603 research outputs found
Galaxy Cluster Mass Estimation from Stacked Spectroscopic Analysis
We use simulated galaxy surveys to study: i) how galaxy membership in
redMaPPer clusters maps to the underlying halo population, and ii) the accuracy
of a mean dynamical cluster mass, , derived from stacked
pairwise spectroscopy of clusters with richness . Using galaxy pairs patterned after the SDSS redMaPPer cluster sample study
of Rozo et al. (2015 RMIV), we show that the pairwise velocity PDF of
central--satellite pairs with in the simulation matches the form
seen in RMIV. Through joint membership matching, we deconstruct the main
Gaussian velocity component into its halo contributions, finding that the
top-ranked halo contributes of the stacked signal. The halo mass
scale inferred by applying the virial scaling of Evrard et al. (2008) to the
velocity normalization matches, to within a few percent, the log-mean halo mass
derived through galaxy membership matching. We apply this approach, along with
mis-centering and galaxy velocity bias corrections, to estimate the log-mean
matched halo mass at of SDSS redMaPPer clusters. Employing the velocity
bias constraints of Guo et al. (2015), we find with and .
Systematic uncertainty in the velocity bias of satellite galaxies
overwhelmingly dominates the error budget.Comment: 14 pages, 7 figure
Looking Into the Fireball: ROTSE-III and Swift Observations of Early GRB Afterglows
We report on a complete set of early optical afterglows of gamma-ray bursts
(GRBs) obtained with the ROTSE-III telescope network from March 2005 through
June 2007. This set is comprised of 12 afterglows with early optical and
Swift/XRT observations, with a median ROTSE-III response time of 45 s after the
start of gamma-ray emission (8 s after the GCN notice time). These afterglows
span four orders of magnitude in optical luminosity, and the contemporaneous
X-ray detections allow multi-wavelength spectral analysis. Excluding X-ray
flares, the broadband synchrotron spectra show that the optical and X-ray
emission originate in a common region, consistent with predictions of the
external forward shock in the fireball model. However, the fireball model is
inadequate to predict the temporal decay indices of the early afterglows, even
after accounting for possible long-duration continuous energy injection. We
find that the optical afterglow is a clean tracer of the forward shock, and we
use the peak time of the forward shock to estimate the initial bulk Lorentz
factor of the GRB outflow, and find 100<Gamma_0<1000, consistent with
expectations.Comment: 31 pages, 5 figures, submitted to Ap
\u3cem\u3eSwift\u3c/em\u3e Monitoring of Cygnus X-2: Investigating the Near-ultraviolet-X-ray Connection
The neutron star X-ray binary (NSXRB) Cyg X-2 was observed by the Swift satellite 51 times over a 4 month period in 2008 with the X-ray Telescope (XRT), UV/optical telescope, and Burst Alert Telescope (BAT) instruments. During this campaign, we observed Cyg X-2 in all three branches of the Z track (horizontal, normal, and flaring branches). We find that the NUV emission is uncorrelated with the soft X-ray flux detected with the XRT and is anticorrelated with the BAT X-ray flux and the hard X-ray color. The observed anticorrelation is inconsistent with simple models of reprocessing as the source of the NUV emission. The anticorrelation may be a consequence of the high inclination angle of Cyg X-2, where NUV emission is preferentially scattered by a corona that expands as the disk is radiatively heated. Alternatively, if the accretion disk thickens as Cyg X-2 goes down the normal branch toward the flaring branch, this may be able to explain the observed anticorrelation. In these models, the NUV emission may not be a good proxy for \dot{m} in the system. We also discuss the implications of using Swift/XRT to perform spectral modeling of the continuum emission of NSXRBs
GRB 081008: from burst to afterglow and the transition phase in between
We present a multi-wavelength study of GRB 081008, at redshift 1.967, by
Swift, ROTSE-III and GROND. Compared to other Swift GRBs, GRB 081008 has a
typical gamma-ray isotropic equivalent energy output (10^53 erg) during the
prompt phase, and displayed two temporally separated clusters of pulses. The
early X-ray emission seen by the Swift/XRT was dominated by the softening tail
of the prompt emission, producing multiple flares during and after the
Swift/BAT detections. Optical observations that started shortly after the first
active phase of gamma-ray emission showed two consecutive peaks. We interpret
the first optical peak as the onset of the afterglow associated with the early
burst activities. A second optical peak, coincident with the later gamma-ray
pulses, imposes a small modification to the otherwise smooth lightcurve and
thus suggests a minimal contribution from a probable internal component. We
suggest the early optical variability may be from continuous energy injection
into the forward shock front by later shells producing the second epoch of
burst activities. These early observations thus provide a potential probe for
the transition from prompt to the afterglow phase. The later lightcurve of GRB
081008 displays a smooth steepening in all optical bands and X-ray. The
temporal break is consistent with being achromatic at the observed wavelengths.
Our broad energy coverage shortly after the break constrains a spectral break
within optical. However, the evolution of the break frequency is not observed.
We discuss the plausible interpretations of this behavior.Comment: 16 pages, 4 figures, accepted for publication in Ap
Physical Parameters Of The Relativistic Shock Waves In GRBs: The Case Of 30 GRBs
Using the modified internal shock wave model we fit the gamma ray burst (GRB)
light and spectral curves of 30 GRBs observed with BATSE. From the best fitting
we obtain basic parameters of the relativistic shells which are in good
agreement with predictions given earlier. We compare measured GRB parameters
with those obtained from the model and discuss connections between them in the
frame of the physical processes laying behind GRB events.Comment: 18 pages, 30 figures, 2 tables, Accepted for publication in IJMP
Exploiting Cross Correlations and Joint Analyses
In this report, we present a wide variety of ways in which information from
multiple probes of dark energy may be combined to obtain additional information
not accessible when they are considered separately. Fundamentally, because all
major probes are affected by the underlying distribution of matter in the
regions studied, there exist covariances between them that can provide
information on cosmology. Combining multiple probes allows for more accurate
(less contaminated by systematics) and more precise (since there is
cosmological information encoded in cross-correlation statistics) measurements
of dark energy. The potential of cross-correlation methods is only beginning to
be realized. By bringing in information from other wavelengths, the
capabilities of the existing probes of dark energy can be enhanced and
systematic effects can be mitigated further. We present a mixture of work in
progress and suggestions for future scientific efforts. Given the scope of
future dark energy experiments, the greatest gains may only be realized with
more coordination and cooperation between multiple project teams; we recommend
that this interchange should begin sooner, rather than later, to maximize
scientific gains.Comment: Report from the "Dark Energy and CMB" working group for the American
Physical Society's Division of Particles and Fields long-term planning
exercise ("Snowmass"
A Search for Untriggered GRB Afterglows with ROTSE-III
We present the results of a search for untriggered gamma-ray burst (GRB)
afterglows with the Robotic Optical Transient Search Experiment-III (ROTSE-III)
telescope array. This search covers observations from September 2003 to March
2005. We have an effective coverage of 1.74 deg^2 yr for rapidly fading
transients that remain brighter than ~ 17.5 magnitude for more than 30 minutes.
This search is the first large area survey to be able to detect typical
untriggered GRB afterglows. Our background rate is very low and purely
astrophysical. We have found 4 previously unknown cataclysmic variables (CVs)
and 1 new flare star. We have not detected any candidate afterglow events or
other unidentified transients. We can place an upper limit on the rate of
fading optical transients with quiescent counterparts dimmer than ~ 20th
magnitude at a rate of less than 1.9 deg^-2 yr-1 with 95% confidence. This
places limits on the optical characteristics of off-axis (orphan) GRB
afterglows. As a byproduct of this search, we have an effective ~ 52 deg^2 yr
of coverage for very slowly decaying transients, such as CVs. This implies an
overall rate of outbursts from high galactic latitude CVs of 0.1 deg^2 yr^-1.Comment: Seven pages, five figures, uses emulateapj class file. Accepted to
Astrophysical Journa
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