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|Title:||Observing the Transient Pulsations of SMC X-1 with NuSTAR||Authors:||Pike, Sean N.
Harrison, Fiona A.
Brumback, McKinley C.
Fürst, Felix S.
Madsen, Kristin K.
Tomsick, John A.
|Issue Date:||2019||Journal:||THE ASTROPHYSICAL JOURNAL||Number:||875||Issue:||2||First Page:||144||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 <SUB>ref</SUB> = 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 <SUB>X</SUB>(2-10 keV) ≈ 5 × 10<SUP>38</SUP> erg s<SUP>-1</SUP>. 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.||URI:||http://hdl.handle.net/20.500.12386/29427||URL:||https://iopscience.iop.org/article/10.3847/1538-4357/ab0f2b||ISSN:||0004-637X||DOI:||10.3847/1538-4357/ab0f2b||Bibcode ADS:||2019ApJ...875..144P||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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checked on Jan 16, 2021
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