The relativistic pulsar-white dwarf binary PSR J1738+0333 - I. Mass determination and evolutionary history

Abstract

PSR J1738+0333 is one of the four millisecond pulsars known to be orbited by\na white dwarf companion bright enough for optical spectroscopy. Of these, it\nhas the shortest orbital period, making it especially interesting for a range\nof astrophysical and gravity related questions. We present a spectroscopic and\nphotometric study of the white dwarf companion and infer its radial velocity\ncurve, effective temperature, surface gravity and luminosity. We find that the\nwhite dwarf has properties consistent with those of low-mass white dwarfs with\nthick hydrogen envelopes, and use the corresponding mass-radius relation to\ninfer its mass; M_WD = 0.181 +/- +0.007/-0.005 solar masses. Combined with the\nmass ratio q=8.1 +/- 0.2 inferred from the radial velocities and the precise\npulsar timing ephemeris, the neutron star mass is constrained to M_PSR = 1.47\n+/- +0.07/-0.06 solar masses. Contrary to expectations, the latter is only\nslightly above the Chandrasekhar limit. We find that, even if the birth mass of\nthe neutron star was only 1.20 solar masses, more than 60% of the matter that\nleft the surface of the white dwarf progenitor escaped the system. The accurate\ndetermination of the component masses transforms this system in a laboratory\nfor fundamental physics by constraining the orbital decay predicted by general\nrelativity. Currently, the agreement is within 1 sigma of the observed decay.\nFurther radio timing observations will allow precise tests of white dwarf\nmodels, assuming the validity of general relativity.\n