The neutron star transient XTE J1701-407 has returned to quiescence after a 3-year long outburst
ATel #3604; N. Degenaar, R. Wijnands, D. Altamirano (University of Amsterdam), E. Cackett (Cambridge), M. Linares, J. Homan and J. Fridriksson (MIT)
on 29 Aug 2011; 17:24 UT
Credential Certification: Nathalie Degenaar (firstname.lastname@example.org)
Subjects: X-ray, Binary, Neutron Star, Transient
Referred to by ATel #: 3654
The neutron star low-mass X-ray binary and long thermonuclear burst source XTE J1701-407 (ATel #1618) has been active ever since its discovery in 2008 June (ATel #1569), displaying a typical 2-10 keV luminosity of ~5E36 erg/s (assuming a distance D=5.0 kpc; ATel #2814). However, routine monitoring of the Galactic bulge with the PCA aboard RXTE (Swank & Markwardt 2001, ASP conference series 251, 94) indicates that the outburst has now ceased: the source intensity has remained below the detection limit of the PCA since 2011 August 9 (see the link provided below).
We have performed follow-up observations with the Swift/XRT to investigate the current state of XTE J1701-407. The source is clearly detected during a 2.0-ks pointing carried out on 2011 August 23, with an average (PC mode) count rate of ~3.9E-2 c/s. The XRT spectrum can be fit with a simple absorbed powerlaw with a hydrogen column density of ~(3+/-2)E22 cm-2 and a photon index of ~2.4+/-1.5. The resulting 0.5-10 keV luminosity is ~4E34 erg/s (for D=5 kpc).
XTE J1701-407 is not detected during our second Swift/XRT observation that was performed on 2011 August 27 for 1.9 ks. Applying the small number prescription of Gehrels 1986 (ApJ 303, 336), we infer a 95% confidence upper limit on the source count rate of ~5.5E-3 c/s (PC mode). Using pimms with the spectral shape found for the first Swift observation (an absorbed powerlaw with nH=3E22 cm-2 and an index of 2.4), we estimate an upper limit on the 0.5-10 keV luminosity of ~4E33 erg/s (D=5 kpc). This is a factor ~10 lower than the 0.5-10 keV luminosity measured with Swift 4 days earlier. If we instead assume a blackbody spectral shape with kT=0.2-0.3 keV, the Swift non-detection implies an upper limit on the 0.5-10 keV thermal luminosity of ~(0.6-2)E34 erg/s (D=5 kpc).
Our Swift observations confirm the decrease in intensity seen in the RXTE/PCA bulge scans and strongly suggest that XTE J1701-407 has returned to quiescence after a long outburst that persisted for ~3 years. Further X-ray observations are planned to monitor the source.
RXTE/PCA bulge scan lightcurve of XTE_J1701-407