Observing the Transient Pulsations of SMC X-1 with NuSTAR

Pike SN, Harrison FA, Bachetti M, Brumback MC, Furst FS, Madsen KK, Pottschmidt K, Tomsick JA, Wilms J (2019)


Publication Status: Published

Publication Type: Journal article

Publication year: 2019

Journal

Publisher: IOP PUBLISHING LTD

Book Volume: 875

Article Number: ARTN 144

Journal Issue: 2

DOI: 10.3847/1538-4357/ab0f2b

Abstract

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 T-ref = 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 L-x (2-10 keV) approximate to 5 x 10(38) 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.

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APA:

Pike, S.N., Harrison, F.A., Bachetti, M., Brumback, M.C., Furst, F.S., Madsen, K.K.,... Wilms, J. (2019). Observing the Transient Pulsations of SMC X-1 with NuSTAR. Astrophysical Journal, 875(2). https://doi.org/10.3847/1538-4357/ab0f2b

MLA:

Pike, Sean N., et al. "Observing the Transient Pulsations of SMC X-1 with NuSTAR." Astrophysical Journal 875.2 (2019).

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