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Observing the Transient Pulsations of SMC X-1 with NuSTAR

Pike, Sean N. and Harrison, Fiona A. and Bachetti, Matteo and Brumback, McKinley C. and Fürst, Felix S. and Madsen, Kristin K. and Pottschmidt, Katja and Tomsick, John A. and Wilms, Jörn (2019) Observing the Transient Pulsations of SMC X-1 with NuSTAR. Astrophysical Journal, 875 (2). Art. No. 144. ISSN 1538-4357. doi:10.3847/1538-4357/ab0f2b.

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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 × 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Pike, Sean N.0000-0002-8403-0041
Harrison, Fiona A.0000-0003-2992-8024
Bachetti, Matteo0000-0002-4576-9337
Brumback, McKinley C.0000-0002-4024-6967
Fürst, Felix S.0000-0003-0388-0560
Madsen, Kristin K.0000-0003-1252-4891
Pottschmidt, Katja0000-0002-4656-6881
Tomsick, John A.0000-0001-5506-9855
Wilms, Jörn0000-0003-2065-5410
Additional Information:© 2019 The American Astronomical Society. Received 2018 October 8; revised 2019 March 10; accepted 2019 March 11; published 2019 April 24. This work was supported under NASA grant No. NNG08FD60C, and made use of data obtained with NuSTAR, a project led by Caltech, funded by NASA and managed by NASA/JPL and has utilized the NUSTARDAS software package, jointly developed by the ASDC (Italy) and Caltech (USA). We would also like to thank the anonymous referee for providing helpful comments that helped to improve the quality of this paper. Software: Stingray (Huppenkothen et al. 2016), HENDRICS (Bachetti 2015), NUSTARDAS, MaLTPyNT (Bachetti 2015), DS9 (Joye & Mandel 2003).
Group:NuSTAR, Space Radiation Laboratory, Astronomy Department
Funding AgencyGrant Number
Subject Keywords:accretion, accretion disks – stars: neutron – X-rays: binaries – X-rays: individual (SMC X-1)
Issue or Number:2
Record Number:CaltechAUTHORS:20190424-160205673
Persistent URL:
Official Citation:Sean N. Pike et al 2019 ApJ 875 144
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:94952
Deposited By: Tony Diaz
Deposited On:24 Apr 2019 23:10
Last Modified:16 Nov 2021 17:09

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