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
.
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
.
For a magnetic field
strength of
1-5
G as derived from the X-ray
spectrum the implied intrinsic X-ray luminosity is
2-4
.
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
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 I5876. The
presence of an extra absorption component in the same part of the
spectrum and the relatively low resolution (
75 km/s for
)
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
.
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