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  1. OA@INAF
  2. PRODOTTI RICERCA INAF
  3. 1 CONTRIBUTI IN RIVISTE (Journal articles)
  4. 1.01 Articoli in rivista
Please use this identifier to cite or link to this item: http://hdl.handle.net/20.500.12386/29427
Title: Observing the Transient Pulsations of SMC X-1 with NuSTAR
Authors: Pike, Sean N.
Harrison, Fiona A.
BACHETTI, Matteo 
Brumback, McKinley C.
Fürst, Felix S.
Madsen, Kristin K.
Pottschmidt, Katja
Tomsick, John A.
Wilms, Jörn
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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