NuSTAR Observations of SMC X-3

ATel #9404; K. Pottschmidt (GSFC/UMBC), R. Ballhausen (ECAP), J. Wilms (ECAP), A. Zezas (FORTH), F. Fuerst (Caltech), M. A. Nowak (MIT), V. Grinberg (MIT), M. Kuehnel (ECAP), P. Kretschmar (ESA-ESAC), J. A. Tomsick (UCBerkeley), V. Antoniou (SAO), J. Kennea (PSU), J. Hong (Harvard), F. Haberl (MPE), T. Maccarone (TTU), A. Hornschemeier (NASA/GSFC), A. Ptak (NASA/GSFC), M. Yukita (JHU), D. Wik (NASA/GSFC), B. Lehmer (UARK), F. Fornasini (Berkeley), A. Bodaghee (GCSU), V. McBride (UCT/SAAO)
on 24 Aug 2016; 16:16 UT
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Credential Certification: Joern Wilms (j.wilms@sternwarte.uni-erlangen.de)

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The X-ray binary SMC X-3 has been in outburst since 2016 July 30 (ATEL 9362, ATEL 9370). We report on a 25ks NuSTAR observation performed between 2016 August 13, 18:39 UT, and 2016 August 14, 07:49, with the aim of measuring the hard X-ray spectrum of the source.

The 3-50keV phase averaged spectrum can be well described (reduced chi^2=1.01 for 1867 degrees of freedom) by an exponentially cutoff power law with photon index Gamma=0.49pm0.04, a folding energy of 12.05pm0.25keV, and a black body component (kT=1.78pm0.05 keV), which contributes 6 per cent of the total 3-50keV flux. The photon index is consistent with that seen by Swift (ATEL 9370). An emission line from neutral iron at 6.39pm0.06keV with equivalent width of 70eV and a line width of about 0.4keV is also present.

No significant intrinsic or foreground absorption is detected (the Galactic NH to the SMC is 6E20cm-2 [Coe et al., 2011, MNRAS 414, 3281] and not detectable with NuSTAR). No other obvious spectral features are seen.

The measured pulse period is 7.8106 pm 0.0013 s with a pulsed fraction of 63.8 pm 0.7 percent. The pulse profile is only weakly energy dependent with a broader main peak and a narrower secondary peak which have almost the same amplitude.

The 3-50keV flux of 1.8e-9 cgs corresponds to a luminosity of 8.4e38 erg/s at the mean distance of the SMC of 62.1 kpc (Graczyk et al., 2014, ApJ 780, 59). It has a systematic uncertainty of about 20% due to the elongation of the SMC, which might put the source up to 5 kpc closer to us (Scowcroft et al., 2016, ApJ 816, 49). This luminosity puts SMC X-3 into the supercritical accretion rate regime of magnetized neutron stars, where radiation breaking dominates the dynamics of the accretion column (Becker et al., 2012, A&A 544, A123), and makes this outburst one of the most luminous outbursts of an accreting neutron star observed in this decade.

The Swift-BAT lightcurve shows SMC X-3 to continue to rise in flux, albeit at a smaller rate. Continued further monitoring at all wavelengths is encouraged.