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Swift/XRT reports an improved position for XTE J1810-189

ATEL # 1432; H.A. Krimm (CRESST/GSFC/USRA), J. Kennea (PSU), J. Tueller (GSFC)
on 17 Mar 2008; 22:19 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

Subjects: X-ray, Request for Observations, Binaries, Transients

The XRT on Swift began observing the field containing XTE J1810-189 (Markwardt and Swank, ATel #1424) at 14:08 UT on March 17, 2008 collecting 1.5 ks of Photon Counting mode data. A bright point source, Swift J181020.7-190411 , was found at the following position:
RA(J2000) = 18h 10m 20.7s
Dec(J2000) = -19d 04m 11.0s
with an estimated error of 5'' radius (90% containment), 54 arcseconds from the INTEGRAL position and 74'' from the BAT position (Neronov et al, ATel #1430). We note that the source point spread function lies on a hot column on the XRT, so the estimated uncertainty is larger than normal.

The XRT spectrum can be well fit (reduced chi2 = 1.1 for 38 degrees of freedom) by an absorbed power-law model with the following parameters: N_H = 4.2 0.7 x 10^22 H atoms/cm^2 and photon index = 1.8 0.3. The 0.3-10 keV flux is ~1.3 x 10^-10 erg/s/cm^2, not corrected for absorption. Fixing the spectrum to the INTEGRAL fitted power-law photon index of 2.26 produces a poorer, although acceptable, fit (reduced chi^2 = 1.3 for 39 d.o.f.), and a higher absorption, N_H = (5.3 0.4) x 10^22 H atoms/cm^2.

We note that both absorption values found are higher than the expected Galactic value of 1.32 x 10^22 H atoms/cm^2. Attempting to fix the absorption at this value or lower, produces a considerably poorer fit (reduced chi^2 = 2.7 for 39 d.o.f.), suggesting that there is another source of absorption other than Galactic, e.g. local absorption in the system.

A single infrared source is found within the XRT error circle. This source, 2MASS 18102072-1904093, has no optical counterpart and reported magnitudes J=14.694, H=12.990, K=12.360. Given the high absorption and lack of an optical counterpart, we conclude that XTE J1810-189 is unlikely to be a nearby source, and thus is most likely a high mass X-ray binary.


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