Swift and RXTE observations of the on-going outburst of the accreting ms pulsar SAX J1808.4-3658

ATel #3736; A. Papitto (Univ. Cagliari, Italy), E. Bozzo, C. Ferrigno (ISDC, Switzerland), L. Burderi (Univ. Cagliari, Italy), T. Di Salvo (Univ. Palermo, Italy), A. Riggio (INAF OAC, Italy)
on 5 Nov 2011; 12:27 UT
Credential Certification: E. Bozzo (enrico.bozzo@unige.ch)

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After the announcement of a new outburst of the 401 Hz accreting ms pulsar SAX J1808.4-3658 (Markwardt et al. 2011, ATel #3733), follow-up observations were performed by Swift and RXTE.

A Swift observation performed in WT mode started on 2011 November 4.660 for an exposure of 1.0 ks. The 1.0-10 keV spectrum (only grade 0 photons) is dominated by a power law with photon index of 2.27(8) absorbed by a medium with nH=0.20(2)E22 cm**-2, a value compatible with that observed during past outbursts of the source (e.g., Papitto et al. 2009, A&A, 493, L39). We found evidences for the presence of a blackbody component with temperature of 0.34(2) keV and radius 18(4) d_3.5 km km, where d_3.5 is the distance to the source in units of 3.5 kpc (Galloway & Cumming 2006, ApJ, 652, 559). The absorbed 2-10 keV flux is 1.62(4)E-9 erg cm**-2 s**-1. Errors on the spectral parameters are given at the 90% confidence level.

RXTE observed the source starting on 2011 November 4.902 for 2.9 ks. A net count-rate of 182.7(4) c/s is recorded by the the PCU2 of the Proportional Counter Array (PCA).

A signal at a frequency between 400.94 and 400.96 Hz is detected at a significance exceeding 6 sigma by performing power spectra over 256 s time intervals and taking into account the number of frequencies searched (2**20). The RMS amplitude (not background subtracted) of the signal varies between 3 and 5 per cent. Considering that no correction was applied to take into account the motion of the observing spacecraft and the orbital motion of the source, we conclude how the observed frequency and its variation over different time intervals is compatible with the one showed by SAX J1808.4-3658 during past outbursts (e.g. Hartman et al. 2009, ApJ, 702, 1673).

The 2.5-25 keV spectrum observed by the top layer of the PCU2 of the PCA is modeled by a power law with index of 1.98(3) and a blackbody with temperature of 0.72(4) keV and radius of 3.7(6) d_3.5 km. A broad (sigma=0.8(2) keV) emission line at an energy compatible with the neutral iron K-alpha transition is also observed. However, the strength and shape of the line, as well as the observed spectral distribution at the low end of the PCA energy band, possibly suffer from contamination of the Galactic ridge emission falling into the field of view of the PCA. The absorption column was fixed to the value determined by Papitto et al. 2009 (nH=0.214(3)E22 cm**-2) in the fitting. The absorbed flux in the 2-10 keV and 2.5-25 keV energy band is 1.71(4)E-9 and 2.35(4)E-9 erg cm**-2 s**-1, respectively, indicating a flux compatible with that observed by Swift, as well as with those observed at the peak of previous outbursts (e.g. Hartman et al. 2008, 675, 1468).

Further observations at all wavelengths are encouraged. We thanks the Swift and RXTE team for the rapid scheduling of follow-up observations.