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Further Swift observations of XTE J1810-189

ATEL # 1441; R. Starling (U. Leicester), J. Kennea (PSU), H. Krimm (CRESST/GSFC/USRA)
on 25 Mar 2008; 23:48 UT
Password Certification: Rhaana Starling (rlcs1@star.le.ac.uk)

Subjects: Infra-Red, Optical, Ultra-Violet, X-ray, Binaries, Transients
Referred to by ATEL #: 1443, 1446

Swift re-observed the possible high mass X-ray binary XTE J1810-189 (Krimm, Kennea and Tueller, ATEL#1432; Markwardt and Swank, ATEL#1424) at 16:55 UT on March 18, 2008.
Here we report an improved position based on 1.3 ks of XRT Photon Counting mode data from these observations of RA,Dec = 272.58535, -19.07026 deg, equivalent to:
RA (J2000) = 18h 10m 20.48s
Dec (J2000) = -19d 04' 12.9"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This is within the previously reported error circle, 3.7 arcseconds from its centre. We note that this observation is not affected by the hot columns.

The X-ray spectrum is consistent with that observed previously (ATEL#1432), well fitted with an absorbed power law of photon index Gamma = 2.0+/-0.3 and total absorption (at z=0) N_H = (3.8+/-0.6)e22 cm^-2 (including the Galactic contribution of approximately 1.3e22 cm^-2). A correction for pile-up has been applied.
We measure an observed (unabsorbed) 0.3-10 keV flux of 4.3e-10 (5.6e-10) erg cm^-2 s^-1, somewhat higher than previously measured.

This new position is inconsistent (using the 90% error circle) with the proposed IR counterpart 2MASS 18102072-1904093 (ATEL#1432), lying 5 arcseconds away, but is consistent with another IR source: 2MASS 1810233-1904136 which has 2MASS reported magnitudes of J=14.68+/-0.05, H=13.01+/-0.06, K=12.32+/-0.05. We propose that 2MASS 1810233-1904136 is the more likely IR counterpart of XTE J1810-189.

Swift UVOT observed the field beginning with a long exposure in v band, thereafter cycling through all six lenticular filters. The source was not detected in any filter, though we note the very high Galactic extinction in this direction of E(B-V) = 18.3, and we derive the following 3-sigma upper limits on the magnitudes:
filterTexp (s)mag limit
v393.4718.95
b19.4518.05
u19.4517.64
uvw119.4417.63
uvm219.4617.35
uvw219.4617.67


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