64 research outputs found
Mapping the allowed parameter space for decaying dark matter models
I consider constraints on a phenomenological decaying-dark-matter model, in
which two weakly-interacting massive particle (WIMP) species have a small mass
splitting, and in which the heavier particle decays to the lighter particle and
a massless particle on cosmological timescales. The decay parameter space is
parameterized by , the speed of the lighter particle in the center-of-mass
frame of the heavier particle prior to decay, and the decay time . Since
I consider the case in which dark-matter halos have formed before there has
been significant decay, I focus on the effects of decay in already-formed
halos. I show that the parameter space may be constrained by
observed properties of dark-matter halos. I highlight which set of observations
is likely to yield the cleanest constraints on parameter space, and
calculate the constraints in those cases in which the effect of decay on the
observables can be calculated without N-body simulations of decaying dark
matter. I show that for km s, the z=0 galaxy
cluster mass function and halo mass-concentration relation constrain 40 Gyr, and that precise constraints on for smaller will
require N-body simulations.Comment: 14 pages, 5 figures, references added, replaced to match version
published in Phys. Rev.
The discovery of a new non-thermal X-ray filament near the Galactic Centre
We report the discovery by XMM-Newton and Chandra of a hard extended X-ray
source (XMM J174540-2904.5) associated with a compact non-thermal radio
filament (the Sgr A-E `wisp'=1LC 359.888-0.086= G359.88-0.07), which is located
within ~4 arcmin of the Galactic Centre. The source position is also coincident
with the peak of the molecular cloud, M -0.13-0.08 (the `20 km/s' cloud). The
X-ray spectrum is non-thermal with an energy index of 1.0 (+1.1 -0.9) and
column density of 38 (+7 -11) x 10^22 H/cm2. The observed 2--10 keV flux of 4 x
10^-13 erg/s/cm2 converts to an unabsorbed X-ray luminosity of 1 x 10^34 erg/s
assuming a distance of 8.0 kpc. The high column density strongly suggests that
this source is located in or behind the Galactic Centre Region. Taking account
of the broad-band spectrum, as well as the source morphology and the positional
coincidence with a molecular cloud, we concluded that both the radio and X-ray
emission are the result of synchrotron radiation. This is the first time a
filamentary structure in the Galactic Centre Region. has been shown,
unequivocally, to have a non-thermal X-ray spectrum.Comment: 5 pages, 4 figures, Accepted for publication in MNRAS, also found in
http://www.star.le.ac.uk/~mas/research/paper/#Sakano2002mnra
A detailed look at the thermal and non-thermal X-ray emission from the Vela supernova remnant with SRG/eROSITA
Aims. Our goal is the characterization of the hot ejecta and shocked
interstellar medium (ISM) associated to the Vela supernova remnant (SNR), as
well as the relativistic electrons injected into the ambient medium by its
central pulsar. To achieve this, we analyze the X-ray data set of Vela acquired
by SRG/eROSITA during its first four all-sky surveys. Methods. Apart from
multi-band imaging, a quantitative view of the physical parameters affecting
the observed thermal and non-thermal emission is obtained by performing
spatially resolved X-ray spectroscopy of over 500 independent regions using
multi-component spectral models. Results. Imaging demonstrates that the X-ray
emission of the Vela SNR consists of at least three morphologically distinct
components, with shell-like structures dominating below 0.6 keV, radial
outward-directed features becoming apparent at medium energies, and the pulsar
wind nebula (PWN) dominating the hard emission above 1.4 keV. Our spectroscopy
reveals a highly structured distribution of X-ray absorption column densities,
which intriguingly appears anticorrelated with optical extinction measurements.
We find evidence for multiple ejecta clumps inside and outside the shell,
within which we find a strongly supersolar concentration of neon and magnesium
relative to oxygen. This includes the bright shrapnel D, in which we separate
shocked ISM in the soft bow-shock from a hot, ejecta-rich clump at its apex,
based on the new data. Finally, we find an extremely extended, smoothly
decreasing distribution of synchrotron emission from the PWN, which extends up
to 14 pc from the pulsar, with a total X-ray luminosity of
of the pulsar's spin-down power. The extended emission likely traces a
relativistic electron population in an ISM-level magnetic field, which requires
the existence of a TeV counterpart powered by inverse Compton radiation.Comment: 24+3 pages, 13+4 figures, accepted for publication in Astronomy &
Astrophysics; Abstract abridge
Dark-Matter Decays and Self-Gravitating Halos
We consider models in which a dark-matter particle decays to a slightly less
massive daughter particle and a noninteracting massless particle. The decay
gives the daughter particle a small velocity kick. Self-gravitating dark-matter
halos that have a virial velocity smaller than this velocity kick may be
disrupted by these particle decays, while those with larger virial velocities
will be heated. We use numerical simulations to follow the detailed evolution
of the total mass and density profile of self-gravitating systems composed of
particles that undergo such velocity kicks as a function of the kick speed
(relative to the virial velocity) and the decay time (relative to the dynamical
time). We show how these decays will affect the halo mass-concentration
relation and mass function. Using measurements of the halo mass-concentration
relation and galaxy-cluster mass function to constrain the
lifetime--kick-velocity parameter space for decaying dark matter, we find
roughly that the observations rule out the combination of kick velocities
greater than 100 km/s and decay times less than a few times the age of the
Universe.Comment: 17 pages, 10 figures, replaced with published versio
The X-ray Halo of GX5-1
Using Chandra observations we have measured the energy-resolved
dust-scattered X-ray halo around the low-mass X-ray binary GX5-1, which shows
signs of both singly- and multiply-scattered X-rays. We compared the observed
X-ray halo at various energies to predictions from a range of dust models.
These fits used both smoothly-distributed dust as well as dust in clumped
clouds, with CO and 21 cm observations helping to determine the position of the
clouds along the line of sight. We found that the BARE-GR-B model of Zubko,
Dwek & Arendt (2004 generally led to the best results, although inadequacies in
both the overall model and the data limit our conclusions. We also found that
the composite dust models of Zubko, Dwek & Arendt (2004), especially the ``no
carbon'' models, gave uniformly poor results. Although models using cloud
positions and densities derived naively from CO and 21 cm data gave generally
poor results, plausible adjustments to the distance of the largest cloud and
the mass of a cloud in the expanding 3 kpc Arm lead to significantly improved
fits. We suggest that combining X-ray halo, CO, and 21 cm observations will be
a fruitful method to improve our understanding of both the gas and dust phases
of the interstellar medium.Comment: 10 pages, accepted by Ap
SRG/eROSITA and XMM-Newton observations of Vela Jr
The Vela supernova remnant complex is a region containing at least three
supernova remnants: Vela, Puppis A, and Vela Jr. With the launch of the
spectro-imaging X-ray telescope eROSITA on board the Spectrum Roentgen Gamma
(SRG) mission, it became possible to observe the one degree wide Vela Jr in its
entirety. Although several previous pointed Chandra and XMM-Newton observations
are available, it is only the second time after the ROSAT all-sky survey that
the whole remnant was observed in X-rays with homogeneous sensitivity. Vela Jr
is one of the few remnants emitting in the TeV band, making it an important
object in shock acceleration studies. However, the age and distance
determination using X-ray emission is largely hampered by the presence of the
Vela SNR along the same line. With the eROSITA data set our aim is to
characterize the emission of Vela Jr and distinguish it from Vela emission, and
also to characterize the spectral emission of the inner remnant. We processed
the eROSITA data dividing the whole remnant into seven different regions. In
addition, images of the whole remnant were employed to pinpoint the position of
the geometric center and constrain the proper motion of the CCO. We also
employed archival XMM-Newton pointed observations of the NW rim to determine
the cutoff energy of the electrons and the expansion velocity. We find the
magnetic field can vary between 2 G and 16 G in the NW rim. We also
find that the remnant spectrum is uniformly featureless in most regions, except
for two inner regions where an extra thermal model component improves the fit.
We obtain new coordinates for the geometric remnant center, resulting in a
separation of only 35.2 15.8" from the position of the CCO. As a result,
we reinforce the association between the CCO and a proposed faint optical/IR
counterpart.Comment: Accepted for publication in A&
A Huge Drop in X-ray Luminosity of the Non-Active Galaxy RXJ1242.6-1119A, and First Post-Flare Spectrum - Testing the Tidal Disruption Scenario
It has been suggested that an unavoidable consequence of the existence of
supermassive black holes, and the best diagnostic of their presence in
non-active galaxies, would be occasional tidal disruption of stars captured by
the black holes. These events manifest themselves in form of luminous flares
powered by accretion of debris from the disrupted star into the black hole.
Candidate events among optically non-active galaxies emerged in the past few
years. For the first time, we have looked with high spatial and spectral
resolution at one of these most extreme variability events ever recorded among
galaxies. Here, we report measuring a factor ~200 drop in luminosity of the
X-ray source RXJ 1242-1119 with the X-ray observatories Chandra and XMM-Newton,
and perform key tests of the favored outburst scenario, tidal disruption of a
star by a supermassive black hole. We show that the detected `low-state'
emission has properties such that it must still be related to the flare. The
power-law shaped post-flare X-ray spectrum indicates a `hardening' compared to
outburst. The inferred black hole mass, the amount of liberated energy, and the
duration of the event favor an accretion event of the form expected from the
(partial or complete) tidal disruption of a star (abstract abbreviated).Comment: to appear in March 1 issue of ApJ Letters (submitted Nov. 10,
accepted in Dec. 2003); background information available at
http://www.xray.mpe.mpg.de/~skomossa
Precise Localization of the Soft Gamma Repeater SGR 1627-41 and the Anomalous X-ray Pulsar AXP 1E1841-045 with Chandra
We present precise localizations of AXP 1E1841-045 and SGR 1627-41 with
Chandra. We obtained new infrared observations of SGR 1627-41 and reanalyzed
archival observations of AXP 1E1841-045 in order to refine their positions and
search for infrared counterparts. A faint source is detected inside the error
circle of AXP 1E1841-045. In the case of SGR 1627-41, several sources are
located within the error radius of the X-ray position and we discuss the
likelihood of one of them being the counterpart. We compare the properties of
our candidates to those of other known AXP and SGR counterparts. We find that
the counterpart candidates for SGR 1627-41 and SGR 1806-20 would have to be
intrinsically much brighter than AXPs to have detectable counterparts with the
observational limits currently available for these sources. To confirm the
reported counterpart of SGR 1806-20, we obtained new IR observations during the
July 2003 burst activation of the source. No brightening of the suggested
counterpart is detected, implying that the counterpart of SGR 1806-20 remains
yet to be identified.Comment: 29 pages, 4 figures, accepted for publication in Ap
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