EVIDENCE FOR A MASSIVE NEUTRON STAR FROM A RADIAL-VELOCITY STUDY OF THE COMPANION TO THE BLACK-WIDOW PULSAR PSR B1957+20

By M. V. van Kerkwijk, R. Breton, Shrinivas R. Kulkarni. 2010

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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 Kobs = 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 K2 = 353 ± 4 km s-1 and a mass ratio q = MPSR/M2 = 69.2 ± 0.8. Combined with the inclination i = 65° ± 2° inferred from models of the light curve, our best-fit pulsar mass is MPSR = 2.40 ± 0.12 M☉. 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, K2>343 km s-1 (q>67.3), we infer a lower limit to the pulsar mass of MPSR>1.66 M☉.

Subfields: Astronomy and Astrophysics, Oceanography

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Citations in the corpus

• From hadrons to quarks in neutron stars: a review (2017, background): “9 In addition the extreme black widow millisecond pulsars PSR J1957 + 20 [43], PSR J2215 + 5135 [44], and PSR J1311-3430 [45, 46] possibly have masses as large as 2.”
• Peering into the Dark Side: Magnesium Lines Establish a Massive Neutron Star in PSR J2215+5135 (2018, methodology): “There may still be unknown or highly uncertain systematic effects, however, biasing the best-fit model inclination and thus the dynamical mass measurements (see, e.g., the discussion in van Kerkwijk et al. 2011, their Section 4).”
• On the astrophysical robustness of the neutron star merger r-process (2012, background): “For example, the mass of the black widow pulsar has recently been determined as 2.4 ± 0.12 M (van Kerkwijk et al. 2011).”
• QUARK MATTER IN MASSIVE COMPACT STARS (2011, background): “Recently, van Kerkwijk et al. (2011) reported the possible existence of a 2.4 M⊙ star.”
• EQUATION OF STATE AND NEUTRON STAR PROPERTIES CONSTRAINED BY NUCLEAR PHYSICS AND OBSERVATION (2013, background): “In Figure 7 we present the individual EOSs that fulfill both constraints for two cases: M̂ = 1.97 M (left panel), the mass of the heaviest known neutron star (Demorest et al. 2010), and M̂ = 2.4 M (right panel), an estimated mass of the black widow pulsar B1957+20 (van Kerkwijk et al. 2011).”
• Grids of stellar models with rotation - II. WR populations and supernovae/GRB progenitors at Z = 0.014 (2012, background): “This choice is somewhat supported by the recent discovery of a very massive 2.4 M⊙ NS by van Kerkwijk et al. (2011).”
• REFINED NEUTRON STAR MASS DETERMINATIONS FOR SIX ECLIPSING X-RAY PULSAR BINARIES (2011, unknown): “Other high-mass neutron stars include PSR B1516+02B with M = 2.10 ± 0.19 M (Kiziltan et al. 2010), PSR J1748−2446I with M = 1.91+0.02−0.10,M (Kiziltan et al. 2010), and possibly the “Black Widow Pulsar” PSR B1957+20 with M = 2.40 ± 0.12 M (van Kerkwijk et al. 2011).”