652 research outputs found
Disc-loss episode in the Be shell optical counterpart to the high-mass X-ray binary IGR J21343+4738
The main goal of this work is to determine the properties of the optical
counterpart to the INTEGRAL source IGR J21343+4738, and study its long-term
optical variability. We present optical photometric BVRI and spectroscopic
observations covering the wavelength band 4000-7500 A. We find that the optical
counterpart to IGR J21343+4738 is a V=14.1 B1IVe shell star located at a
distance of ~8.5 kpc. The Halpha line changed from an absorption dominated
profile to an emission dominated profile, and then back again into absorption.
In addition, fast V/R asymmetries were observed once the disc developed.
Although the Balmer lines are the most strongly affected by shell absorption,
we find that shell characteristics are also observed in He I lines. The optical
spectral variability of IGR J21343+4738 is attributed to the formation of an
equatorial disc around the Be star and the development of an enhanced density
perturbation that revolves inside the disc. We have witnessed the formation and
dissipation of the circumstellar disc. The strong shell profile of the Halpha
and He I lines and the fact that no transition from shell phase to a pure
emission phase is seen imply that we are seeing the system near edge-on.Comment: accepted for publication in A&
Double Neutron Star Formation: Merger Times, Systemic Velocities, and Travel Distances
The formation and evolution of double neutron stars (DNS) have traditionally
been studied using binary population synthesis. In this work, we take an
alternative approach by focusing only on the second supernova (SN) forming the
DNS and the subsequent orbital decay and merger due to gravitational wave
radiation. Using analytic and numerical methods, we explore how different NS
natal kick velocity distributions, pre-SN orbital separations, and progenitor
He-star masses affect the post-SN orbital periods, eccentricities, merger
times, systemic velocities, and distances traveled by the system before
merging. Comparison with the set of 17 known DNSs in the Milky Way shows that
DNSs have pre-SN orbital separations ranging between 1 and 44 .
Those DNSs with pre-SN separations 1 have merger time
distributions that peak 10-100 Myr after formation, regardless of the
kick velocity received by the NS. These DNSs are typically formed with systemic
velocities 10 km s and may travel 1-10 kpc before
merging. Depending on progenitor mass of the second-born NS, the short merger
time can account for the -process enrichment observed in compact stellar
systems such as ultra-faint dwarf galaxies. For Milky Way-mass galaxies only
DNSs with the tightest pre-SN orbits and large kick velocities (10
km s) can escape. However, those DNSs that do escape may travel as far
as Mpc before merging, which as previous studies have pointed out has
implications for identifying the host galaxies to short gamma ray bursts and
gravitational wave events.Comment: 16 pages, 10 figures, accepted for publication in MNRA
ROSAT observations of the dwarf starforming galaxy Holmerg II (UGC 4305)
We present ROSAT PSPC and HRI observations of the dwarf irregular galaxy
Holmberg II (UGC4305). This is one of the most luminous dwarf galaxies (Lx~
10^{40} erg s^{-1} cm^{-2}) detected in the ROSAT All-Sky Survey. The X-ray
emission comes from a single unresolved point source, coincident with a large
HII region which emits intense radio emission. The source is variable on both
year and day timescales, clearly favouring accretion into a compact object
rather than a supernova remnant or a superbubble interpretation for the origin
of the X-ray emission. However, its X-ray spectrum is well-fit by a a
Raymond-Smith spectrum with kT~0.8 keV, lower than the temperature of X-ray
binaries in nearby spiral galaxies.Comment: Accepted for publication in MNRA
Broad-band X-ray spectra of anomalous X-ray pulsars and soft -ray repeaters: pulsars in a weak-accretion regime ?
We present the results from the analysis of the broad-band X-ray spectra of 5
Anomalous X-ray Pulsars (AXPs) and Soft -ray Repeaters (SGRs). We fit
their Suzaku and INTEGRAL spectra with models appropriate for the X-ray
emission from the accretion flow onto a pulsar. We find that their X-ray
spectra can be well described with this model. In particular we find that: (a)
the radius of the accretion column is m resulting in a transverse
optical depth of ; (b) the vertical Thompson optical depth is , and (c) their luminosity translates in accretion rates
. These results are in good agreement with the
predictions from the fall-back disk model, providing further support in the
interpretation of AXPs and SGRs as accreting pulsars.Comment: Accepted for publication in MNRAS, 10 pages, 2 figure
- …