791 research outputs found
GX 339-4: back to life
We report preliminary results of a RossiXTE campaign on the 2002 outburst of
the black-hole candidate GX 339-4. We show power density spectra of five
observations during the early phase of the outburst. The first four power
spectra show a smooth transition between a Low State and a Very High State. The
fifth power spectrum resembles a High State, but a strong 6 Hz QPO appears
suddenly within 16 seconds.Comment: 3 pages, 3 figures; to appear in Proceedings of the 4th Microquasar
Workshop, eds. Ph Durouchoux, Y. Fuchs and J. Rodriguez, published by the
Center for Space Physics: Kolkat
Black hole candidate XTE J1752-223: Swift observations of canonical states during outburst
We present Swift broadband observations of the recently discovered black hole
candidate, X-ray transient, XTE J1752-223, obtained over the period of outburst
from October 2009 to June 2010. From Swift-UVOT data we confirm the presence of
an optical counterpart which displays variability correlated, in the soft
state, to the X-ray emission observed by Swift-XRT. The optical counterpart
also displays hysteretical behaviour between the states not normally observed
in the optical bands, suggesting a possible contribution from a synchrotron
emitting jet to the optical emission in the rising hard state. We offer a
purely phenomenological treatment of the spectra as an indication of the
canonical spectral state of the source during different periods of the
outburst. We find that the high energy hardness-intensity diagrams over two
separate bands follows the canonical behavior, confirming the spectral states.
Our XRT timing analysis shows that in the hard state there is significant
variability below 10Hz which is more pronounced at low energies, while during
the soft state the level of variability is consistent with being minimal. These
properties of XTE J1752-223 support its candidacy as a black hole in the
Galactic centre region.Comment: 8 pages, 8 figures; MNRAS in pres
Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
We propose a hybrid model to fit the X-ray spectra of atoll-type X-ray
transients in the soft and hard states. This model uniquely produces luminosity
tracks that are proportional to T^4 for both the accretion disk and boundary
layer. The model also indicates low Comptonization levels for the soft state,
gaining a similarity to black holes in the relationship between Comptonization
level and the strength of integrated rms variability in the power density
spectrum. The boundary layer appears small, with a surface area that is roughly
constant across soft and hard states. This result may suggestion that the NS
radius is smaller than its inner-most stable circular orbit.Comment: 15 pages, 15 figures, accepted for publication in the Ap
Emotional reactions to deviance in groups: the relation between number of angry reactions, felt rejection, and conformity
How many members of a group need to express their anger in order to influence a deviant group member's behavior? In two studies, we examine whether an increase in number of angry group members affects the extent to which a deviant individual feels rejected, and we investigate downstream effects on conformity. We show that each additional angry reaction linearly increases the extent to which a deviant individual feels rejected, and that this relation is independent of the total number of majority members (Study 1). This felt rejection is then shown to lead to anti-conformity unless two conditions are met: (1) the deviant is motivated to seek reacceptance in the group, and (2) conformity is instrumental in gaining reacceptance because it is observable by the majority (Study 2). These findings show that angry reactions are likely to trigger anti-conformity in a deviant, but they are also consistent with a motivational account of conformity, in which conformity is strategic behavior aimed at gaining reacceptance from the group
Relativistic Beaming and the Intrinsic Properties of Extragalactic Radio Jets
Relations between the observed quantities for a beamed radio jet, apparent
transverse speed and apparent luminosity (beta_app,L), and the intrinsic
quantities, Lorentz factor and intrinsic luminosity (gamma,L_o), are
investigated. The inversion from measured to intrinsic values is not unique,
but approximate limits to gamma and L_o can be found using probability
arguments. Roughly half the sources in a flux density--limited, beamed sample
have a value of gamma close to the measured beta_app. The methods are applied
to observations of 119 AGN jets made with the VLBA at 15 GHz during 1994-2002.
The results strongly support the common relativistic beam model for an
extragalactic radio jet. The (beta_app,L) data are closely bounded by a
theoretical envelope, an aspect curve for gamma=32, L_o= 10^25 W/Hz. This gives
limits to the maximum values of gamma and L_o in the sample: gamma_max about
32, and L_o,max ~ 10^26 W/Hz. No sources with both high beta_app and low L are
observed. This is not the result of selection effects due to the observing
limits, which are flux density S>0.5 Jy, and angular velocity mu<4 mas/yr. Many
of the fastest quasars have a pattern Lorentz factor gamma_p close to that of
the beam, gamma_b, but some of the slow quasars must have gamma_p<<gamma_b.
Three of the 10 galaxies in the sample have a superluminal feature, with speeds
up to beta_app about 6. The others are at most mildly relativistic. The
galaxies are not off-axis versions of the powerful quasars, but Cygnus A might
be an exception.Comment: 12 pages, 9 figures, 1 table, accepted for publication in the
Astrophysical Journa
INTEGRAL/RossiXTE high-energy observation of a state transition of GX 339-4
On 2004 August 15, we observed a fast (shorter than 10 hours) state
transition in the bright black-hole transient GX 339-4 simultaneously with
RossiXTE and INTEGRAL. This transition was evident both in timing and spectral
properties. Combining the data from PCA, HEXTE and IBIS, we obtained good
quality broad-band (3-200 keV) energy spectra before and after the transition.
These spectra indicate that the hard component steepened. Also, the high-energy
cutoff that was present at ~70 keV before the transition was not detected after
the transition. This is the first time that an accurate determination of the
broad-band spectrum across such a transition has been measured on a short time
scale. It shows that, although some spectral parameters do not change abruptly
through the transition, the high-energy cutoff increases/disappears rather
fast. These results constitute a benchmark on which to test theoretical models
for the production of the hard component in these systems.Comment: Accepted for publication in MNRAS (9 pages, 6 figures
Chandra and Swift observations of the quasi-persistent neutron star transient EXO 0748-676 back to quiescence
The quasi-persistent neutron star X-ray transient and eclipsing binary EXO
0748-676 recently started the transition to quiescence following an accretion
outburst that lasted more than 24 years. We report on two Chandra and twelve
Swift observations performed within five months after the end of the outburst.
The Chandra spectrum is composed of a soft, thermal component that fits to a
neutron star atmosphere model with kT^inf~0.12 keV, joined by a hard powerlaw
tail that contributes ~20% of the total 0.5-10 keV unabsorbed flux. The
combined Chandra/Swift data set reveals a relatively hot and luminous quiescent
system with a temperature of kT^inf~0.11-0.13 keV and a bolometric thermal
luminosity of ~8.1E33-1.6E34 (d/7.4 kpc)^2 erg/s. We discuss our results in the
context of cooling neutron star models.Comment: Accepted for publication in MNRAS Letters, moderate revision
according to referee report, added one plot to figure 2 and included new
Swift observations, 5 pages, 2 figure
The correlation timescale of the X-ray flux during the outbursts of soft X-ray transients
Recent studies of black hole and neutron star low mass X-ray binaries (LMXBs)
show a positive correlation between the X-ray flux at which the
low/hard(LH)-to-high/soft(HS) state transition occurs and the peak flux of the
following HS state. By analyzing the data from the All Sky Monitor (ASM)
onboard the Rossi X-ray Timing Explorer (RXTE), we show that the HS state flux
after the source reaches its HS flux peak still correlates with the transition
flux during soft X-ray transient (SXT) outbursts. By studying large outbursts
or flares of GX 339-4, Aql X-1 and 4U 1705-44, we have found that the
correlation holds up to 250, 40, and 50 d after the LH-to-HS state transition,
respectively. These time scales correspond to the viscous time scale in a
standard accretion disk around a stellar mass black hole or a neutron star at a
radius of ~104-5 Rg, indicating that the mass accretion rates in the accretion
flow either correlate over a large range of radii at a given time or correlate
over a long period of time at a given radius. If the accretion geometry is a
two-flow geometry composed of a sub-Keplerian inflow or outflow and a disk flow
in the LH state, the disk flow with a radius up to ~105 Rg would have
contributed to the nearly instantaneous non-thermal radiation directly or
indirectly, and therefore affects the time when the state transition occurs.Comment: 7 pages, 3 figure
Chandra/HETGS Spectroscopy of the Galactic Black Hole GX 339-4: A Relativistic Iron Line and Evidence for a Seyfert-like Warm Absorber
We observed the Galactic black hole GX 339-4 with the Chandra High Energy
Transmission Grating Spectrometer (HETGS) for 75 ksec during the decline of its
2002-2003 outburst. The sensitivity of this observation provides an
unprecedented glimpse of a Galactic black hole at about a tenth of the
luminosity of the outburst peak. The continuum spectrum is well described by a
model consisting of multicolor disk blackbody (kT = 0.6 keV) and power-law
(Gamma = 2.5) components. X-ray reflection models yield improved fits. A
strong, relativistic Fe K-alpha emission line is revealed, indicating that the
inner disk extends to the innermost stable circular orbit. The breadth of the
line is sufficient to suggest that GX 339-4 may harbor a black hole with
significant angular momentum. Absorption lines from H-like and He-like O, and
He-like Ne and Mg are detected, as well as lines which are likely due to Ne II
and Ne III. The measured line properties make it difficult to associate the
absorption with the coronal phase of the interstellar medium. A scenario
wherein the absorption lines are due to an intrinsic AGN-like warm-absorber
geometry -- perhaps produced by a disk wind in an extended disk-dominated state
-- may be more viable. We compare our results to Chandra observations of the
Galactic black hole candidate XTE J1650-500, and discuss our findings in terms
of prominent models for Galactic black hole accretion flows and connections to
supermassive black holes.Comment: 20 pages, 11 postscript figure files (many in color), uses
emulateapj.sty and apjfonts.sty, slightly expanded, accepted for publication
in Ap
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