A Caltech Library Service

Radius Constraints from Reflection Modeling of Cygnus X-2 with NuSTAR and NICER

Ludlam, R. M. and Cackett, E. M. and García, J. A. and Miller, J. M. and Stevens, A. L. and Fabian, A. C. and Homan, J. and Ng, M. and Guillot, S. and Buisson, D. J. K. and Chakrabarty, D. (2022) Radius Constraints from Reflection Modeling of Cygnus X-2 with NuSTAR and NICER. Astrophysical Journal, 927 (1). Art. No. 112. ISSN 0004-637X. doi:10.3847/1538-4357/ac5028.

[img] PDF - Published Version
Creative Commons Attribution.

[img] PDF - Accepted Version
See Usage Policy.


Use this Persistent URL to link to this item:


We present a spectral analysis of NuSTAR and NICER observations of the luminous, persistently accreting neutron star (NS) low-mass X-ray binary Cygnus X-2. The data were divided into different branches that the source traces out on the Z-track of the X-ray color–color diagram; namely, the horizontal branch, the normal branch, and the vertex between the two. The X-ray continuum spectrum was modeled in two different ways that produced comparable quality fits. The spectra showed clear evidence of a reflection component in the form of a broadened Fe K line, as well as a lower-energy emission feature near 1 keV likely due to an ionized plasma located far from the innermost accretion disk. We account for the reflection spectrum with two independent models (relxillns and rdblur*rfxconv). The inferred inclination is in agreement with earlier estimates from optical observations of ellipsoidal lightcurve modeling (relxillns: i = 67° ± 4°; rdblur*rfxconv: i = 60° ± 10°). The inner disk radius remains close to the NS (R_(in) ≤ 1.15 R_(ISCO)) regardless of the source position along the Z-track or how the 1 keV feature is modeled. Given the optically determined NS mass of 1.71 ± 0.21 M_⊙, this corresponds to a conservative upper limit of R_(in) ≤ 19.5 km for M = 1.92 M_⊙ or R_(in) ≤ 15.3 km for M = 1.5 M_⊙. We compare these radius constraints to those obtained from NS gravitational wave merger events and recent NICER pulsar lightcurve modeling measurements.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Ludlam, R. M.0000-0002-8961-939X
Cackett, E. M.0000-0002-8294-9281
García, J. A.0000-0003-3828-2448
Stevens, A. L.0000-0002-5041-3079
Fabian, A. C.0000-0002-9378-4072
Homan, J.0000-0001-8371-2713
Ng, M.0000-0002-0940-6563
Guillot, S.0000-0002-6449-106X
Chakrabarty, D.0000-0001-8804-8946
Additional Information:© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 October 20; revised 2022 January 6; accepted 2022 January 27; published 2022 March 9. Support for this work was provided by NASA through the NASA Hubble Fellowship grant #HST-HF2-51440.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. 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.A.G. acknowledges support from NASA grant 80NSSC20K0540 and from the Alexander von Humboldt Foundation. S.G. acknowledges the support of the Centre National d'Etudes Spatiales (CNES).
Group:Space Radiation Laboratory, NuSTAR
Funding AgencyGrant Number
NASA Hubble FellowshipHST-HF2-51440.001
Alexander von Humboldt FoundationUNSPECIFIED
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Subject Keywords:Neutron stars; Low-mass x-ray binary stars; Accretion
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Neutron stars (1108); Low-mass x-ray binary stars (939); Accretion (14)
Record Number:CaltechAUTHORS:20220309-982137000
Persistent URL:
Official Citation:R. M. Ludlam et al 2022 ApJ 927 112
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:113812
Deposited By: George Porter
Deposited On:09 Mar 2022 23:19
Last Modified:09 Mar 2022 23:19

Repository Staff Only: item control page