Radio and X-ray observations of the AMXP MAXI J1816-195 in quiescence

ATel #15769; A. Marino (ICE-CSIC), T. D. Russell, M. Del Santo (INAF/IASF Palermo), F. Coti Zelati, N. Rea (ICE-CSIC), A. Manca, A. Sanna (Univ. Cagliari)
on 21 Nov 2022; 11:37 UT
Credential Certification: Alessio Marino (marino@ice.csic.es)

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MAXI J1816-195 is a recently discovered accreting millisecond X-ray pulsar (Bult et al., 2022) that underwent a 1-month outburst in June 2022 (ATEL #15418, #15421, #15425, #15468, #15484). Subsequently, reports of non-detection in near-IR (ATEL #15501) and of a rapid breakdown in X-rays (ATEL #15506) in early July suggested that the source had transitioned to quiescence.

On November 1st and 2nd, 2022 we observed MAXI J1816-195 with the Australia Telescope Compact Array (ATCA), for a total on-source time of approximately 2.5 hrs. Data were recorded at central frequencies of 5.5 and 9 GHz (with 2 GHz of bandwidth at each central frequency). We used PKS 1934-638 for bandpass and flux calibration and B1829-207 for phase calibration. Data were calibrated and imaged following standard procedures with the Common Astronomy Software Applications (CASA, version 5.1.2; The CASA team et al., 2022). MAXI J1816-195 was undetected at both frequencies, with 3-sigma upper limits on the brightness of 105 and 90 uJy/beam at 5.5 and 9 GHz, respectively. Stacking the two frequency bands together yields a 3-sigma upper-limit of 84 uJy/beam.

Between November 6th and 7th, 2022 we performed a follow-up X-ray observation with Swift/XRT in PC mode for a total exposure time of 4.6 ks. The source was also undetected in X-rays, with a 3-sigma upper limit to the net count-rate of 0.0036 cts/s (0.3-10 keV). This count-rate upper-limit converts to an upper-limit on the observed 0.3-10 keV flux of 3e-13 erg/cm^2/s, using an absorbed (NH=2.3e22 cm^-2, e.g. Chen et al., 2022) power-law with photon index Gamma=1.5 in WebPIMMS. This result confirms that the system is currently in quiescence.

In order to estimate the X-ray and radio luminosity of the source, we considered the upper limit to the distance of 6.3 kpc (Chen et al., 2022). The results reported here translate to an X-ray luminosity upper limit of 1.4e33 erg/s and a radio luminosity upper limit of 3.0e28 erg/s. The position of the source on the radio:X-ray luminosity plane seems consistent with the Neutron Stars Low Mass X-ray Binaries (NS LMXBs) population at low accretion regimes (see link below).

The ATCA is part of the ATNF, 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.

MAXI 1816 in the radio:X-ray diagram