133,386 research outputs found
Correlated X-ray Spectral and Timing Behavior of the Black Hole Candidate XTE J1550-564: A New Interpretation of Black Hole States
We present an analysis of RXTE data of the X-ray transient XTE J1550-564. The
source went through several states, which were divided into spectrally soft and
hard states. These states showed up as distinct branches in the color-color
diagram, forming a structure with a comb-like topology; the soft state branch
forming the spine and the hard state branches forming the teeth. Variability
was strongly correlated with the position on the branches. The broad band noise
became stronger, and changed from power law like to band limited, as the
spectrum became harder. Three types of QPOs were found: 1-18 Hz and 102-284 Hz
QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. The
frequencies of the high and low frequency QPOs on the hard branches were
correlated with each other, and anti-correlated with spectral hardness. The
changes in QPO frequency suggest that the inner disc radius only increases by a
factor of 3-4 as the source changes from a soft to a hard state. Our results on
XTE J1550-564 strongly favor a 2-dimensional description of black hole
behavior, where the regions near the spine of the comb in the color-color
diagram can be identified with the high state, and the teeth with transitions
from the high state, via the intermediate state (which includes the very high
state) to the low state, and back. The two physical parameters underlying this
behavior vary to a large extent independently and could for example be the mass
accretion rate through the disk and the size of a Comptonizing region.Comment: 49 pages (inlcuding 26 figures and 4 tables), accepted for
publication in ApJ Supplement
RXTE observations of the dipping low-mass X-ray binary 4U 1624-49
We analyse ~ 360 ks of archival data from the Rossi X-Ray Timing Explorer
(RXTE) of the 21 hr orbital period dipping low-mass X-ray binary 4U 1624-49. We
find that outside the dips the tracks in the colour-colour and
hardness-intensity diagrams (CDs and HIDs) are reminiscent of those of atoll
sources in the middle and upper parts of the banana branch. The tracks show
secular shifts up to ~ 10%. We study the power spectrum of 4U 1624-49 as a
function of the position in the CD. This is the first time power spectra of
this source are presented. No quasi-periodic oscillations (QPOs) are found. The
power spectra are dominated by very low frequency noise (VLFN), characteristic
for atoll sources in the banana state, and band limited noise (BLN) which is
not reliably detected but may, uncharacteristically, strengthen and increase in
frequency with spectral hardness. The VLFN fits to a power law, which becomes
steeper when the source moves to the harder part of the CD. We conclude that 4U
1624-49 is an atoll source which in our observations is in the upper banana
branch. Combining this with the high (0.5-0.7 L_Edd) luminosity, the long-term
flux stability of the source as seen with the RXTE All-Sky Monitor (ASM), and
with the fact that it is an X-ray dip source, we conclude that 4U 1624-49 is
most likely a GX atoll source such as GX 3+1 and GX 9+9, but seen edge on.Comment: 8 pages, 5 figures, 2 tables, accepted for publication in A&A. This
version: a few typos correcte
Multi-Conjugate Adaptive Optics images of the Trapezium Cluster
Multi-Conjugate Adaptive Optics (MCAO) combines the advantages of standard
adaptive optics, which provides high contrast and high spatial resolution, and
of wide field ~1' imaging. Up to recently, MCAO for astronomy was limited to
laboratory experiments. In this paper, we present the first scientific results
obtained with the first MCAO instrument put on the sky. We present a new study
of the Trapezium cluster using deep MCAO images with a field of view of 1'x1'
obtained at the VLT. We have used deep J, H and Ks images recently obtained
with the prototype MCAO facility MAD at the VLT in order to search for new
members and new multiple systems in the Trapezium cluster. On bright targets
(Ks~9mag), these images allow us to reach DeltaKs~6mag as close as 0.4" We
report the detection of 128 sources, including 10 new faint objects in the
magnitude range between 16.1<Ks<17.9mag. In addition to all previously known
multiple systems with separations greater than 0.1", we confirm the
multiplicity of TCC-055. We also report the detection in J, H and Ks of a very
red extended embedded protostellar object, HC419, previously detected in the
thermal infrared only. The analysis of the first MCAO images obtained on the
sky demonstrates not only the technical feasibility of MCAO but also its great
potential and versatility in terms of scientific outputs.Comment: High resolution version available on
http://arrakeen.free.fr/pub/madorion.pdf Accepted 25 Sep. 2007 for
publication in A&A, 14 pages, 11 figure
The Star Formation Rate of Turbulent Magnetized Clouds: Comparing Theory, Simulations, and Observations
We derive and compare six theoretical models for the star formation rate
(SFR) - the Krumholz & McKee (KM), Padoan & Nordlund (PN), and Hennebelle &
Chabrier (HC) models, and three multi-freefall versions of these, suggested by
HC - all based on integrals over the log-normal distribution of turbulent gas.
We extend all theories to include magnetic fields, and show that the SFR
depends on four basic parameters: (1) virial parameter alpha_vir; (2) sonic
Mach number M; (3) turbulent forcing parameter b, which is a measure for the
fraction of energy driven in compressive modes; and (4) plasma beta=2(M_A/M)^2
with the Alfven Mach number M_A. We compare all six theories with MHD
simulations, covering cloud masses of 300 to 4x10^6 solar masses and Mach
numbers M = 3 to 50 and M_A = 1 to infinity, with solenoidal (b=1/3), mixed
(b=0.4) and compressive turbulent (b=1) forcings. We find that the SFR
increases by a factor of four between M=5 and 50 for compressive forcing and
alpha_vir~1. Comparing forcing parameters, we see that the SFR is more than 10x
higher with compressive than solenoidal forcing for Mach 10 simulations. The
SFR and fragmentation are both reduced by a factor of two in strongly
magnetized, trans-Alfvenic turbulence compared to hydrodynamic turbulence. All
simulations are fit simultaneously by the multi-freefall KM and multi-freefall
PN theories within a factor of two over two orders of magnitude in SFR. The
simulated SFRs cover the range and correlation of SFR column density with gas
column density observed in Galactic clouds, and agree well for star formation
efficiencies SFE = 1% to 10% and local efficiencies epsilon = 0.3 to 0.7 due to
feedback. We conclude that the SFR is primarily controlled by interstellar
turbulence, with a secondary effect coming from magnetic fields.Comment: 34 pages, 12 figures, ApJ in press, movies at
http://www.ita.uni-heidelberg.de/~chfeder/pubs/sfr/sfr.shtm
Placing limits on the stochastic gravitational-wave background using European Pulsar Timing Array data
Direct detection of low-frequency gravitational waves (
Hz) is the main goal of pulsar timing array (PTA) projects. One of the main
targets for the PTAs is to measure the stochastic background of gravitational
waves (GWB) whose characteristic strain is expected to approximately follow a
power-law of the form , where is the
gravitational-wave frequency. In this paper we use the current data from the
European PTA to determine an upper limit on the GWB amplitude as a function
of the unknown spectral slope with a Bayesian algorithm, by modelling
the GWB as a random Gaussian process. For the case , which is
expected if the GWB is produced by supermassive black-hole binaries, we obtain
a 95% confidence upper limit on of , which is 1.8 times
lower than the 95% confidence GWB limit obtained by the Parkes PTA in 2006. Our
approach to the data analysis incorporates the multi-telescope nature of the
European PTA and thus can serve as a useful template for future
intercontinental PTA collaborations.Comment: 14 pages, 8 figures, 3 tables, mnras accepte
Searching for new hypercompact HII regions
Hypercompact (HC) HII regions are, by nature, very young HII regions,
associated with the earliest stages of massive star formation. They may
represent the transition phase as an early B-type star grows into an O-type
star. Unfortunately, so few HCHII regions are presently known that their
general attributes and defining characteristics are based on small number
statistics. A larger sample is needed for detailed studies and good statistics.
Class II methanol masers are one of the best indicators of the early stages of
massive star formation. Using the Arecibo Methanol Maser Galactic Plane Survey
- the most sensitive blind survey for 6.7 GHz methanol masers to date - we
selected 24 HCHII region candidates. We made EVLA continuum observations at 3.6
and 1.3 cm to search for HCHII regions associated with these masers. We
identified six potential HCHII regions in our sample based on the presence of
optically thick free-free emission. Overall, we find that 30% of the methanol
masers have an associated centimeter radio continuum source (separation less
than 0.1 pc), which is in general agreement with previous studies.Comment: Accepted for publication in ApJL, in the Special Issue of the EVL
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