Observing the Transient Pulsations of SMC X-1 with NuSTAR

• Published in: The Astrophysical journal Vol. 875; no. 2; pp. 144 - 153
• Main Authors: Pike, Sean N., Harrison, Fiona A., Bachetti, Matteo, Brumback, McKinley C., Fürst, Felix S., Madsen, Kristin K., Pottschmidt, Katja, Tomsick, John A., Wilms, Jörn
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 20-04-2019; IOP Publishing
• Subjects: Accretion disks; accretion, accretion disks; Astrophysics; Binary stars; Fluctuations; Luminosity; Neutron flux; Neutron stars; Occultation; Power law; Pulsars; Spectral analysis; Spectroscopic telescopes; Spectrum analysis; stars: neutron; Variability; X ray binaries; X ray stars; X-rays: binaries; X-rays: individual (SMC X-1)
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab0f2b

We report on Nuclear Spectroscopic Telescope Array observations of transient pulsations in the neutron star X-ray binary SMC X-1. The transition from nonpulsing to pulsing states was not accompanied by a large change in flux. Instead, both pulsing and nonpulsing states were observed in a single observation during the low-flux super-orbital state. During the high state, we measure a pulse period of P = 0.70117(9) s at Tref = 56145 MJD. Spectral analysis during nonpulsing and pulsing states reveals that the observations can be consistently modeled by an absorbed power law with a phenomenological cutoff resembling a Fermi-Dirac distribution, or by a partially obscured cutoff power law. The shapes of the underlying continua show little variability between epochs, while the covering fraction and column density vary between super-orbital states. The strength of pulsations also varies, leading us to infer that the absence and reemergence of pulsations are related to changing obscuration, such as by a warped accretion disk. SMC X-1 is accreting near or above its Eddington limit, reaching an unabsorbed X-ray luminosity of LX(2-10 keV) 5 × 1038 erg s−1. This suggests that SMC X-1 may be a useful local analog to ultraluminous X-ray pulsars (ULXPs), which likewise exhibit strong variability in their pulsed fractions, as well as flux variability on similar timescales. In particular, the gradual pulse turn-on, which has been observed in M82 X-2, is similar to the behavior we observe in SMC X-1. Thus we propose that pulse fraction variability of ULXPs may also be due to variable obscuration.