XMM-Newton detects a relativistically broadened Iron Line in SAX J1808.4-3658 spectrum.

ATel #1846; A. Papitto (Univ. Roma Tor Vergata, INAF OAR), A. D'Ai, T. Di Salvo, R. Iaria (Univ. Palermo), M. T.Menna (INAF OAR), L. Burderi, A. Riggio (Univ. Cagliari)
on 17 Nov 2008; 00:14 UT
Credential Certification: Alessandro Papitto (papitto@oa-roma.inaf.it)

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The Accreting Millisecond Pulsar SAX J1808.4-3658 (P=2.49 ms) was observed by XMM Newton for 60 ks on 2008 October 1 (Start Time=MJD 54739.99517), during the outburst which began on 2008 September 21 (ATEL #1728).

To model the continuum emission we used data acquired by EPIC PN (timing mode) and RGS1-2, which observed 600c/s and 40c/s respectively from the source. The absorbed 0.3-10 keV continuum emission is well fitted by a model composed by a multitemperature disc blackbody, a blackbody and a power law, but the addition of a relativistically broadened emission line at 6.5 keV is strongly required by the data. The parameters we obtain for the continuum are: nH=0.162(1)E22 cm^-2; kTdisk=0.20(1) keV; kTbb=0.34(1) keV; alpha=2.08(1). The normalization of the blackbody component is compatible with the size of the NS surface. Numbers in parentheses are the uncertainties in the last digit, quoted at the 90% confidence level.

A line with an energy compatible with K emission of Iron is detected at more than 10 sigma. The line is well modeled by a diskline model (Fabian et al. 1989, MNRAS, 238, 729), which gives an energy of LineE=6.48(8)keV, and an equivalent width of 105 +/- 15 eV, for an inclination best fit value of (60+/-10)°. The energy of the line indicates a weakly ionized state for Iron. We note however that larger values are obtained for lower inclinations, which are indicated by the orbital evolution of the binary (Di Salvo et al. 2008, MNRAS, 389, 1851).

We obtain an estimate for the inner disc radius of (11+/-5) GM/c^2; as the corotation radius is Rc=15GM/c^2 for a 1.4Msun NS accreting from a Keplerian disc in SAX J1808.4-3658, this value is perfectly in agreement with the disc being truncated in regions where accretion is not centrifugally inhibited. This is the first time that such an estimate can be made for an Accreting Millisecond Pulsar.

We note that the line detection agrees with Suzaku observations (ATEL #1844).

We thank Dr.N.Schartel and the XMM-Newton Team who made this ToO observation possible.