New ephemeris of the ADC source 2A 1822–371: a stable orbital-period derivative over 30 years

¹ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, Monserrato 09042, Italy e-mail: burderi@dsf.unica.it
² Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, Palermo 90123, Italy e-mail: disalvo@fisica.unipa.it
³ INAF Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy
⁴ INAF Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio Catone 00040, Italy

Received: 13 July 2009
Accepted: 4 March 2010

Abstract

We report on a timing of the eclipse arrival times of the low mass X-ray binary and X-ray pulsar 2A 1822–371 performed using all available observations of the Proportional Counter Array on board the Rossi X-ray Timing Explorer, XMM-Newton pn, and Chandra. These observations span the years from 1996 to 2008.
Combining these eclipse arrival time measurements with those already available covering the period from 1977 to 1996, we obtain an orbital solution valid for more than thirty years. The time delays calculated with respect to a constant orbital period model show a clear parabolic trend, implying that the orbital period in this source constantly increases with time at a rate = 1.50(7) × 10^-10 s/s. This is 3 orders of magnitude larger than what is expected from conservative mass transfer driven by magnetic braking and gravitational radiation. From the conservation of the angular momentum of the system we find that to explain the high and positive value of the orbital period derivative the mass transfer rate must not be less than 3 times the Eddington limit for a neutron star, suggesting that the mass transfer has to be partially non-conservative. With the hypothesis that the neutron star accretes at the Eddington limit we find a consistent solution in which at least 70% of the transferred mass has to be expelled from the system.