2,264 research outputs found
Bounds on Compactness for LMXB Neutron Stars from X-ray Burst Oscillations
We have modelled X-ray burst oscillations observed with the Rossi X-ray
Timing Explorer (RXTE) from two low mass X-ray binaries (LMXB): 4U 1636-53 with
a frequency of 580 Hz, and 4U 1728-34 at a frequency of 363 Hz. We have
computed least squares fits to the oscillations observed during the rising
phase of bursts using a model which includes emission from either a single
circular hot spot or a pair of circular antipodal hot spots on the surface of a
neutron star. We model the spreading of the thermonuclear hot spots by assuming
that the hot spot angular size grows linearly with time. We calculate the flux
as a function of rotational phase from the hot spots and take into account
photon deflection in the relativistic gravitational field of the neutron star
assuming the exterior spacetime is the Schwarzschild metric. We find acceptable
fits with our model and we use these to place constraints on the compactness of
the neutron stars in these sources. For 4U 1636-53, in which detection of a 290
Hz sub-harmonic supports the two spot model, we find that the compactness
(i.e., mass/radius ratio) is constrained to be M/R < 0.163 at 90% confidence (G
= c = 1). This requires a relatively stiff equation of state (EOS) for the
stellar interior. For example, if the neutron star has a mass of 1.4 Msun then
its radius must be > 12.8 km. Fits using a single hot spot model are not as
highly constraining. We discuss the implications of our findings for recent
efforts to calculate the EOS of dense nucleon matter and the structure of
neutron stars.Comment: 27 pages, 6 figures, AASTeX. Revised and expanded version.
Resubmitted to Astrophysical Journa
Sus scrofa: Population Structure, Reproduction and Condition in Tropical North Eastern Australia
Three feral pig populations inhabiting contrasting environments along the north easterncoast of Australia have been investigated with respect to population structure, individual condition andreproduction. The population on Prince of Wales Island contains a large proportion of juvenile andsub-adult pigs but lacks pigs in the higher age classes. Individuals also breed at an earlier age thananimals of the mainland populations. Pig populations on Cape York Peninsula show a largerproportion of older animals and feral pigs living in rainforest habitats show a low proportion ofanimals in very young and very old age classes. Pigs from the lowland rainforest population are inbetter condition than those of the other populations for most of the year, reflecting the availability offood all year round in this environment. Differences in the population structure of the threepopulations are discussed with respect to fecundity and several mortality factors such as predation anddiseases/parasites
A Search for X-Ray Flashes with XMM-Newton
We searched for X-ray flashes (XRFs) -- which we defined as ~10s duration
transient X-ray events observable in the 0.4-15 keV passband -- in fields
observed using XMM-Newton with the EPIC/pn detector. While we find two
non-Poissonian events, the astrophysical nature of the events is not confirmed
in fully simultaneous observations with the EPIC/MOS detectors, and we conclude
that the events are anomalous to the EPIC/pn detector. We find a 90% upper
limit on the number of flashes per sky per year at two different incoming flash
fluxes: 4.0x10^9 events / sky / year for a flux of 7.1x10^-13 erg / cm^2 / s
and 6.8x10^7 events / sky / year for 1.4x10^-11 erg / cm^2 / s. These limits
are consistent with an extrapolation from the BeppoSAX/WFC XRF rate at much
higher fluxes (about a factor of 10^5), assuming an homogenous population, and
with a previous, more stringent limit derived from ROSAT pointed observations.Comment: Version accepted by MNRAS (minor changes
New BeppoSAX-WFC results on superbursts
Presently seven superbursters have been identified representing 10% of the
total Galactic X-ray burster population. Four superbursters were discovered
with the Wide Field Cameras (WFCs) on BeppoSAX and three with the All-Sky
Monitor and Proportional Counter Array on RXTE. We discuss the properties of
superbursters as derived from WFC observations. There are two interesting
conclusions. First, the average recurrence time of superbursts among X-ray
bursters that are more luminous than 10% of the Eddington limit is 1.5 yr per
object. Second, superbursters systematically have higher alpha values and
shorter ordinary bursts than most bursters that have not exhibited superbursts,
indicating a higher level of stable thermonuclear helium burning. Theory
predicts hitherto undetected superbursts from the most luminous neutron stars.
We investigate the prospects for finding these in GX~17+2.Comment: Submitted in January 2004 for the Proceedings of the meeting 'X-Ray
Timing 2003: Rossi and Beyond', eds. P. Kaaret, F. K. Lamb, & J. H. Swank
(Melville, NY: American Institute of Physics
Carbon Flashes in the Heavy Element Ocean on Accreting Neutron Stars
We show that burning of a small mass fraction of carbon in a neutron star
ocean is thermally unstable at low accumulated masses when the ocean contains
heavy ashes from the hydrogen burning rapid proton (rp) process. The key to
early unstable ignition is the low thermal conductivity of a heavy element
ocean. The instability requires accretion rates in excess of one-tenth the
Eddington limit when the carbon mass fraction is 0.1 or less. The unstable
flashes release 10^{42} to 10^{43} ergs over hours to days, and are likely the
cause of the recently discovered large Type I X-ray bursts (so-called
``superbursts'') from six Galactic low mass X-ray binaries. In addition to
explaining the energetics, recurrence times, and durations of the superbursts,
these mixed carbon/heavy element flashes have an accretion rate dependence of
unstable burning similar to that observed. Though the instability is present at
accretion rates near Eddington, there is less contrast with the accretion
luminosity there, explaining why most detections are made at accretion rates
between 0.1 and 0.3 Eddington. Future comparisons of time dependent
calculations with observations will provide new insights into the rp process.Comment: Submitted to Astrophysical Journal Letters (6 pages, 3 figures
Discovery of a Second Millisecond Accreting Pulsar: XTE J1751-305
We report the discovery by the RXTE PCA of a second transient accreting
millisecond pulsar, XTE J1751-305, during regular monitoring observations of
the galactic bulge region. The pulsar has a spin frequency of 435 Hz, making it
one of the fastest pulsars. The pulsations contain the signature of orbital
Doppler modulation, which implies an orbital period of 42 minutes, the shortest
orbital period of any known radio or X-ray millisecond pulsar. The mass
function, f_x = (1.278 +/- 0.003) x 10^{-6} M_sun, yields a minimum mass for
the companion of between 0.013 and 0.017 M_sun, depending on the mass of the
neutron star. No eclipses were detected. A previous X-ray outburst in June,
1998, was discovered in archival All-Sky Monitor data. Assuming mass transfer
in this binary system is driven by gravitational radiation, we constrain the
orbital inclination to be in the range 30-85 deg, and the companion mass to be
0.013-0.035 M_sun. The companion is most likely a heated helium dwarf. We also
present results from the Chandra HRC-S observations which provide the best
known position of XTE J1751-305.Comment: Astrophysical Journal Letters, Accepted, (AASTeX
Radio Monitoring of the January 11, 1997 Gamma-Ray Burst
We report on a comprehensive radio monitoring program of the bright gamma-ray
burster GRB970111. These VLA observations were made at a frequency of 1.4 GHz
and span a range of post-burst timescales between 28 hours and one month.
Despite extensive sampling at sub-milliJansky sensitivities, no radio source
was detected above 0.5 mJy in the current best error box (~14 arcmin^2) for
GRB970111. A highly unusual radio source, VLA J1528.7+1945, was seen to drop in
flux density by a factor of two in our monitoring period but it lies outside
the error box and thus it is unlikely to be related to GRB970111. Cosmological
fireball models of gamma-ray bursts make predictions of late-time emission
occurring at longer wavelengths. The absence of a flaring or fading radio
counterpart to GRB970111 provides strong constraints on these models.Comment: ApJ Let (accepted
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