133 research outputs found
The gamma-ray spectrum of Centaurus A: A high-resolution observation between 70 keV and 8 MeV
The NASA/Goddard Space Flight Center Low Energy Gamma ray Spectrometer (LEGS) observed the nearby active nucleus galaxy Centaurus A (NGC 5128) during a balloon flight on 1981 November 19. There is no evidence of a break in the spectrum or of any line features. The 1.6 MeV limit is a factor of 8 lower than the 1974 line flux, indicating that, if the 1974 feature was real, and, if it was narrow, then the line intensity decreased significantly between 1974 and 1981. The lack of observed annihilation radiation from Cen A, combined with the temporal variations that are seen in the X-ray and gamma-ray intensities, constrain the size of the emission region to be between 10 to the 13th power and 5 x 10 to the 17th power cm
The Accretion Flows and Evolution of Magnetic Cataclysmic Variables
We have used a model of magnetic accretion to investigate the accretion flows
of magnetic cataclysmic variables. Numerical simulations demonstrate that four
types of flow are possible: discs, streams, rings and propellers. The
fundamental observable determining the accretion flow, for a given mass ratio,
is the spin-to-orbital period ratio of the system. If IPs are accreting at
their equilibrium spin rates, then for a mass ratio of 0.5, those with
Pspin/Porb < 0.1 will be disc-like, those with 0.1 < Pspin/Porb < 0.6 will be
stream-like, and those with Pspin/Porb ~ 0.6 will be ring-like. The spin to
orbital period ratio at which the systems transition between these flow types
increases as the mass ratio of the stellar components decreases.
For the first time we present evolutionary tracks of mCVs which allow
investigation of how their accretion flow changes with time. As systems evolve
to shorter orbital periods and smaller mass ratios, in order to maintain spin
equilibrium, their spin-to-orbital period ratio will generally increase. As a
result, the relative occurrence of ring-like flows will increase, and the
occurrence of disc-like flows will decrease, at short orbital periods. The
growing number of systems observed at high spin-to-orbital period ratios with
orbital periods below 2h, and the observational evidence for ring-like
accretion in EX Hya, are fully consistent with this picture.Comment: Accepted for publication in ApJ. 6 figures - included here at low
resolutio
High and low states of the system AM Herculis
Context: We investigate the distribution of optically high and low states of
the system AM Herculis (AM Her).
Aims: We determine the state duty cycles, and their relationships with the
mass transfer process and binary orbital evolution of the system.
Methods: We make use of the photographic plate archive of the Harvard College
Observatory between 1890 and 1953 and visual observations collected by the
American Association of Variable Star Observers between 1978 and 2005. We
determine the statistical probability of the two states, their distribution and
recurrence behaviors.
Results: We find that the fractional high state duty cycle of the system AM
Her is 63%. The data show no preference of timescales on which high or low
states occur. However, there appears to be a pattern of long and short duty
cycle alternation, suggesting that the state transitions retain memories. We
assess models for the high/low states for polars (AM Her type systems). We
propose that the white-dwarf magnetic field plays a key role in regulating the
mass transfer rate and hence the high/low brightness states, due to variations
in the magnetic-field configuration in the system.Comment: 8 pages, 5 figures, accepted for publication in A&
A 150MG magnetic white dwarf in the cataclysmic variable RX J1554.2+2721
We report the detection of Zeeman-split Lalpha absorption pi and sigma+ lines
in the far-ultraviolet Hubble Space Telescope/Space Telescope Imaging
Spectrograph spectrum of the magnetic cataclysmic variable RX J1554.2+2721.
Fitting the STIS data with magnetic white dwarf model spectra, we derive a
field strength of B~144MG and an effective temperature of 17000K<Teff<23000K.
This measurement makes RX J1554.2+2721 only the third cataclysmic variable
containing a white dwarf with a field exceeding 100MG. Similar to the other
high-field polar AR UMa, RX J1554.2+2721 is often found in a state of feeble
mass transfer, which suggests that a considerable number of high-field polars
may still remain undiscovered.Comment: 4 pages, accepted for ApJ Letter
The origin and fate of short-period low-mass black-hole binaries
We present results of a population synthesis study for semidetached short
orbital period binaries which contain low-mass(<1.5 Msun) donors and black hole
(>4 Msun) accretors. Evolution of these binaries is determined by nuclear
evolution of the donors and/or orbital angular momentum loss due to magnetic
braking by the stellar wind of the donors and gravitational wave radiation.
According to our model, the estimated total number of this type of black-hole
binaries in the Galaxy is about 10000. If the magnetic braking is described by
the Verbunt & Zwaan formula, the model predicts around 3000 transient systems
with periods >2 hours and around 300 luminous stable systems with periods
between 3 and 8 hours. Several dozens of these bright systems should be above
the RXTE ASM sensitivity limit. The absence of such systems implies that
angular momentum losses are reduced by a factor more than 2 with respect to the
Verbunt & Zwaan prescription. We show that it is unlikely that the transient
behaviour of black-hole short-period X-ray binaries is explained by the evolved
nature of the stellar companion. A substantial fraction of black-hole binaries
with periods >3 hours could be faint with truncated, stable cold accretion
discs as proposed by Menou et al. Most of the semidetached black-hole binaries
are expected to have periods shorter than ~2 hours. Properties of such, still
to be observed, very small mass-ratio (q<0.02) binaries are different from
those of their longer period cousins.Comment: 13 pages, 6 figures, accepted for publication in A&
Evolution of low-mass binaries with black-hole components
We consider evolutionary models for the population of short-period (<10 hr)
low-mass black-hole binaries (LMBHB) and compare them with observations of soft
X-ray transients (SXT). Evolution of LMBHB is determined by nuclear evolution
of the donors and/or orbital angular momentum loss due to magnetic braking by
the stellar wind of the donors and gravitational wave radiation. We show that
the absence of observed stable luminous LMBHB implies that upon RLOF by the
low-mass donor angular momentum losses are substantially reduced with respect
to the Verbunt and Zwaan "standard" prescription for magnetic braking. Under
this assumption masses and effective temperatures of the model secondaries of
LMBHB are in a satisfactory agreement with the masses and effective
temperatures (as inferred from their spectra) of the observed donors in LMBHB.
Theoretical mass-transfer rates in SXTs are consistent with the observed ones
if one assumes that accretion discs in these systems are truncated ("leaky").
We find that the population of short-period SXT is formed mainly by systems
which had unevolved or slightly evolved (X_c > 0.35) donors at the RLOF. Longer
period (0.5 - 1 day) SXT might descend from systems with initial donor mass
about 1 solar and X_c < 0.35. It is unnecessary to invoke donors with almost
hydrogen-depleted cores to explain the origin of LMBHB. Our models suggest that
a very high efficiency of common-envelopes ejection is necessary to form LMBHB,
unless currently commonly accepted empirical estimates of mass-loss rates by
winds for pre-WR and WR-stars are significantly over-evaluated.Comment: 11 pages. To appear in New Astronomy Review, vol. 51, issues 10-12,
Proceedings of "Jean-Pierre Lasota, X-ray binaries, accretion disks and
compact stars" (October 2007); Ed. M. Abramowicz; v3: Eq. (8) for upper limit
on mass-transfer rate and Figs. 4 and 7 correcte
The accretion flow in the discless intermediate polar V2400 Ophiuchi
RXTE observations confirm that the X-ray lightcurve of V2400 Oph is pulsed at
the beat cycle, as expected in a discless intermediate polar. There are no
X-ray modulations at the orbital or spin cycles, but optical line profiles vary
with all three cycles. We construct a model for line-profile variations in a
discless accretor, based on the idea that the accretion stream flips from one
magnetic pole to the other, and show that this accounts for the observed
behaviour over the spin and beat cycles. The minimal variability over the
orbital cycle implies that 1) V2400 Oph is at an inclination of only ~10 deg,
and 2) much of the accretion flow is not in a coherent stream, but is circling
the white dwarf, possibly as a ring of denser, diamagnetic blobs. We discuss
the light this sheds on disc formation in intermediate polars.Comment: 10 pages, 12 figures, To appear in MNRAS, includes low-res figures to
reduce siz
A half-a-day long thermonuclear X-ray burst from KS 1731-260
We report on an approximately twelve hour long X-ray flare from the low-mass
X-ray binary KS 1731-260. The flare has a rise time of less than 13 min and
declines exponentially with a decay time of 2.7 hours. The flare emission is
well described by black-body radiation with peak temperature of 2.4 keV. The
total energy release from the event is 10^{42} erg (for an assumed distance of
7 kpc). The flare has all the characteristics of thermo-nuclear X-ray bursts
(so-called type I X-ray bursts), except for its very long duration and
therefore large energy release (factor of 1500-4000 longer and 250-425 more
energy than normal type I X-ray bursts from this source). The flare is preceded
by a short and weak X-ray burst, possibly of type I. Days to weeks before the
flare, type I X-ray bursts were seen at a rate of ~3 per day. However, after
the flare type I X-ray bursting ceased for at least a month, suggesting that
the superburst affected the type I bursting behaviour. The persistent emission
is not significantly different during the non-bursting period. We compare the
characteristics of this event with similar long X-ray flares, so-called
superbursts, seen in other sources (4U 1735-44, 4U 1820-30, 4U 1636-53, Ser
X-1, GX 3+1). The event seen from KS 1731-260 is the longest reported so far.
We discuss two possible mechanisms that might cause these superbursts, unstable
carbon burning (as proposed recently) and electron capture by protons with
subsequent capture of the resulting neutrons by heavy nuclei.Comment: 10 pages, 4 figures, accepted for publication in A&
The fight for accretion: discovery of intermittent mass transfer in BB Doradus in the low state
Our long-term photometric monitoring of southern nova-like cataclysmic
variables with the 1.3-m SMARTS telescope found BB Doradus fading from V ~ 14.3
towards a deep low state at V ~ 19.3 in April 2008. Here we present
time-resolved optical spectroscopy of BB Dor in this faint state in 2009. The
optical spectrum in quiescence is a composite of a hot white dwarf with Teff =
30000 +- 5000 K and a M3-4 secondary star with narrow emission lines (mainly of
the Balmer series and HeI) superposed. We associate these narrow profiles with
an origin on the donor star. Analysis of the radial velocity curve of the
H-alpha emission from the donor star allowed the measurement of an orbital
period of 0.154095 +- 0.000003 d (3.69828 +- 0.00007 h), different from all
previous estimates. We detected episodic accretion events which veiled the
spectra of both stars and radically changed the line profiles within a
timescale of tens of minutes. This shows that accretion is not completely
quenched in the low state. During these accretion episodes the line wings are
stronger and their radial velocity curve is delayed by ~ 0.2 cycle, similar to
that observed in SW Sex and AM Her stars in the high state, with respect to the
motion of the white dwarf. Two scenarios are proposed to explain the extra
emission: impact of the material on the outer edge of a cold, remnant accretion
disc, or the combined action of a moderately magnetic white dwarf (B1 <~ 5 MG)
and the magnetic activity of the donor star.Comment: 10 pages, 10 figures, accepted by MNRA
The Nature of the Compact X-ray Source in Supernova Remnant RCW 103
The discovery of the 6.67 hr periodicity in the X-ray source 1E 161348-5055
associated with the supernova remnant RCW 103 has raised interesting
suggestions about the nature of the X-ray source. Here we argue that in either
accreting neutron star or magnetar model, a supernova fallback disk may be a
critical ingredient in theoretical interpretations of 1E 161348-5055. We
further emphasize the effect of fallback disks on the evolution of young
compact objects in various ways, and suggest that even SS 433 could also be
powered by fallback disk accretion process.Comment: 12 pages, 1 figure, accepted to ApJ
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