Observing the Transient Pulsations of SMC X-1 with NuSTAR

Journal article

Publication Details

Author(s): Pike SN, Harrison FA, Bachetti M, Brumback MC, Furst FS, Madsen KK, Pottschmidt K, Tomsick JA, Wilms J
Journal: Astrophysical Journal
Publication year: 2019
Volume: 875
Journal issue: 2
ISSN: 0004-637X
eISSN: 1538-4357


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.

FAU Authors / FAU Editors

Wilms, Jörn Prof. Dr.
Professur für Astronomie und Astrophysik

External institutions with authors

California Institute of Technology (Caltech)
Dartmouth College
European Space Astronomy Centre (ESAC)
INAF-Osservatorio Astronomico di Cagliari
University of California, Berkeley
University of Maryland, Baltimore County

How to cite

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://dx.doi.org/10.3847/1538-4357/ab0f2b

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


Last updated on 2019-23-08 at 08:33