3,224 research outputs found
Pancyclicity of Hamiltonian line graphs
Let f(n) be the smallest integer such that for every graph G of order n with minimum degree 3(G)>f(n), the line graph L(G) of G is pancyclic whenever L(G) is hamiltonian. Results are proved showing that f(n) = ®(n 1/3)
Could 2S 0114+650 be a magnetar?
We investigate the spin evolution of the binary X-ray pulsar 2S 0114+650,
which possesses the slowest known spin period of hours. We argue
that, to interpret such long spin period, the magnetic field strength of this
pulsar must be initially \gsim 10^{14} G, that is, it was born as a magnetar.
Since the pulsar currently has a normal magnetic field G, our
results present support for magnetic field decay predicted by the magnetar
model.Comment: 7 pages, 1 figure, accepted for publication in ApJ
Long Gamma-Ray Burst Progenitors: Boundary Conditions and Binary Models
The observed association of Long Gamma-Ray Bursts (LGRBs) with peculiar Type
Ic supernovae gives support to Woosley`s collapsar/hypernova model, in which
the GRB is produced by the collapse of the rapidly rotating core of a massive
star to a black hole. The association of LGRBs with small star-forming galaxies
suggests low-metallicity to be a condition for a massive star to evolve to the
collapsar stage. Both completely-mixed single star models and binary star
models are possible. In binary models the progenitor of the GRB is a massive
helium star with a close companion. We find that tidal synchronization during
core-helium burning is reached on a short timescale (less than a few
millennia). However, the strong core-envelope coupling in the subsequent
evolutionary stages is likely to rule out helium stars with main-sequence
companions as progenitors of hypernovae/GRBs. On the other hand, helium stars
in close binaries with a neutron-star or black-hole companion can, despite the
strong core-envelope coupling in the post-helium burning phase, retain
sufficient core angular momentum to produce a hypernova/GRB.Comment: 8 pp., 2 figs, Proceedings of 5th Stromlo Symposiu
New Direct Observational Evidence for Kicks in SNe
We present an updated list of direct strong evidence in favour of kicks being
imparted to newborn neutron stars. In particular we discuss the new cases of
evidence resulting from recent observations of the X-ray binary Circinus X-1
and the newly discovered binary radio pulsar PSR J1141-6545. We conclude that
the assumption that neutron stars receive a kick velocity at their formation is
unavoidable (van den Heuvel & van Paradijs 1997).Comment: 2 pages, to appear in the proceedings of the IAU Colloq. 177 "Pulsar
Astronomy - 2000 and beyond
A catalogue of low-mass X-ray binaries in the Galaxy, LMC, and SMC (Fourth edition)
We present a new edition of the catalogue of the low-mass X-ray binaries in
the Galaxy and the Magellanic Clouds. The catalogue contains source name(s),
coordinates, finding chart, X-ray luminosity, system parameters, and stellar
parameters of the components and other characteristic properties of 187
low-mass X-ray binaries, together with a comprehensive selection of the
relevant literature. The aim of this catalogue is to provide the reader with
some basic information on the X-ray sources and their counterparts in other
wavelength ranges (-rays, UV, optical, IR, and radio). Some sources,
however, are only tentatively identified as low-mass X-ray binaries on the
basis of their X-ray properties similar to the known low-mass X-ray binaries.
Further identification in other wavelength bands is needed to finally determine
the nature of these sources. In cases where there is some doubt about the
low-mass nature of the X-ray binary this is mentioned. Literature published
before 1 October 2006 has, as far as possible, been taken into account.Comment: 45 pages, catalogue include
Discovery of Two Relativistic Neutron Star-White Dwarf Binaries
We have discovered two recycled pulsars in relativistic orbits as part of the
first high-frequency survey of intermediate Galactic latitudes. PSR J1157-5112
is a 44 ms pulsar and the first recycled pulsar with an ultra-massive (M > 1.14
Mo) white dwarf companion. Millisecond pulsar J1757-5322 is a relativistic
circular-orbit system which will coalesce due to the emission of gravitational
radiation in less than 9.5 Gyr. Of the ~40 known circular orbit pulsars,
J1757-5322 and J1157-5112 have the highest projected orbital velocities. There
are now three local neutron-star/white-dwarf binaries that will coalesce in
less than a Hubble time, implying a large coalescence rate for these objects in
the local Universe. Systems such as J1141-6545 (Kaspi et al. 2000) are
potential gamma-ray burst progenitors and dominate the coalescence rate, whilst
lighter systems make excellent progenitors of millisecond pulsars with
planetary or ultra-low mass companions.Comment: 4 pages, to appear in ApJ Letters. Uses aastex v 5.0,
emulateapj5.sty, apjfonts.st
Degree Sequences and the Existence of -Factors
We consider sufficient conditions for a degree sequence to be forcibly
-factor graphical. We note that previous work on degrees and factors has
focused primarily on finding conditions for a degree sequence to be potentially
-factor graphical.
We first give a theorem for to be forcibly 1-factor graphical and, more
generally, forcibly graphical with deficiency at most . These
theorems are equal in strength to Chv\'atal's well-known hamiltonian theorem,
i.e., the best monotone degree condition for hamiltonicity. We then give an
equally strong theorem for to be forcibly 2-factor graphical.
Unfortunately, the number of nonredundant conditions that must be checked
increases significantly in moving from to , and we conjecture that
the number of nonredundant conditions in a best monotone theorem for a
-factor will increase superpolynomially in .
This suggests the desirability of finding a theorem for to be forcibly
-factor graphical whose algorithmic complexity grows more slowly. In the
final section, we present such a theorem for any , based on Tutte's
well-known factor theorem. While this theorem is not best monotone, we show
that it is nevertheless tight in a precise way, and give examples illustrating
this tightness.Comment: 19 page
The Peculiar Evolutionary History of IGR J17480-2446 in Terzan 5
The low mass X-ray binary (LMXB) IGR J17480-2446 in the globular cluster
Terzan 5 harbors an 11 Hz accreting pulsar. This is the first object discovered
in a globular cluster with a pulsar spinning at such low rate. The accreting
pulsar is anomalous because its characteristics are very different from the
other five known slow accreting pulsars in galactic LMXBs. Many features of the
11 Hz pulsar are instead very similar to those of accreting millisecond
pulsars, spinning at frequencies >100 Hz. Understanding this anomaly is
valuable because IGR J17480-2446 can be the only accreting pulsar discovered so
far which is in the process of becoming an accreting millisecond pulsar. We
first verify that the neutron star (NS) in IGR J17480-2446 is indeed spinning
up by carefully analyzing X-ray data with coherent timing techniques that
account for the presence of timing noise. We then study the present Roche lobe
overflow epoch and the two previous spin-down epochs dominated by magneto
dipole radiation and stellar wind accretion. We find that IGR J17480-2446 is
very likely a mildly recycled pulsar and suggest that it has started a spin-up
phase in an exceptionally recent time, that has lasted less than a few 10^7 yr.
We also find that the total age of the binary is surprisingly low (<10^8 yr)
when considering typical parameters for the newborn NS and propose different
scenarios to explain this anomaly.Comment: Accepted by ApJ, in pres
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