536 research outputs found
First evidence for spectral state transitions in the ESO243-49 hyper luminous X-ray source HLX-1
The brightest Ultra-Luminous X-ray source (ULX), ESO 243-49 HLX-1, with a 0.2
- 10 keV X-ray luminosity of up to 10^42 erg s^-1, provides the strongest
evidence to date for the existence of intermediate mass black holes. Although
small scale X-ray spectral variability has already been demonstrated, we have
initiated a monitoring campaign with the X-ray Telescope onboard the Swift
satellite to search for luminosity-related spectral changes and to compare its
behavior with the better studied stellar mass black holes. In this paper, we
report a drop in the XRT count rate by a factor of ~8 which occurred
simultaneously with a hardening of the X-ray spectrum. A second observation
found that the source had re-brightened by a factor of ~21 which occurred
simultaneously with a softening of the X-ray spectrum. This may be the first
evidence for a transition between the low/hard and high/soft states.Comment: Accepted by ApJ Letter, 2 figure
Monte-Carlo simulations of the background of the coded-mask camera for X- and Gamma-rays on-board the Chinese-French GRB mission SVOM
For several decades now, wide-field coded mask cameras have been used with
success to localise Gamma-ray bursts (GRBs). In these instruments, the event
count rate is dominated by the photon background due to their large field of
view and large effective area. It is therefore essential to estimate the
instrument background expected in orbit during the early phases of the
instrument design in order to optimise the scientific performances of the
mission. We present here a detailed study of the instrument background and
sensitivity of the coded-mask camera for X- and Gamma-rays (CXG) to be used in
the detection and localisation of high-redshift GRBs on-board the international
GRB mission SVOM. To compute the background spectrum, a Monte-Carlo approach
was used to simulate the primary and secondary interactions between particles
from the main components of the space environment that SVOM will encounter
along its Low Earth Orbit (LEO) (with an altitude of 600 km and an inclination
of ~ 30 deg) and the body of the CXG. We consider the detailed mass model of
the CXG in its latest design. According to our results, i) the design of the
passive shield of the camera ensures that in the 4-50 keV imaging band the
cosmic X-Gamma-ray background is dominant whilst the internal background should
start to become dominant above 70-90 keV; ii) the current camera design ensures
that the CXG camera will be more sensitive to high-redshift GRBs than the Swift
Burst Alert Telescope thanks to a low-energy threshold of 4 keV.Comment: 16 pages, 10 figures (1 colour), accepted for publication in Nuclear
Instruments and Methods in Physics Research: Section
Investigating slim disk solutions for HLX-1 in ESO 243-49
The hyper luminous X-ray source HLX-1 in the galaxy ESO 243-49, currently the
best intermediate mass black hole candidate, displays spectral transitions
similar to those observed in Galactic black hole binaries, but with a
luminosity 100-1000 times higher. We investigated the X-ray properties of this
unique source fitting multi-epoch data collected by Swift, XMM-Newton & Chandra
with a disk model computing spectra for a wide range of sub- and
super-Eddington accretion rates assuming a non-spinning black hole and a
face-on disk (i = 0 deg). Under these assumptions we find that the black hole
in HLX-1 is in the intermediate mass range (~2 x 10^4 M_odot) and the accretion
flow is in the sub-Eddington regime. The disk radiation efficiency is eta =
0.11 +/-0.03. We also show that the source does follow the L_X ~ T^4 relation
for our mass estimate. At the outburst peaks, the source radiates near the
Eddington limit. The accretion rate then stays constant around 4 x 10^(-4)
M_odot yr^(-1) for several days and then decreases exponentially. Such
"plateaus" in the accretion rate could be evidence that enhanced mass transfer
rate is the driving outburst mechanism in HLX-1. We also report on the new
outburst observed in August 2011 by the Swift-X-ray Telescope. The time of this
new outburst further strengthens the ~1 year recurrence timescale.Comment: 24 pages, 10 figures, accepted for publication in Ap
The origin of variability of the intermediate-mass black-hole ULX system HLX-1 in ESO 243-49
The ultra-luminous intermediate-mass black-hole system HLX-1 in the ESO
243-49 galaxy exhibits variability with a possible recurrence time of a few
hundred days. Finding the origin of this variability would constrain the still
largely unknown properties of this extraordinary object. Since it exhibits an
intensity-hardness behavior characteristic of black-hole X-ray transients, we
have analyzed the variability of HLX-1 in the framework of the disk instability
model that explains outbursts of such systems. We find that the long-term
variability of HLX-1 is unlikely to be explained by a model in which outbursts
are triggered by thermal-viscous instabilities in an accretion disc. Possible
alternatives include the instability in a radiation-pressure dominated disk but
we argue that a more likely explanation is a modulated mass-transfer due to
tidal stripping of a star on an eccentric orbit around the intermediate-mass
black hole. We consider an evolutionary scenario leading to the creation of
such a system and estimate the probability of its observation. We conclude,
using a simplified dynamical model of the post-collapse cluster, that no more
than 1/100 to 1/10 of Mbh < 10^4 Msun IMBHs - formed by run-away stellar
mergers in the dense collapsed cores of young clusters - could have a few times
1 Msun Main-Sequence star evolve to an AGB on an orbit eccentric enough for
mass transfer at periapse, while avoiding collisional destruction or being
scattered into the IMBH by 2-body encounters. The finite but low probability of
this configuration is consistent with the uniqueness of HLX-1. We note,
however, that the actual response of a standard accretion disk to bursts of
mass transfer may be too slow to explain the observations unless the orbit is
close to parabolic (and hence even rarer) and/or additional heating, presumably
linked to the highly time-dependent gravitational potential, are invoked.Comment: 8 pages, 2 figures. Additional figure, extended discussion. To be
published in ApJ, June 10, 2011, v734 -
Optical variability of the accretion disk around the intermediate mass black hole ESO 243-49 HLX-1 during the 2012 outburst
We present dedicated quasi-simultaneous X-ray (Swift) and optical (Very Large
Telescope (VLT), V- and R-band) observations of the intermediate mass black
hole candidate ESO 243-49 HLX-1 before and during the 2012 outburst. We show
that the V-band magnitudes vary with time, thus proving that a portion of the
observed emission originates in the accretion disk. Using the first quiescent
optical observations of HLX-1, we show that the stellar population surrounding
HLX-1 is fainter than V~25.1 and R~24.2. We show that the optical emission may
increase before the X-ray emission consistent with the scenario proposed by
Lasota et al. (2011) in which the regular outbursts could be related to the
passage at periastron of a star circling the intermediate mass black hole in an
eccentric orbit, which triggers mass transfer into a quasi-permanent accretion
disk around the black hole. Further, if there is indeed a delay in the X-ray
emission we estimate the mass-transfer delivery radius to be ~1e11 cm.Comment: 9 pages, 2 figures, accepted for publication in ApJ
Further Observations of the Intermediate Mass Black Hole Candidate ESO 243-49 HLX-1
The brightest Ultra-Luminous X-ray source HLX-1 in the galaxy ESO 243-49
currently provides strong evidence for the existence of intermediate mass black
holes. Here we present the latest multi-wavelength results on this intriguing
source in X-ray, UV and radio bands. We have refined the X-ray position to
sub-arcsecond accuracy. We also report the detection of UV emission that could
indicate ongoing star formation in the region around HLX-1. The lack of
detectable radio emission at the X-ray position strengthens the argument
against a background AGN.Comment: 4 pages, 2 figures. Accepted 11th of Feb 2010. Contributed talk to
appear in Proceedings of "X-ray Astronomy 2009: Present Status,
Multi-Wavelength Approach and Future Perspectives", Bologna, Italy, September
7-11, 2009, AIP, eds. A. Comastri, M. Cappi, and L. Angelin
Testing the standard fireball model of GRBs using late X-ray afterglows measured by Swift
We show that all X-ray decay curves of GRBs measured by Swift can be fitted
using one or two components both of which have exactly the same functional form
comprised of an early falling exponential phase followed by a power law decay.
The 1st component contains the prompt gamma-ray emission and the initial X-ray
decay. The 2nd component appears later, has a much longer duration and is
present for ~80% of GRBs. It most likely arises from the external shock which
eventually develops into the X-ray afterglow. In the remaining ~20% of GRBs the
initial X-ray decay of the 1st component fades more slowly than the 2nd and
dominates at late times to form an afterglow but it is not clear what the
origin of this emission is.
The temporal decay parameters and gamma/X-ray spectral indices derived for
107 GRBs are compared to the expectations of the standard fireball model
including a search for possible "jet breaks". For ~50% of GRBs the observed
afterglow is in accord with the model but for the rest the temporal and
spectral indices do not conform to the expected closure relations and are
suggestive of continued, late, energy injection. We identify a few possible jet
breaks but there are many examples where such breaks are predicted but are
absent.
The time, T_a, at which the exponential phase of the 2nd component changes to
a final powerlaw decay afterglow is correlated with the peak of the gamma-ray
spectrum, E_peak. This is analogous to the Ghirlanda relation, indicating that
this time is in some way related to optically observed break times measured for
pre-Swift bursts.Comment: submitted to Ap
The Combinatorial PP1-Binding Consensus Motif (R/K)x (0,1)V/IxFxx(R/K)x(R/K) Is a New Apoptotic Signature
BACKGROUND: Previous studies established that PP1 is a target for Bcl-2 proteins and an important regulator of apoptosis. The two distinct functional PP1 consensus docking motifs, R/Kx((0,1))V/IxF and FxxR/KxR/K, involved in PP1 binding and cell death were previously characterized in the BH1 and BH3 domains of some Bcl-2 proteins. PRINCIPAL FINDINGS: In this study, we demonstrate that DPT-AIF(1), a peptide containing the AIF(562-571) sequence located in a c-terminal domain of AIF, is a new PP1 interacting and cell penetrating molecule. We also showed that DPT-AIF(1) provoked apoptosis in several human cell lines. Furthermore, DPT-APAF(1) a bi-partite cell penetrating peptide containing APAF-1(122-131), a non penetrating sequence from APAF-1 protein, linked to our previously described DPT-sh1 peptide shuttle, is also a PP1-interacting death molecule. Both AIF(562-571) and APAF-1(122-131) sequences contain a common R/Kx((0,1))V/IxFxxR/KxR/K motif, shared by several proteins involved in control of cell survival pathways. This motif combines the two distinct PP1c consensus docking motifs initially identified in some Bcl-2 proteins. Interestingly DPT-AIF(2) and DPT-APAF(2) that carry a F to A mutation within this combinatorial motif, no longer exhibited any PP1c binding or apoptotic effects. Moreover the F to A mutation in DPT-AIF(2) also suppressed cell penetration. CONCLUSION: These results indicate that the combinatorial PP1c docking motif R/Kx((0,1))V/IxFxxR/KxR/K, deduced from AIF(562-571) and APAF-1(122-131) sequences, is a new PP1c-dependent Apoptotic Signature. This motif is also a new tool for drug design that could be used to characterize potential anti-tumour molecules
Modelling the spectral response of the Swift-XRT CCD camera: Experience learnt from in-flight calibration
(Abbreviated) We show that the XRT spectral response calibration was
complicated by various energy offsets in photon counting (PC) and windowed
timing (WT) modes related to the way the CCD is operated in orbit (variation in
temperature during observations, contamination by optical light from the sunlit
Earth and increase in charge transfer inefficiency). We describe how these
effects can be corrected for in the ground processing software. We show that
the low-energy response, the redistribution in spectra of absorbed sources, and
the modelling of the line profile have been significantly improved since launch
by introducing empirical corrections in our code when it was not possible to
use a physical description. We note that the increase in CTI became noticeable
in June 2006 (i.e. 14 months after launch), but the evidence of a more serious
degradation in spectroscopic performance (line broadening and change in the
low-energy response) due to large charge traps (i.e. faults in the Si crystal)
became more significant after March 2007. We describe efforts to handle such
changes in the spectral response. Finally, we show that the commanded increase
in the substrate voltage from 0 to 6V on 2007 August 30 reduced the dark
current, enabling the collection of useful science data at higher CCD
temperature (up to -50C). We also briefly describe the plan to recalibrate the
XRT response files at this new voltage.Comment: 27 pages, 29 figures (many in colour), accepted for publication in
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