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MAXI J1810-222: Swift/XRT localization, possible optical counterpart with GAIA distance

ATel #12487; Jamie A. Kennea (Penn State) and Hitoshi Negoro (Nihon U.)
on 9 Feb 2019; 16:30 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Neutron Star, Transient, Pulsar

Referred to by ATel #: 12496, 12521, 12910, 13540, 16016

On February 9th, 2019 at 06:24UT, Swift began a 1ks Target of Opportunity observation of the transient source MAXI J1810-222 (ATEL #12254). Inside the XRT field of view we find a bright uncatalogued point source at the following location: RA/Dec(J2000) = 273.16526, -22.32362, which is equivalent to:

RA(J2000) = 18h 12m 39.66s,
Dec(J2000) = -22d 19m 25.0s,
with an estimated error radius of 1.8 arc-seconds (90% confidence), after UVOT enhancement. This position lies 2.4 arc-minutes from the reported NuSTAR position (ATEL #12283), and inside the MAXI error region.

Analysis of Swift UVOT data in the u, b and v bands, reveals an optical source close (2.6 arc-seconds) to the XRT position. Although this source is outside of the formal XRT error region, we propose that this may be the optical counterpart of MAXI J1810-222. Analysis of the optical brightness of this source is complicated by the proximity to the star HD 314107. A catalog search at the UVOT position reveals that this star is in the GAIA DR2 catalog with an estimated distance of approximately 730 +/- 30 pc (Bailer-Jones et al., 2018).

Spectral analysis of the PC mode data reveals a spectral fit consistent with the reported NuSTAR spectrum (ATEL #12398), with an absorbed black-body plus power-law model giving N_H = 1.4 +/- 0.6 x 10^22 cm^-2, kT = 0.42 +/- 0.03 keV, and a photon index of 4.2 +/- 2.4. The fitted flux is 4 x 10^-10 erg/s/cm^2 (0.5 - 10 keV), which assuming a distance of 730 pc gives a luminosity of 2.5 x 10^34 erg/s (0.5 - 10 keV).

Observations to confirm the link between MAXI J1810-222 and the possible optical counterpart are encouraged.