Evidence for a Massive Neutron Star from a Radial-velocity Study of the Companion to the Black-widow Pulsar PSR B1957+20
Abstract
The most massive neutron stars constrain the behavior of ultra-dense matter, with larger masses possible only for increasingly stiff equations of state. Here, we present evidence that the black-widow pulsar, PSR B1957+20, has a high mass. We took spectra of its strongly irradiated companion and found an observed radial-velocity amplitude of K obs = 324 ± 3 km s-1. Correcting this for the fact that, due to the irradiation, the center of light lies inward relative to the center of mass, we infer a true radial-velocity amplitude of K 2 = 353 ± 4 km s-1 and a mass ratio q = M PSR/M 2 = 69.2 ± 0.8. Combined with the inclination i = 65° ± 2° inferred from models of the light curve, our best-fit pulsar mass is M PSR = 2.40 ± 0.12 M sun. We discuss possible systematic uncertainties, in particular, in the light curve modeling. Taking an upper limit of i < 85° based on the absence of radio eclipses at high frequency, combined with a conservative lower limit to the motion of the center of mass, K 2>343 km s-1 (q>67.3), we infer a lower limit to the pulsar mass of M PSR>1.66 M sun.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2011
- DOI:
- 10.1088/0004-637X/728/2/95
- arXiv:
- arXiv:1009.5427
- Bibcode:
- 2011ApJ...728...95V
- Keywords:
-
- pulsars: individual: PSR B1957+20;
- stars: neutron;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 7 pages, 3 figures, 1 table, accepted for publication in ApJ