136 research outputs found
The Temporal and Spectral Characteristics of "Fast Rise and Exponential Decay" Gamma-Ray Burst Pulses
In this paper we have analyzed the temporal and spectral behavior of 52 Fast
Rise and Exponential Decay (FRED) pulses in 48 long-duration gamma-ray bursts
(GRBs) observed by the CGRO/BATSE, using a pulse model with two shape
parameters and the Band model with three shape parameters, respectively. It is
found that these FRED pulses are distinguished both temporally and spectrally
from those in long-lag pulses. Different from these long-lag pulses only one
parameter pair indicates an evident correlation among the five parameters,
which suggests that at least 4 parameters are needed to model burst
temporal and spectral behavior. In addition, our studies reveal that these FRED
pulses have correlated properties: (i) long-duration pulses have harder spectra
and are less luminous than short-duration pulses; (ii) the more asymmetric the
pulses are the steeper the evolutionary curves of the peak energy () in
the spectrum within pulse decay phase are. Our statistical
results give some constrains on the current GRB models.Comment: 18 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Pulse Width Evolution of Late Time X-rays Flares in GRBs: Evidence For Internal Shocks
We study the duration and variability of late time X-ray flares following
gamma-ray bursts (GRBs) observed by the narrow field X-ray telescope (XRT)
aboard the {\it Swift} spacecraft. These flares are thought to be indicative of
late time activity by the central engine that powers the GRB and produced by
means similar to those which produce the prompt emission. We use a
non-parametric procedure to study the overall temporal properties of the flares
and a structure function analysis to look for an evolution of the fundamental
variability time-scale between the prompt and late time emission. We find a
strong correlation in 28 individual x-ray flares in 18 separate GRBs between
the flare duration and their time of peak flux since the GRB trigger. We also
find a qualitative trend of decreasing variability as a function of time since
trigger, with a characteristic minimum variability timescale
for most flares. We interpret these results as evidence of internal shocks at
collision radii that are larger than those that produced the prompt emission.
Contemporaneous detections of high energy emission by GLAST could be a crucial
test in determining if indeed these X-ray flares originate as internal shocks
behind the afterglow, as any X-ray emission originating from behind the
afterglow is expected to undergo inverse Compton scattering as it passes
through the external shock.Comment: 26 pages, 5 figures, 1 table. Submitted to ApJ. This work expands
upon and formalizes our previous report at the October 2006 AAS HEAD Meeting
of the discovery of pulse width evolutio
Temporal and Spectral Evolution of Gamma-ray Burst Broad Pulses: Identification of High Latitude Emission in the Prompt Emission
We perform a detailed analysis on broad pulses in bright Gamma-ray bursts
(GRBs) to understand the evolution of GRB broad pulses. Using the temporal and
spectral properties, we test the high latitude emission (HLE) scenario in the
decaying phase of broad pulses. The HLE originates from the curvature effect of
a relativistic spherical jet, where higher latitude photons are delayed and
softer than the observer's line-of-sight emission. The signature of HLE has not
yet been identified undisputedly during the prompt emission of GRBs. The HLE
theory predicts a specific relation, F E_{p}\!^{2},
between the peak energy in F spectra and the spectral flux
F measured at , F. We search for evidence of this
relation in 2157 GRBs detected by the Gamma-ray Burst Monitor (GBM) on board
the Fermi Gamma-ray Space Telescope (Fermi) from the years 2008 to 2017. After
imposing unbiased selection criteria in order to minimize contamination in a
signal by background and overlaps of pulses, we build a sample of 32 broad
pulses in 32 GRBs. We perform a time-resolved spectral analysis on each of
these 32 broad pulses and find that the evolution of 18 pulses (56%) is clearly
consistent with the HLE relation. For the 18 broad pulses, the exponent
in the relation of F E_{p}\!^{\delta} is
distributed as a Gaussian function with median and width of 1.99 and 0.34,
respectively. This result provides constraint on the emission radius of GRBs
with the HLE signature.Comment: 30 pages, 36 figures, accepted to Ap
A test of the power law relationship between gamma-ray burst pulse width ratio and energy expected in fireballs or uniform jets
Recently, under the assumption that the Doppler effect of the
relativistically expanding fireball surface is important, Qin et al. showed
that in most cases the power law relationship between the pulse width and
energy of gamma-ray bursts (GRBs)would exist in a certain energy range. We
check this prediction with two GRB samples which contain well identified
pulses. A power law anti-correlation between the full pulse width and energy
and a power law correlation between the pulse width ratio and energy are seen
in the light curves of the majority (around 65%) of bursts of the two samples
within the energy range of BATSE, suggesting that these bursts are likely to
arise from the emission associated with the shocks occurred on a
relativistically expanding fireball surface. For the rest of the bursts, the
relationships between these quantities were not predicted previously. We
propose to consider other spectral evolutionary patterns or other radiation
mechanisms such as a varying synchrotron or Comptonized spectrum to check if
the observed relationships for these rest bursts can also be accounted for by
the Doppler model. In addition, we find that the upper limits of the width
ratio for the two samples do not exceed 0.9, in agrement with what predicted
previously by the Doppler model. The plateau/power law/plateau and the peaked
features predicted and detected previously by Qin et al. are generally
observed, with the exceptions being noticed only in a few cases. According to
the distinct values of two power law indices of FWHM and ratio and energy, we
divide the bursts into three subsets which are located in different areas of
the two indices plane. We suspect that different locations of the two indices
might correspond to different mechanisms.Comment: 16 pages, 7 figures, MNRAS accepte
Influence of major mergers on the radio emission of elliptical galaxies
We investigate the influence of major mergers on the radio emission of
elliptical galaxies. We use a complete sample of close pairs, which contains
475 merging and 1828 non-merging paired elliptical galaxies of M_r<-21.5
selected from the Sloan Digital Sky Survey. In addition, a control sample of
2000 isolated field galaxies is used for comparison. We cross-identify the
optical galaxies with the radio surveys of FIRST and NVSS. We find that the
radio fraction of merging paired galaxies is about 6%, which is slightly higher
than the 5% obtained for non-merging paired galaxies, although these values are
consistent with each other owing to the large uncertainty caused by the limited
sample. The radio fraction is twice as that of isolated galaxies, which is less
than 3%. Radio emission of elliptical galaxies is only slightly affected by
major mergers, but predominantly depends on their optical luminosities.
Therefore, merging is not important in triggering the radio emission of
elliptical galaxies.Comment: 5 pages, 5 figures, 1 table, accepted for publication in A&A, minor
change
Two types of softening detected in X-ray afterglows of Swift bursts: internal and external shock origins?
The softening process observed in the steep decay phase of early X-ray
afterglows of Swift bursts has remained a puzzle since its discovery. The
softening process can also be observed in the later phase of the bursts and its
cause has also been unknown. Recently, it was suggested that, influenced by the
curvature effect, emission from high latitudes would shift the Band function
spectrum from higher energy band to lower band, and this would give rise to the
observed softening process accompanied by a steep decay of the flux density.
The curvature effect scenario predicts that the terminating time of the
softening process would be correlated with the duration of the process. In this
paper, based on the data from the UNLV GRB group web-site, we found an obvious
correlation between the two quantities. In addition, we found that the
softening process can be divided into two classes: the early type softening
() and the late type softening ().
The two types of softening show different behaviors in the duration vs.
terminating time plot. In the relation between the variation rates of the flux
density and spectral index during the softening process, a discrepancy between
the two types of softening is also observed. According to their time scales and
the discrepancy between them, we propose that the two types are of different
origins: the early type is of internal shock origin and the late type is of
external shock origin. The early softening is referred to the steep decay just
following the prompt emission, whereas the late decay typically conceives the
transition from flat decay to late afterglow decay. We suspect that there might
be a great difference of the Lorentz factor in two classes which is responsible
for the observed discrepancy.Comment: 20 pages, 5 figures, 2 tables, Accepted for Publication to Journal of
Cosmology and Astroparticle Physics (JCAP
The Chandra COSMOS Survey: III. Optical and Infrared Identification of X-ray Point Sources
The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that
has imaged the central 0.9 deg^2 of the COSMOS field down to limiting depths of
1.9 10^-16 erg cm^-2 s-1 in the 0.5-2 keV band, 7.3 10^-16 erg cm^-2 s^-1 in
the 2-10 keV band, and 5.7 10^-16 erg cm^-2 s-1 in the 0.5-10 keV band. In this
paper we report the i, K and 3.6micron identifications of the 1761 X-ray point
sources. We use the likelihood ratio technique to derive the association of
optical/infrared counterparts for 97% of the X-ray sources. For most of the
remaining 3%, the presence of multiple counterparts or the faintness of the
possible counterpart prevented a unique association. For only 10 X-ray sources
we were not able to associate a counterpart, mostly due to the presence of a
very bright field source close by. Only 2 sources are truly empty fields.
Making use of the large number of X-ray sources, we update the "classic locus"
of AGN and define a new locus containing 90% of the AGN in the survey with full
band luminosity >10^42 erg/s. We present the linear fit between the total i
band magnitude and the X-ray flux in the soft and hard band, drawn over 2
orders of magnitude in X-ray flux, obtained using the combined C-COSMOS and
XMM-COSMOS samples. We focus on the X-ray to optical flux ratio (X/O) and we
test its known correlation with redshift and luminosity, and a recently
introduced anti-correlation with the concentration index (C). We find a strong
anti-correlation (though the dispersion is of the order of 0.5 dex) between C
and X/O, computed in the hard band, and that 90% of the obscured AGN in the
sample with morphological information live in galaxies with regular morphology
(bulgy and disky/spiral), suggesting that secular processes govern a
significant fraction of the BH growth at X-ray luminosities of 10^43- 10^44.5
erg/s.Comment: 21 pages, 17 figures, 4 tables; accepted for publication in ApJS. The
catalog is available at the urls listed in the pape
Probing the low-redshift star formation rate as a function of metallicity through the local environments of type II supernovae
Type II SNe can trace star formation to probe its global metallicity
distribution at low-redshift. We present oxygen and iron abundance
distributions of SN II progenitor regions that avoid many previous sources of
bias. Because Fe (rather than O) abundance drives the late stage evolution of
the massive stars that are the progenitors of CCSNe, and because Fe enrichment
lags O enrichment, we find a general conversion from O abundance to Fe
abundance. The distributions we present here are the best yet standard of
comparison for evaluating how rare classes of SNe depend on progenitor
metallicity. We measure the gas-phase O abundance of a representative subsample
of the hosts of SNe II from the first-year PTF SN search, using a combination
of SDSS spectra near the SN location (9) and new long slit spectroscopy (25).
The median metallicity of these 34 hosts is 12+log(O/H) = 8.65, with a median
error of 0.09. The median host galaxy stellar mass from fits to SDSS photometry
is 10^9.9 solar masses. They do not show a systematic offset in metallicity or
mass from a redshift-matched sample of the MPA/JHU value-added catalog. In
contrast to previous SN host metallicity studies, this sample is drawn from a
single, areal survey. SNe in the lowest-mass galaxies are not systematically
excluded. The metallicity distribution we find is statistically
indistinguishable from the metallicity distribution of SN II hosts found by
targeted surveys and by samples from multiple surveys with different selection
functions. Using the relationship between Fe and O abundances found for Milky
Way disk, bulge, and halo stars, we translate our O abundance distribution of
SN II environments into Fe abundance estimates. We find that though this sample
spans only 0.65 dex in O abundance, the gap between the Fe and O abundance is
50% wider at the low-metallicity end of our sample than at the high-metallicity
end. (abridged)Comment: 22 pages, 14 figures, 7 tables, ApJ accepte
Fermi observations of high-energy gamma-ray emission from GRB 090217A
The Fermi observatory is advancing our knowledge of Gamma-Ray Bursts (GRBs)
through pioneering observations at high energies, covering more than 7 decades
in energy with the two on-board detectors, the Large Area Telescope (LAT) and
the Gamma-ray Burst Monitor (GBM). Here we report on the observation of the
long GRB 090217A which triggered the GBM and has been detected by the LAT with
a significance greater than 9 sigma. We present the GBM and LAT observations
and on-ground analyses, including the time-resolved spectra and the study of
the temporal profile from 8 keV up to 1 GeV. All spectra are well reproduced by
a Band model. We compare these observations to the first two LAT-detected, long
bursts GRB 080825C and GRB 080916C. These bursts were found to have
time-dependent spectra and exhibited a delayed onset of the high-energy
emission, which are not observed in the case of GRB 090217A. We discuss some
theoretical implications for the high-energy emission of GRBs.Comment: 17 pages, 4 figures. Contact Authors: Fred, Piron; Sara, Cutini;
Andreas, von Kienli
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