolution. MAXI J1810-222 was detected at both frequencies. Fitting for a point source in the image plane, we measured flux densities of 354 +/- 12 μJy at 5.5 GHz and 346 +/- 10 μJy at 9 GHz. This corresponds to a radio spectral index, α, of -0.05 +/- 0.10 (where S_ν ∝ ν^α), consistent with emission from a compact jet, which are associated with the hard X-ray spectral state.

We conclude that the system has performed another spectral transition within the last two weeks and is now again in a hard X-ray spectral state. Multi-wavelength follow-up observations are encouraged.

We thank Jamie Stevens and ATCA staff for scheduling these observations. We acknowledge the Gomeroi people as the Traditional Owners of the ATCA observatory site.

References
Russell, T. D. et al., 2022, MNRAS, 513, 6196
Del Santo, M. et al., 2023, MNRAS Letters, 523, Issue 1, pp.L15-L20
CASA Team, et al. 2022, PASP, 134, 114501