Observing the Transient Pulsations of SMC X-1 with NuSTAR

Journal
Date Issued
2019
Author(s)
Pike, Sean N.
•
Harrison, Fiona A.
•
•
Brumback, McKinley C.
•
Fürst, Felix S.
•
Madsen, Kristin K.
•
Pottschmidt, Katja
•
Tomsick, John A.
•
Wilms, Jörn
DOI
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) ≈ 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.
Volume
875
Issue
2
Start page
144
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2019ApJ...875..144P
Rights
open.access
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