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Photometry and the light curve of the optical counterpart of the eclipsing millisecond pulsar 1957+20

Djorgovski, S. and Evans, Charles R. (1988) Photometry and the light curve of the optical counterpart of the eclipsing millisecond pulsar 1957+20. Astrophysical Journal, 335 . L61-L65. ISSN 0004-637X. doi:10.1086/185340.

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We confirm the discovery of the optical counterpart to the eclipsing millisecond pulsar 1957 + 20 recently reported by Fruchter, Stinebring, and Taylor and identified earlier by Kulkarni et al. and Fruchter and colleagues. A nearly complete light curve derived from 58 CCD images taken in the Gunn r band has been obtained and found to match the 9.17 hr orbital period of the pulsar. As expected, minimum and maximum light correspond to orbital phases 0.25 and 0.75, respectively. The amplitude of the variation is 0.85 mag with apparent magnitudes of Gunn-Thuan r = 19.36, or V ≃ 19.7, and colors (g - r) = 0.66, or (B -V) ≃ 1.2, at maximum light, and uncorrected for interstellar extinction. However, the centroid of the previously proposed optical counterpart displays a 9.17 hr periodic wobble with amplitude of ~0.”5, indicating the presence of a field star separated by ≃ 1". Assuming that minimum light is due entirely to the field star, the pulsar companion would have peak apparent magnitudes of r = 20.05, or V ≃ 20.3, and colors (g - r) = 0.45, or (B - V) ≃ 1.0, again not including extinction. If the extinction is assumed to be = 1 mag, the corrected color is found to be (g - r) = 0.11, or (B - V) ≃ 0.7, which corresponds to an effective temperature of 5800 K. Based on modeling the heating of and reradiation from the companion, we obtain a synthetic light curve that matches the observed sharp rise to maximum light and flat, broad minimum. By matching the model to the observed brightness and color of the counterpart it is possible to rule out a high extinction (A_V ≃ 3 mag) to the source due to the high luminosity required. For A_V between 1 and 2 mag the inferred stellar radius lies between 0.07 R⊙ and 0.2 R⊙, depending on distance. Assuming the distance 0.9 kpc derived from the radio dispersion measure, and an extinction of A_V = 1 mag, we derive an absolute visual magnitude of the pulsar companion of 10.5 and its radius to be ≃ 0.16 R_⊙, close to that of a degenerate hydrogen dwarf.

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Djorgovski, S.0000-0002-0603-3087
Additional Information:© 1988 American Astronomical Society. Provided by the NASA Astrophysics Data System. Received 1988 August 8; accepted 1988 September 16. We thank J. Roth, S. Staples, and the staff at Palomar Observatory for assistance with the observations. We acknowledge stimulating discussions with S. Kulkarni and E. S. Phinney. The Palomar 60 inch telescope is operated jointly by the California Institute of Technology and the Carnegie Institution of Washington. This work was supported in part by NSF grant AST 85-14911 (C. R. E.). S. D. acknowledges partial support from California Institute of Technology.
Funding AgencyGrant Number
NSFAST 85-14911
Subject Keywords:photometry — planets: general — pulsars — stars: binaries — stars: variable
Record Number:CaltechAUTHORS:20190806-154125598
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:97684
Deposited By: Tony Diaz
Deposited On:07 Aug 2019 16:40
Last Modified:16 Nov 2021 17:33

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