MAXI J0903-531: Swift localization and optical counterpart

ATel #14557; J. A. Kennea (PSU), A. Bahramian (ICRAR), P. A. Evans, A. P. Beardmore (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano (INAF-IASFPA), K. Yamaoka (Nagoya U.), M. Serino (RIKEN), H. Negoro (Nihon U.)
on 16 Apr 2021; 18:20 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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Starting at 12:02UT on April 16th, 2021, Swift began a target of opportunity observation of the error region of MAXI J0903-531 (ATEL #14555), consisting of 10 paintings to cover the MAXI error regions with 200s per pointing. In the Photon Counting mode Swift/XRT data, we find a previous uncatalogued X-ray source at the following location: RA/Dec(J2000) = 136.27866, -53.50518 which is equivalent to:

RA(J2000) = 09h 05m 06.9s
Dec(J2000) = -53d 30m 18.6s

with an estimated uncertainty of 2.8 arc-seconds radius (90% confidence). This lies 26 arc-minutes from the center of the MAXI error region, inside the error ellipse. This source is the only one detected that lies inside the MAXI error region, and has an average count rate over the 200s observation of approximately 1.34 +/- 0.12 XRT count/s. No significant variability is seen over the short observation. We note also that this position is consistent with a UVOT source detected in the u filter, with magnitude u=12.54 +/- 0.02 (Vega system).

A catalogue search in Vizier finds that this optical counterpart is known, and has an estimated distance from Gaia of around 10 kpc (e.g. Bailer-Jones et al., 2021), the GAIA counterpart of MAXI J0903-531 is Source 5311384333263075840 (GAIA EDR3).

The PC mode spectrum can be well fit by an absorbed power-law model, with photon index = 0.6 +/- 0.3 and N_H = 2.5 +/- 3.5 x 10^21 cm^-2. This hard spectrum is suggestive of the source being an accreting pulsar, such as a Be/XRBs and other HMXBs.

Further observations of this source have been requested.

This analysis is support by the Swift Guest Investigator Program.