NuSTAR observation of the latest outburst of SAX J1808.4-3658

ATel #13022; A. Sanna (Univ. of Cagliari), T. Di Salvo (Univ. of Palermo), L. Burderi, A. Riggio, A. Gambino (Univ. of Cagliari), R. Iaria (Univ. of Palermo)
on 13 Aug 2019; 17:38 UT
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Credential Certification: Andrea Sanna (andrea.sanna@dsf.unica.it)

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Following the renewed X-ray activity of the accreting millisecond X-ray pulsar SAX J1808.4-3658 reported by NICER (ATel #13001), we triggered a NuSTAR ToO observation. NuSTAR observed the source between 2019 August 10 11:26 UTC and 2019 August 11 12:06 UTC, for a total exposure time of roughly 44 ks. The source was clearly detected across the full NuSTAR band (3-79 keV), with an average count rate of 23 c/s (FPMA and FPMB combined). The light curve showed two clear X-ray bursts.

The NuSTAR X-ray spectrum is remarkably similar to the NuSTAR spectrum of SAX J1808.4-3658 obtained during the 2015 outburst (Di Salvo et al. 2019) and is well fitted by an absorbed comptonized continuum (Nthcomp in Xspec) plus a blackbody component and a broad iron line, giving a reduced chi^2 = 1756 for 1753 d.o.f. We fix the absorption column density to nH = 0.21E22 cm^-2 (Di Salvo et al. 2019), and we find a blackbody temperature of kT = 0.59(2) keV, a power-law photon index of Gamma = 1.89(1), an electron temperature of kTe of about 30 keV. Fixing the energy of the iron line at 6.4 keV, we find the inner disk radius at less than ~16 Rg (corresponding to ~32 km for a 1.4 Msun neutron star) and an inclination angle larger than 58 deg. The X-ray flux is ~8e-10 erg/s/cm^2 in the 3-70 keV energy band.

We barycentered the NuSTAR photon arrival times adopting the radio source coordinates (Rupen et al. 2002, IAU Circ., 7997, 2) and we corrected the delays of the photon time of arrivals caused by the X-ray pulsar orbital motion under the hypothesis of a circular orbit and assuming the latest orbital ephemerides reported in Sanna et al. 2017 (MNRAS 471, 463-477). Given the peculiar orbital evolution of the source (see e.g Patruno et al. 2017, ApJ, 841, 98 and Sanna et al. 2017, MNRAS, 471, 463), we applied timing techniques exploring a sample of values for the time of passage at the ascending node (T_asc) corresponding to +/- 5 sigmas with respect to the predicted value obtained following Sanna et al. 2017 (MNRAS 471, 463-477). We obtained the most significant pulse profile for T_asc = 58705.37335 +/- 0.00003 MJD (TDB) which differs roughly 7 seconds with respect to the preliminary value obtained with a short NICER observation (Atel #13001). Similar to Bult et al. 2019 (Atel #13001), the T_asc value obtained here confirms a slower orbital expansion with respect to the quadratic model estimated from the previous outbursts (see Sanna et al. 2017, MNRAS, 471, 463 for detailed modeling). Folding the data with the updated value of T_asc, we retrieved a pulsation detection (~15 sigmas) around the expected frequency 401 Hz. The pulse profile is well described by a sinusoid with a fractional amplitude of 2%. A more detailed analysis is underway.

We thank the NuSTAR team for the prompt scheduling of the reported observation.