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A Chandra Position for the Neutron Star X-ray Transient XTE J1701-407

ATEL # 1795; David L. Kaplan (UCSB) and Deepto Chakrabarty (MIT)
on 21 Oct 2008; 17:55 UT
Password Certification: David L. Kaplan (dkaplan@kitp.ucsb.edu)

Subjects: X-ray, Binaries, Neutron Stars, Transients

We acquired a 5 ks HRC-I Chandra Director's Discretionary Time observation of the neutron star X-ray transient XTE J1701-407 (ATEL #1569, #1572, #1616, #1618, #1621, and #1635) on 2008 August 26, following a Target of Opportunity ACIS-S observation on 2008 August 07 that was severely compromised by photon pileup. In the HRC-I observation the source is well-detected, with a count-rate of 1.32 count/s (background-subtracted) within a 20-pixel (2.6 arcsec) radius. The radial profile appears slightly asymmetric on small (sub-arcsec) scales, but we believe this to be due to problems in screening events and reconstructing event positions (see http://cxc.harvard.edu/cal/Hrc/filter.html and Murray et al. 2000, SPIE, 4140, 144; Juda et al. 2000, SPIE, 4140, 155).

The determination of the position is slightly affected by the observed asymmetry, but the uncertainty associated with that is less than one HRC pixel (0.13 arcsec). We find a J2000 position of:

RA=17:01:44.33 Dec=-40:51:30.1

with an uncertainty of 0.6" due to Chandra aspect uncertainties. There is another X-ray source detected in both the HRC and ACIS images, with about 0.014 count/s in the HRC, located at:

RA=17:01:56.85 Dec=-40:49:38.3

or 3 arcmin away from XTE J1701-407. We designate this source as CXOU J170156.8-404938. We could not use this source to correct the absolute astrometry as it is outside the field of our ground-based imaging and there is no obvious counterpart in 2MASS Point Source Catalog (the nearest source is 2 arcsec away, which is larger than the expected astrometric uncertainty).

Using the spectrum of XTE J1701-407 measured by SWIFT on 2008 June 11 (Degenaar & Wijnands 2008, ATEL #1572) the inferred 2-10 keV flux in the HRC-I observation is ~1.8e-10 erg/s/cm^2, or approximately approximately 65% of the measured SWIFT flux. The SWIFT BAT monitor shows that the source was increasing in flux somewhat during the time of our HRC-I observation. We find no evidence for pulsations in a power-spectrum going from 0 Hz to 200 Hz (limited by the intrinsic timing uncertainty of the HRC) with an upper limit to the pulsed fraction of about 10% (99% confidence), although quasiperiodic oscillations at 30 Hz were detected by RXTE (Strohmayer et al. 2008, ATEL #1635).

Of the four possible infrared counterparts to XTE J1701-407 discussed in Kaplan & Chakrabarty (2008; ATEL #1630), we find that only the two brighter sources (K-band magnitudes of 13.8 and 14.2) are consistent with the Chandra position. However, they are 0.8 arcsec from the X-ray position and hence only marginally consistent with the typical 0.6 arcsec astrometric uncertainty of Chandra.

We thank S. Murray for help in interpreting the HRC data, and H. Tananbaum for the Director's Discretionary observation.


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