Chandra Positions for the Neutron Star X-ray Transients XTE J1810-189 and SAX J1750.8-2900

ATel #1490; Deepto Chakrabarty (MIT), Peter G. Jonker (SRON/CfA), Craig B. Markwardt (NASA/GSFC)
on 28 Apr 2008; 12:08 UT
Credential Certification: Deepto Chakrabarty (deepto@space.mit.edu)

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As part of an ongoing Chandra program for precise localization of neutron star X-ray transients in low-mass X-ray binaries, we have obtained short Chandra/HRC-S observations of XTE J1810-189 and SAX J1750.8-2900. XTE J1810-189 was observed for a livetime of 1145.3 s on 2008 April 23, 00:15 TT. The best-fit position is: RA(J2000) = 18h 10m 20.86s Dec(J2000) = -19d 04' 11.2" with an uncertainty radius of 0.6 arcsec (90% confidence). This is outside the revised 3.5 arcsec (90% containment) Swift X-ray error circle reported by Starling et al. (ATEL #1441), lying 5.7 arcsec from its center. The Chandra position is not obviously coincident with any of the infrared point sources in the K-band finder image of Torres et al. (ATEL #1446). The HRC-S source intensity was 1.86(4) count/s. Assuming the previously reported absorbed power-law X-ray spectrum (ATEL #1441) with photon index gamma = 2.0 and absorption column N_H = 3.8e22 cm^(-2), this corresponds to an absorbed (unabsorbed) X-ray flux of 2.7e-10 (1.0e-9) erg/cm^2/s in the 0.1-10 keV band. SAX J1750.8-2900 was observed for a livetime of 1183.8 s on 2008 April 24, 00:13 TT. The best-fit position is: RA(J2000) = 17h 50m 24.42s Dec(J2000) = -29d 02' 15.4" with an uncertainty radius of 0.6 arcsec (90% confidence). This is consistent with the revised Swift X-ray error circle reported by Torres et al. (ATEL #1446), lying 1.6 arcsec from its center. It also confirms the variable infrared counterpart proposed by Torres et al. (ATEL #1472), lying 0.4 arcsec away. The HRC-S source intensity was 4.54(6) count/s. Assuming an absorbed power-law spectrum with gamma = 2.0 and N_H = 3.1e22 cm^(-2) as measured with 3-30 keV RXTE PCA archival data from April 10, the Chandra count rate corresponds to an absorbed (unabsorbed) X-ray flux of 5.6e-10 (2.0e-9) erg/cm^2/s in the 0.1-10 keV band. Both observations were taken in HRC-S timing mode. No X-ray pulsations were detected from either source.