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A Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with NuSTAR

Ludlam, R. M. and Miller, J. M. and Bachetti, M. and Barret, D. and Bostrom, A. C. and Cackett, E. M. and Degenaar, N. and Di Salvo, T. and Natalucci, L. and Tomsick, J. A. and Paerels, F. and Parker, M. L. (2017) A Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with NuSTAR. Astrophysical Journal, 836 (1). Art. No. 140. ISSN 1538-4357. doi:10.3847/1538-4357/836/1/140.

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We present NuSTAR observations of neutron star (NS) low-mass X-ray binaries: 4U 1636-53, GX 17+2, and 4U 1705-44. We observed 4U 1636-53 in the hard state, with an Eddington fraction, F_(Edd), of 0.01; GX 17+2 and 4U 1705-44 were in the soft state with fractions of 0.57 and 0.10, respectively. Each spectrum shows evidence for a relativistically broadened Fe K α line. Through accretion disk reflection modeling, we constrain the radius of the inner disk in 4U 1636-53 to be R_(in) = 1.03 ± 0.03 ISCO (innermost stable circular orbit), assuming a dimensionless spin parameter a* = cJ/GM^2 = 0.0, and R_(in) = 1.08 ± 0.06 ISCO for a* = 0.3 (errors quoted at 1σ). This value proves to be model independent. For a* = 0.3 and M = 1.4 M⊙, for example, 1.08 ± 0.06 ISCO translates to a physical radius of R = 10.8 ± 0.6 km, and the NS would have to be smaller than this radius (other outcomes are possible for allowed spin parameters and masses). For GX 17+2, R_(in) = 1.00-1.04 ISCO for a* = 0.0 and R_(in) = 1.03-1.30 ISCO for a* = 0.3. For a* = 0.3 and M = 1.4 M⊙, R_(in) = 1.03-1.30 ISCO translates to R = 10.3-13.0 km. The inner accretion disk in 4U 1705-44 may be truncated just above the stellar surface, perhaps by a boundary layer or magnetosphere; reflection models give a radius of 1.46–1.64 ISCO for a* = 0.0 and 1.69–1.93 ISCO for a* = 0.3. We discuss the implications our results may have on the equation of state of ultradense, cold matter and our understanding of the innermost accretion flow onto NSs with low surface magnetic fields, and systematic errors related to the reflection models and spacetime metric around less idealized NSs.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Ludlam, R. M.0000-0002-8961-939X
Bachetti, M.0000-0002-4576-9337
Barret, D.0000-0002-0393-9190
Cackett, E. M.0000-0002-8294-9281
Natalucci, L.0000-0002-6601-9543
Tomsick, J. A.0000-0001-5506-9855
Parker, M. L.0000-0002-8466-7317
Additional Information:© 2017 The American Astronomical Society. Received 2016 August 5; revised 2017 January 4; accepted 2017 January 6; published 2017 February 14. This research has made use of the NuSTAR. Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (Caltech, USA). J.M.M. gratefully acknowledges support through the NuSTAR guest observer program. N.D. is supported via an NWO Vidi grant and an EU Marie Curie Intra-European fellowship under contract no. FP-PEOPLE- 2013-IEF-627148. E.M.C. gratefully acknowledges support from the National Science Foundation through CAREER award number AST-1351222.
Subject Keywords:accretion, accretion disks – stars: neutron – X-rays: binaries – X-rays: individual (4U 1636-53, GX 17 +2, 4U 1705-44)
Issue or Number:1
Record Number:CaltechAUTHORS:20170111-113848710
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Official Citation:R. M. Ludlam et al 2017 ApJ 836 140
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:73424
Deposited By: Joy Painter
Deposited On:17 Jan 2017 18:41
Last Modified:11 Nov 2021 05:16

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