XMM-Newton gives spectral modeling and updated ephemeris of the Accreting Millisecond Pulsar IGR J17511-3057

ATel #2220; A. Papitto, A. Riggio (INAF OAC), L. Burderi (Univ. Cagliari), T. Di Salvo, A. D'Aì, R. Iaria (Univ. Palermo), M. T.Menna (INAF OAR)
on 1 Oct 2009; 19:06 UT
Credential Certification: Alessandro Papitto (papitto@oa-roma.inaf.it)

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We report on an XMM-Newton Director Discretionary Time ToO observation of the newly discovered Accreting Millisecond Pulsar IGR J17511-3057 (ATEL #2196, #2197, #2198, #2199, and #2215) during its ongoing outburst. XMM-Newton spotted the source for 70 ks starting on 2009 Sep 21.01.

The EPIC MOS1 camera, operated in Small Window mode, allowing a precise estimate of the source position:

RA = 17 51 08.55
DEC = -30 57 41.7

affected by an uncertainty of 1.1 arcsec. This estimate is consistent with the one obtained from a Chandra observation (ATEL #2215)

The absorbed flux of the persistent emission of the source is 2.8(4)E-10 erg cm^-2 s^-1 (2-10 keV). A preliminary analysis of the EPIC pn spectrum shows it is consistent with being fitted by a model composed of a multicolor disk black body ( kTin=0.13(1) keV ), a single temperature black body ( kT=0.73(6)keV ) and a power law of index 1.71(5). The slope of the power law is almost consistent with the value (1.8) estimated by the PCA on board RXTE at the beginning of the outburst (ATEL #2197). Modeling of the ISM absorption indicates an nH equal to 1.16(4)E22 cm^-2. A 6.7keV fluorescence Iron line is barely detected. Errors on flux estimates and spectral parameters are quoted at 90% confidence level.

The 245 Hz pulsation first reported by RXTE (ATEL #2197) is clearly detected throughout the observation. Using the best available position estimate given by Chandra (ATEL #2215), a timing analysis reveals the source is spinning at a frequency of 244.8339512(1) Hz. Analysis of the Doppler shifts caused by the orbital motion leads to the following ephemeris:

P_orb=12487.51(2) s
a sini/c=0.275196(4) lt-s
T*=55094.9695351(7) MJD

where P_orb is the orbital period of the source, a sini/c is the projected semi-major axis of the neutron star orbit and T* is the epoch of passage of the neutron star at the ascending node of the orbit. The errors in parentheses on the spin and orbital parameters are given at the 1 sigma level. The orbital solution we found is perfectly consistent with the one obtained from RXTE measurements (ATEL #2197).

Two type I X-ray burst are also evident in the X-ray light curve, peaking at fluxes of 1.1(1)E-08 erg/s/cm^2 (2-10 keV). We confirm the detection of burst oscillations made by Watts et al. (ATEL #2199). The burst oscillations we observed thanks to XMM-Newton have the very same frequency of the 'persistent' oscillations, and are also phase locked to them.

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