Neutron star masses and system parameters of 10 eclipsing high-mass X-ray binaries
High-mass X-ray binaries (HMXBs) are among the brightest X-ray sources in
our Galaxy. In these systems, a neutron star is accreting mass from a
massive companion star. Matter is captured directly from the companion wind
and, in some cases, through the mediation of an accretion disk. When it falls
to the neutron star, it is heated up to millions of degrees and therefore
emits large amounts of X-rays.
Among the more than hundred HMXBs, only a few sources have an inclination
high enough so that the companion periodically occults the neutron star,
giving rise to X-ray eclipses. For these sources, it is possible to infer
with higher precision various orbital parameters, as well as the mass of
the neutron star, from the measured duration of the eclipse.
As most HMXBs have rather long (1-10 days) orbital periods, this requires
extensive observations. Taking advantage of the available long-term
monitoring observations of ten eclipsing HMXBs, carried out with the hard
X-ray imager on-board INTEGRAL, IBIS/ISGRI, in the 17-150 keV band, and the
All Sky Monitor (ASM; 2-12 keV) on-board RXTE, it was possible to obtain the
most accurate ephemeris available so far for these sources.
INTEGRAL, with an accumulated observation times ranging from 200 to 1100 hours
for the different sources, has provided for the first time accurate mean
orbital light curves at higher (17-150 keV) energy, revealing that the
eclipses are much less affected and distorted by asymmetric absorption than
at lower energy (see figure, lower left). All high-energy eclipses look
sharp and symmetric, thus permitting to achieve an unprecedented accuracy
in the determination of the occultation time.
More accurate eclipse durations have been derived, leading to revised values
of neutron star masses (see figure, lower right). In most cases, the measured
eclipses are significantly shorter than previously reported in the literature,
and thus the estimated NS masses are generally larger than previously thought.
Though the measured masses still cover a rather wide range of
(1.0-2.1) M⊙, a major fraction is beyond the canonical
1.4 M⊙ value. The exact equation of state (EoS) of neutron
stars is still not known. Very soft (EoSs) predict maximum NS masses in the
1.4-1.5 M⊙ range (this occurs when the NS core is made by
exotic matter as kaons, hyperons, and pions), whereas stiff EoSs can reach
up to 2.4-2.5 M⊙.
The new results seem to favor stiff EoSs. They also constitute a database
to be used for population and evolutionary studies of HMXBs, as well as
theoretical modelling of long-term accretion in wind-fed X-ray binaries.
Credits:
"The ephemeris, orbital decay, and masses of 10 eclipsing HMXBs"
Falanga, M., Bozzo, E., Lutovinov, A., Bonnet-Bidaud, J.M., Fetisova, Y. & Puls, J.,
2015, Accepted for publication on A&A, arXiv:1502.07126
http://xxx.lanl.gov/abs/1502.07126