X-ray and radio observations of MAXI J1810-222 suggest the source is now in a hard intermediate state

ATel #16026; Melania Del Santo, Thomas Russell, Antonino D'Ai' (INAF/IASF Palermo)
on 3 May 2023; 17:35 UT
Credential Certification: Melania Del Santo (melania@ifc.inaf.it)

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Following the recent report of a hard X-ray spectral state (ATel #16016) and radio emission (ATel #16025) from MAXI J1810-222, we triggered coordinated X-ray and radio observations.

Swift/XRT observed MAXI J1810-222 in WT mode between 2023-05-02 19:33:00 and 2023-05-02 21:21:00 UT, with a total exposure time of 1393s. The 0.3-10 keV light curve shows a constant count rate of 4.3 cps. The X-ray spectrum showed a clear softening with respect to the latest XRT April observations (Atel #16016). A power-law fit yielded in fact a value of 2.36 +/- 0.8, while the best-fit is a combination of an absorbed disk blackbody plus a power law. We found an equivalent hydrogen column of (9.3 +/- 0.6)E21 cm^-2, a photon index of 1.74(-0.06, +0.16) and a blackbody temperature of 0.34 +/- 0.05 keV. The derived 0.3-10 keV unabsorbed flux is 3.9(-0.6,+1.0)E-10 erg cm^-2 s^-1.

MAXI J1810-222 was also observed by the Australia Telescope Compact Array (ATCA), with observations occurring between 2023-05-02 17:50:40 and 2023-05-02 21:51:40 UT (strictly simultaneous with the Swift/XRT observations). Data were recorded simultaneously at central frequencies of 5.5GHz and 9GHz, with 2GHz of bandwidth at each frequency. The array was in a compact H214 configuration. We used PKS B1934-638 for bandpass and flux calibration, and B1817-254 for phase calibration. Data flagging, calibration, and imaging were carried out following standard procedures with the Common Astronomy Software Applications for radio astronomy (CASA, version 5.1.2; The CASA Team et. al., 2022). During imaging, we used a Briggs robust parameter of 0 to balance sensitivity and resolution. Fitting for a point source in the image plane, we measure flux densities of 405 +/- 18 uJy at 5.5GHz and 440 +/- 15 uJy at 9GHz, corresponding to a radio spectral index of 0.18 +/- 0.12. These results show the presence of a compact radio jet.

MAXI J1810-222 shows a very peculiar Hardness-Intensity Diagram, going back and forth from the hard to the soft state several times, across intermediate states (see Del Santo et al. 2023; Russell et al. 2022). The radio behaviour combined with the shape of the X-ray spectrum observed on May 2nd are indicative of a hard-intermediate spectral state.

We thank the Swift team, and Jamie Stevens and ATCA staff for the scheduling of these observations. ATCA is part of the ATNF (https://ror.org/05qajvd42) which is funded by the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge the Gomeroi people as the Traditional Owners of the ATCA observatory site.

References:
The CASA Team et al. 2022, PASP, 134, 114501
Del Santo et al, 2023, MNRAS, doi:10.1093/mnrasl/slad048
Russell et al. 2022, MNRAS, 513, 6196