The newly discovered X-ray pulsar 4U 1822-37

Peter G. Jonker(University of Amsterdam) Michiel van der Klis (University of Amsterdam), Paul Groot (CfA),

Abstract

We report the discovery of 0.59 s X-ray pulsations from the low-mass X-ray binary, 5.57 hr dipping and eclipsing ADC source 2A 1822-371. Pulse arrival time analysis indicates a circular orbit with e <0.03 (95% confidence) and an asini for the neutron star of 1.006(5) lightseconds, implying a mass function of $(2.03\pm0.03)\times10^{-2} M_\odot$. The barycentric pulse period was 0.59325(2) s in 1996.270 and 0.59308615(5) s in 1998.205, indicating an average spin up with $\dot P/P = (-1.52\pm0.02)\times10^{-4} {\rm yr}^{-1}$. For a magnetic field strength of $\sim$1-5 $\times10^{12}$ G as derived from the X-ray spectrum the implied intrinsic X-ray luminosity is $\sim$2-4 $\times10^{37}{\rm erg\,s^{-1}}$. The pulse amplitude is low, but increases steeply as a function of energy from a sinusoidal amplitude of 0.25% in 2-5.4 keV to $\sim$3% above 20 keV.

4U 1822-37 is optically bright and has a well-constrained inclination (because it is eclipsing), measuring the radial velocity of the companion star would therefore allow for a full binary solution, providing the first accurate mass of a neutron star in an LMXB. Harlaftis, Charles, & Horne (1997, MNRAS, 285, 673) tried measuring the radial velocity of the companion in 4U 1822-37 using spectra obtained with the 3.9 m AAT using He I$\lambda$5876. The presence of an extra absorption component in the same part of the spectrum and the relatively low resolution ($\sim$75 km/s for $H\alpha$) which kept them from resolving the two components, yielded a lower limit on the radial velocity of the companion star of 225 km/s. This constrains the mass of the neutron star to be $>0.6^{+1.0}_{-0.3}M_\odot$. Here, we report on the discovery of the pulsar and of our recent attempts using the VLT and the 6.5 m Magellan to measure the radial velocity curve.

CATEGORY: BLACK HOLE AND NEUTRON STAR BINARIES