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4U 1608-52 is back to quiescence after its 2007-2009 outburst

ATEL # 2264; M. Linares (Amsterdam & MIT-MKI), D. Altamirano, N. Degenaar, R. Wijnands (Amsterdam)
on 26 Oct 2009; 15:22 UT
Password Certification: Manuel Linares (mlinares@science.uva.nl)

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

After an outburst that lasted more than 2.3 years (starting around June 12th., 2007; Palmer et al., ATel #1113), the neutron star transient and atoll source 4U 1608-52 has returned to quiescence. The source was last detected by Swift-BAT, RXTE-PCA and RXTE-ASM on September 14th. (~0.006 BAT c/s/cm2), September 19th. (~3.5 mCrab) and October 5th. (~1.4 ASM c/s), 2009, respectively.

We obtained daily Swift-XRT observations since October 20th. On October 20th. and 21st. the source was not detected. Combining both observations (for a total of 960s) we estimate using webpimms a 2-10 keV unabsorbed flux upper limit (95% c.l.; Gehrels et al. 1986) of 5.6E-13 erg/cm^{2}/s (for a power law spectrum with photon index 1.5), which corresponds to 8.7E32 erg/s. We use a distance of 3.6 kpc and an absorbing column density fixed at 1.5E22 cm^{-2} (Nakamura et al. 1989; Penninx et al. 1989).

On October 22nd. and 23d. the source was detected in Swift-XRT observations of ~1.2 and ~1.1 ksec with 23 and 5 photons, respectively. By combining these two observations we estimate an unabsorbed 2-10 keV flux of 2.5E-13 erg/cm^{2}/s (power law fit; index ~4.5), corresponding to 3.9E32 erg/s.

We detected 4U 1608-52 at luminosities below 1E33 erg/s about 15 days after the last detection of its 2007-2009 outburst. We thereby confirm that the outburst has finished and the source is back to quiescence.

We thank the Swift team for a rapid scheduling of the observations.


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