[vsnet-alert 25728] MAXI J0903-531 (ATel)

[Taichi Kato]

   IDs:  HMXB

2MASS 090506.824 -533019.50 (2000.0) 12.447 12.339 12.272 
090506.817 -533019.49 (2000.0) Gaia_DR2_5311384333263075840 plx=-0.015(0.020) dismod=- pmra=-2.108(0.034) pmdec=2.252(0.035) G=13.155 BP=13.317 RP=12.808
090506.817 -533019.49 (2000.0) Gaia_EDR3_5311384333263075840 plx=0.053(0.012) dismod=16.4 pmra=-1.996(0.012) pmdec=2.273(0.014) G=13.145 BP=13.324 RP=12.798

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ATEL #14557							     ATEL #14557

Title:	MAXI J0903-531: Swift localization and optical counterpart
Author:	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.)
Queries:	jak51 at psu.edu
Posted:	16 Apr 2021; 18:20 UT
Subjects:Optical, X-ray, Transient

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.