XTE J1810-189 is a Neutron Star

ATel #1443; C. B. Markwardt (U. Md./CRESST/NASA GSFC), T. E. Strohmayer and J. H. Swank (NASA GSFC)
on 27 Mar 2008; 01:03 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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In a pointed observation of XTE J1810-189 (ATEL #1424, #1430, #1432, #1441) on 2008-03-26 at 12:47 UT, the RXTE PCA detected an X-ray burst from the source. The burst shows a clear thermal spectrum and cooling trend, which indicates it is a thermonuclear burst from a neutron star.

No burst oscillations or other high frequency timing properties were detected.

The peak burst X-ray flux was about 2.5E-8 erg/s/cm² (bolometric, unabsorbed). Assuming that the Type-I X-ray burst reached the critical Eddington bolometric peak luminosity of 3.8E38 erg/s (Kuulkers et al. 2003, A&A, 399, 663), the distance upper limit is about 11.5 kpc. The absorbing column density was fixed at the value derived by Swift XRT, 3.8e22 cm^-2(Starling et al., ATEL #1441).

We note the discrepancy between the previously reported position by PCA (ATEL #1424) and follow-up observations by Swift and INTEGRAL (ATEL #1430, #1432, #1441). This discrepancy is likely caused by at least two factors. First, the source is quite variable, which may confuse the PCA scanning technique (i.e. an intensity variation could be mistaken for a position variation). Although the variability was accounted for in an average sense, it is impossible to account for individual fluctuations. Also, the source is near the galactic ridge and several other point sources, which may have also affected the position accuracy.