73 research outputs found
Recent and Future Observations in the X-ray and Gamma-ray Bands: Chandra, Suzaku, GLAST, and NuSTAR
This paper presents a brief overview of the accomplishments of the Chandra
satellite that are shedding light on the origin of high energy particles in
astrophysical sources, with the emphasis on clusters of galaxies. It also
discusses the prospects for the new data to be collected with instruments
recently launched - such as Suzaku - or those to be deployed in the near
future, and this includes GLAST and NuSTAR.Comment: To appear in "Astrophysical Sources of High Energy Particles and
Radiation", eds. T. Bulik, B. Rudak, and G. Madejski, AIP Conference
Proceedings, vol 801 (2005), proc. conf. in Torun, Poland, 20-24 June 200
Rhapsody. II. Subhalo Properties and the Impact of Tidal Stripping From a Statistical Sample of Cluster-Size Halos
We discuss the properties of subhalos in cluster-size halos, using a
high-resolution statistical sample: the Rhapsody simulations introduced in Wu
et al. (2012). We demonstrate that the criteria applied to select subhalos have
significant impact on the inferred properties of the sample, including the
scatter in the number of subhalos, the correlation between the subhalo number
and formation time, and the shape of subhalos' spatial distribution and
velocity structure. We find that the number of subhalos, when selected using
the peak maximum circular velocity in their histories (a property expected to
be closely related to the galaxy luminosity), is uncorrelated with the
formation time of the main halo. This is in contrast to the previously reported
correlation from studies where subhalos are selected by the current maximum
circular velocity; we show that this difference is a result of the tidal
stripping of the subhalos. We also find that the dominance of the main halo and
the subhalo mass fraction are strongly correlated with halo concentration and
formation history. These correlations are important to take into account when
interpreting results from cluster samples selected with different criteria. Our
sample also includes a fossil cluster, which is presented separately and placed
in the context of the rest of the sample.Comment: 15 pages, 10 figures; Paper I: arXiv:1209.3309; replaced to match
published versio
Rhapsody. I. Structural Properties and Formation History From a Statistical Sample of Re-simulated Cluster-size Halos
We present the first results from the Rhapsody cluster re-simulation project:
a sample of 96 "zoom-in" simulations of dark matter halos of 10^14.8 +- 0.05
Msun/h, selected from a 1 (Gpc/h)^3 volume. This simulation suite is the first
to resolve this many halos with ~5x10^6 particles per halo in the cluster-mass
regime, allowing us to statistically characterize the distribution of and
correlation between halo properties at fixed mass. We focus on the properties
of the main halos and how they are affected by formation history, which we
track back to z=12, over five decades in mass. We give particular attention to
the impact of the formation history on the density profiles of the halos. We
find that the deviations from the Navarro-Frenk-White (NFW) model and the
Einasto model depend on formation time. Late-forming halos tend to have
considerable deviations from both models, partly due to the presence of massive
subhalos, while early-forming halos deviate less but still significantly from
the NFW model and are better described by the Einasto model. We find that the
halo shapes depend only moderately on formation time. Departure from spherical
symmetry impacts the density profiles through the anisotropic distribution of
massive subhalos. Further evidence of the impact of subhalos is provided by
analyzing the phase-space structure. A detailed analysis of the properties of
the subhalo population in Rhapsody is presented in a companion paper.Comment: 20 pages, 13 figures, replaced to match published versio
Optical spectroscopic observations of blazars and gamma-ray blazar candidates in the Sloan Digital Sky Survey Data Release Nine
We present an analysis of the optical spectra available in the Sloan Digital
Sky survey data release nine (SDSS DR9) for the blazars listed in the
ROMA-BZCAT and for the gamma-ray blazar candidates selected according to their
IR colors. First, we adopt a statistical approach based on MonteCarlo
simulations to find the optical counterparts of the blazarslisted in the
ROMA-BZCAT catalog. Then we crossmatched the SDSS spectroscopic catalog with
our selected samples of blazars and gamma-ray blazar candidates searching for
those with optical spectra available to classify our blazar-like sources and,
whenever possible, to confirm their redshifts. Our main objectives are
determining the classification of uncertain blazars listed in the ROMA-BZCAT
and discovering new gamma-ray blazars. For the ROMA-BZCAT sources we
investigated a sample of 84 blazars confirming the classification for 20 of
them and obtaining 18 new redshift estimates. For the gamma-ray blazars,
indicated as potential counterparts of unassociated Fermi sources or with
uncertain nature, we established the blazar-like nature of 8 out the 27 sources
analyzed and confirmed 14 classifications.Comment: 7 pages, 2 figures, 4 tables, AJ published in 2014 (pre-proof
version
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GLAST and Suzaku: Study on Cosmic-Ray Acceleration And Interaction in the Cosmos
The Gamma-Ray Large Area Space Telescope (GLAST) is an international and multiagency mission scheduled for launch in the fall 2007. The Large Area Telescope (LAT), the primary instrument of the mission, will survey the high energy sky found to be very dynamic and surprisingly diverse by its predecessor the Energetic Gamma Ray Experiment Telescope (EGRET). GLAST-LAT will have a much improved sensitivity when compared with EGRET and extend the higher energy coverage to {approx} 300 GeV. The instrument is now mounted on the spacecraft and undergoing a suite of pre-flight tests. Data analysis software has been tried out by collaborators in two rounds of 'Data Challenges' using simulated observations including backgrounds. The instrument performance and observational data on selected sources presented here have been obtained through the Data Challenges in the collaborative efforts. There are features in the GLAST-LAT observation possibly unfamiliar to X-ray astronomers: (1) GLAST will operate mostly in the survey mode; (2) the foreground objects (gas, dust, and star-light) become gamma-ray sources; (3) multiple sources will be 'confused' because of the wide point-spread-function. The last two features will pose a challenge for analysis on extended Galactic sources such as supernova remnants and pulsar wind nebulae: multi-wavelength study with X-ray instruments like Suzaku and atmospheric Chrenkov telescopes will become essential to dig out the underlying physics
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Constraints on the Low-Energy Cutoff in the Electron Distributionof the PKS 0637--752 Jet
We re-analyze the Chandra X-ray spectrum of the kpc-scale jet in PKS 0637-752 to investigate the possible low energy cutoff in the relativistic electron spectrum producing the non-thermal radiation in the scenario of inverse Compton emission off the cosmic microwave background. This was among the first objects targeted by the Chandra Observatory and gives a unique opportunity to study the low energy X-ray emission free of contamination. As previously noted, the spectrum can be fit by a power law, with the slope predicted by the radio spectrum, modified by low energy absorption through the Galaxy as determined from the spectrum of the quasar core and by HI 21 cm observations. We report evidence for a broad excess of emission below 1 keV, but are unable to constrain the shape very well. If we assume that this soft excess is unrelated to the electron population responsible for the power law emission, and that the electron spectrum cuts off at an energy of {gamma}{sub min}m{sub e}c{sup 2}, then we must have {gamma}{sub min} ({Lambda}/10) {approx}< 75 due to the absence of any low energy turn-over in the X-ray spectrum. This predicts that the observed radio spectrum should extend unbroken down below 2 MHz for the estimated jet restframe magnetic field B = 10 {micro} G. In addition, the observed optical flux can be used to place a lower limit on {gamma}{sub min}; the constraint is not very strong, but does suggest that {gamma}{sub min} must be higher than 1 to avoid overproducing the optical emission. The implication of these limits on the jet luminosity is discussed. An alternative phenomenological description of the soft excess is offered where the low-energy end of the electron spectrum is modified to account for the excess
Constraining Emission Models of Luminous Blazar Sources
Many luminous blazars which are associated with quasar-type active galactic
nuclei display broad-band spectra characterized by a large luminosity ratio of
their high-energy (gamma-ray) and low-energy (synchrotron) spectral components.
This large ratio, reaching values up to 100, challenges the standard
synchrotron self-Compton models by means of substantial departures from the
minimum power condition. Luminous blazars have also typically very hard X-ray
spectra, and those in turn seem to challenge hadronic scenarios for the high
energy blazar emission. As shown in this paper, no such problems are faced by
the models which involve Comptonization of radiation provided by a broad
line-region, or dusty molecular torus. The lack or weakness of bulk Compton and
Klein-Nishina features indicated by the presently available data favors
production of gamma-rays via up-scattering of infrared photons from hot dust.
This implies that the blazar emission zone is located at parsec-scale distances
from the nucleus, and as such is possibly associated with the extended,
quasi-stationary reconfinement shocks formed in relativistic outflows. This
scenario predicts characteristic timescales for flux changes in luminous
blazars to be days/weeks, consistent with the variability patterns observed in
such systems at infrared, optical and gamma-ray frequencies. We also propose
that the parsec-scale blazar activity can be occasionally accompanied by
dissipative events taking place at sub-parsec distances and powered by internal
shocks and/or reconnection of magnetic fields. These could account for the
multiwavelength intra-day flares occasionally observed in powerful blazars
sources.Comment: 34 pages, accepted for publication in the Astrophysical Journa
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Application of a XMM-Newton EPIC Monte Carlo to Analysis And Interpretation of Data for Abell 1689, RXJ0658-55 And the Centaurus Clusters of Galaxies
We propose a new Monte Carlo method to study extended X-ray sources with the European Photon Imaging Camera (EPIC) aboard XMM Newton. The Smoothed Particle Inference (SPI) technique, described in a companion paper, is applied here to the EPIC data for the clusters of galaxies Abell 1689, Centaurus and RXJ 0658-55 (the ''bullet cluster''). We aim to show the advantages of this method of simultaneous spectral-spatial modeling over traditional X-ray spectral analysis. In Abell 1689 we confirm our earlier findings about structure in temperature distribution and produce a high resolution temperature map. We also confirm our findings about velocity structure within the gas. In the bullet cluster, RXJ 0658-55, we produce the highest resolution temperature map ever to be published of this cluster allowing us to trace what looks like the motion of the bullet in the cluster. We even detect a south to north temperature gradient within the bullet itself. In the Centaurus cluster we detect, by dividing up the luminosity of the cluster in bands of gas temperatures, a striking feature to the north-east of the cluster core. We hypothesize that this feature is caused by a subcluster left over from a substantial merger that slightly displaced the core. We conclude that our method is very powerful in determining the spatial distributions of plasma temperatures and very useful for systematic studies in cluster structure
Upper Bound on the First Star Formation History
Our understanding of the nature of the extragalactic background light (EBL)
has improved with the recent development of gamma-ray observation techniques.
An open subject in the context of the EBL is the reionization epoch, which is
an important probe of the formation history of first stars, the so-called
Population III (Pop III) stars. Although the mechanisms for the formation of
Pop III stars are rather well understood on theoretical grounds, their
formation history is still veiled in mystery because of their faintness. To
shed light into this matter, we study jointly the gamma-ray opacity of distant
objects and the reionization constraints from studies of intergalactic gas. By
combining these studies, we obtain a sensitive upper bound on the Pop III star
formation rate density as at
, where and are the escape fraction of ionizing
photons from galaxies and the clumping factor of the intergalactic hydrogen
gas. This limit is a times tighter constraint compared with previous
studies that take into account gamma-ray opacity constraints only. Even if we
do not include the current gamma-ray constraints, the results do not change.
This is because the detected gamma-ray sources are still at where
the reionization has already finished.Comment: 5 pages, 2 figures, accepted for publication in ApJ
A Hard X-Ray View of Two Distant VHE Blazars: 1ES 1101-232 and 1ES 1553+113
TeV blazars are known as prominent nonthermal emitters across the entire
electromagnetic spectrum with their photon power peaking in the X-ray and TeV
bands. If distant, absorption of gamma-ray photons by the extragalactic
background light (EBL) alters the intrinsic TeV spectral shape, thereby
affecting the overall interpretation. Suzaku observations for two of the more
distant TeV blazars known to date, 1ES 1101-232 and 1ES 1553+113, were carried
out in 2006 May and July, respectively, including a quasi-simultaneous coverage
with the state-of-the-art Cerenkov telescope facilities. We report on the
resulting data sets with emphasis on the X-ray band and set in context to their
historical behavior. During our campaign, we did not detect any significant
X-ray or gamma-ray variability. 1ES 1101-232 was found in a quiescent state
with the lowest X-ray flux ever measured. The combined XIS and HXD PIN data for
1ES 1101-232 and 1ES 1553+113 clearly indicate spectral curvature up to the
highest hard X-ray data point (~30 keV), manifesting as softening with
increasing energy. We describe this spectral shape by either a broken power law
or a log-parabolic fit with equal statistical goodness of fits. The combined
1ES 1553+113 very high energy spectrum (90-500 GeV) did not show any
significant changes with respect to earlier observations. The resulting
contemporaneous broadband spectral energy distributions of both TeV blazars are
discussed in view of implications for intrinsic blazar parameter values, taking
into account the gamma-ray absorption in the EBL.Comment: 9 pages, 10 figure
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