44,748 research outputs found
An Ultra Low Mass and Small Radius Compact Object in 4U 1746-37?
Photospheric radius expansion (PRE) bursts have already been used to
constrain the masses and radii of neutron stars. RXTE observed three PRE bursts
in 4U 1746-37, all with low touchdown fluxes. We discuss here the possibility
of low mass neutron star in 4U 1746-37 because the Eddington luminosity depends
on stellar mass. With typical values of hydrogen mass fraction and color
correction factor, a Monte-Carlo simulation was applied to constrain the mass
and radius of neutron star in 4U 1746-37. 4U 1746-37 has a high inclination
angle. Two geometric effects, the reflection of the far side accretion disc and
the obscuration of the near side accretion disc have also been included in the
mass and radius constraints of 4U 1746-37. If the reflection of the far side
accretion disc is accounted, a low mass compact object (mass of
and radius of at 68% confidence)
exists in 4U 1746-37. If another effect operated, 4U 1746-37 may contain an
ultra low mass and small radius object
( at 68% confidence). Combined
all possibilities, the mass of 4U 1746-37 is at
99.7% confidence. For such low mass NS, it could be reproduced by a self-bound
compact star, i.e., quark star or quark-cluster star.Comment: accepted by Ap
Simultaneous constraints on the mass and radius of Aql X-1 from quiescence and X-ray burst observations
The measurement of neutron star mass and radius is one of the most direct way
to distinguish between various dense matter equations of state. The mass and
radius of accreting neutron stars hosted in low mass X-ray binaries can be
constrained by several methods, including photospheric radius expansion from
type-I X-ray bursts and from quiescent spectra. In this paper, we apply for the
first time these two methods simultaneously to constrain the mass and radius of
Aql X-1, as a reliable distance estimation, high signal-to-noise ratio
quiescent spectra from Chandra and XMM-Newton, and photospheric radius
expansion bursts from RXTE are available. This is also used to verify the
consistency between the two methods, and to narrow down the uncertainties of
the neutron star mass and radius. It is found that the distance to Aql X-1
should be in the range of kpc, based on the overlapping confidence
regions between photospheric radius expansion burst and quiescent spectra
methods. In addition, we show that the mass and radius determined for the
compact star in Aql X-1 are compatible with strange star equations of state and
conventional neutron star models.Comment: 16 pages, 9 figures, accepted by Ap
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