MAXI J1816-195: Swift/XRT detection of a dust scattered halo

ATel #15506; A. P. Beardmore (Leicester) and J. A. Kennea (PSU) report on behalf of the Swift/XRT Team
on 7 Jul 2022; 15:31 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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MAXI J1816-195 is an AMXP that was discovered in outburst by MAXI (ATEL #15418). It has been observed several times by the Swift X-ray telescope. In a recent observation taken on June 23rd, with data in both Photon Counting (PC) and Windowed Timing mode, PSF fitting shows that there is a significant extended region around the bright point source. This combined with the high absorption present in the system (e.g. ATEL #15467) is suggestive of the presence of a dust scattered halo.

A further Swift-XRT observation of MAXIJ1816-195 was taken on 2022-July-06, in which 5.2 ks of data were collected in PC mode. An initial analysis suggests the central source count rate has dropped by a factor of 10000 to 0.0033 +/- 0.0008 c/s, compared with the XRT observation of June 23rd, 2022 (ATEL #15467), confirming reports that the transient is returning to quiescent levels (ATEL #15501).

Closer examination of the latest PC data reveals an approximately 2 - 6 arcmin radius extended source centred on the location of the transient. A preliminary spectral analysis of the extended source emission suggests it can be modelled by an absorbed powerlaw, with photon index 4.3 +/- 0.4 and column density 1.3 +/- 0.2 x 10^22 cm^-2. The highly absorbed nature of the spectrum and steep photon index compared to the results reported in ATEL #15467 for the central source are consistent with emission from a dust scattered halo.

Further Swift observations are planned to monitor the evolution of halo.