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MAXI J1409-619: Swift localization and identification as a new X-ray transient

ATel #2962; J. A. Kennea (PSU), H. Krimm (CRESST/GSFC/USRA), P. Romano, V. Mangano (INAF-IASFPA), P. Curran (CEA-Saclay) and P. Evans (U Leicester)
on 20 Oct 2010; 22:31 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transient

Referred to by ATel #: 2965, 2969, 3060, 3067

At 15:14 UT on Oct 20th, 2010 Swift began a 1ks target of opportunity observation of MAXI J1409-619 (Yamaoka et al, 2010, ATEL #2959). Utilizing photon counting mode data we find a uncatalogued bright X-ray source inside the MAXI error circle at the following UVOT enhanced location, RA, Dec(J2000) = 212.01068, -61.98340, which is equivalent to :

RA(J2000) = 14h 08m 02.56s,
Dec(J2000) = -61d 59m 00.3s,

with an estimated uncertainty of 1.9 arcseconds radius (90% confidence). This source lies 7.3 arcseconds from the MAXI localization, inside the 0.2 degree error circle. We note that this position is inconsistent with any catalogued X-ray source and lies ~0.4 degrees away from the Galactic plane, and therefore conclude that MAXI J1409-619 is a new Galactic X-ray Transient source. The nearest catalogued source is 2MASS 14080271-6159020, a J=15.874 source 2.1 arcseconds from the center of the XRT error circle, and is likely the IR counterpart of the transient.

The photon counting spectrum, corrected for pile-up, is well fit by an absorbed power-law model with an absorption of ~3 x 10^22 cm^-2, and a photon index of -0.5 (+0.1/-0.6). The average flux over the XRT observation is 1.3 x 10^-10 erg/s/cm^-2 (0.3-10 keV), uncorrected for absorption. The source shows significant variability over the short 1ks observation, with the count rate varying between 0.8 and 0.3 XRT counts/s, the lightcurve shows a possible sinusoidal variation with a period of ~720 seconds, although we note that it is impossible to tell if this represents an actual source periodicity given the length of the data.

The BAT hard X-ray transient monitor confirms the detection of MAXI J1409-619 in the 15-50 keV band.  Analysis of archival images at the XRT position shows that the source was first detected on 18-Oct-2010 at a level of 0.0068 +/- 0.0013 ct/s/cm^2, or ~30 mCrab.  The source rate appears to be rising, although we do not yet have the transient monitor data from the past 24 hours.  It was not detected on 17-October-2010 (1-sigma upper limit of 0.004 ct/s/cm^2), the date of the peak in the MAXI light curve (Yamaoka et al, ATel #2959).

Based on the shape of the light curve, the characteristics of the X-ray spectrum (flux and hard photon index), the location close to the Galactic plane, plus the presence of an IR counterpart close to the XRT error circle, MAXI J1409-619 may be a good SFXT candidate.

Further multi-wavelength observations of this transient are encouraged to determine its nature.