593 research outputs found
Cumulative light curves of gamma-ray bursts and relaxation systems
The cumulative light curves of a large sample of gamma-ray bursts (GRBs) were
obtained by summing the BATSE counts. The smoothed profiles are much simpler
than the complex and erratic running light curves that are normally used. For
most GRBs the slope of the cumulative light curve (S) is approximately constant
over a large fraction of the burst. The bursts are modelled as relaxation
systems that continuously accumulate energy in the reservoir and
discontinuously release it. The slope is a measure of the cumulative power
output of the central engine. A plot of S versus peak flux in 64ms (P64ms)
shows a very good correlation over a wide range for both short and long GRBs.
No relationship was found between S and GRBs with known redshift. The standard
slope (S'), which is representative of the power output per unit time, is
correlated separately with P64ms for both sub-classes indicating more powerful
outbursts for the short GRBs. S' is also anticorrelated with GRB duration.
These results imply that GRBs are powered by accretion into a black hole.Comment: 4 pages, 2 figures. Accepted for publication in Astronomy and
Astrophysics Letter
Temporal properties of short and long gamma-ray bursts
A temporal analysis was performed on a sample of 100 bright short GRBs with
T90 < 2s from the BATSE Current Catalog along with a similar analysis on 319
long bright GRBs with T90 > 2s from the same catalog. The short GRBs were
denoised using a median filter and the long GRBs were denoised using a wavelet
method. Both samples were subjected to an automated pulse selection algorithm
to objectively determine the effects of neighbouring pulses. The rise times,
fall times, FWHM, pulse amplitudes and areas were measured and their frequency
distributions are presented. The time intervals between pulses were also
measured. The frequency distributions of the pulse properties were found to be
similar and consistent with lognormal distributions for both the short and long
GRBs. The time intervals between the pulses and the pulse amplitudes of
neighbouring pulses were found to be correlated with each other. The same
emission mechanism can account for the two sub-classes of GRBs.Comment: 3 pages, 8 figures; Proceedings of "Gamma-Ray Burst and Afterglow
Astronomy 2001", Woods Hol
Temporal properties of the short gamma-ray bursts
A temporal analysis has been performed on a sample of 100 bright gamma-ray
bursts (GRBs) with T90<2s from the BATSE current catalog. The GRBs were
denoised using a median filter and subjected to an automated pulse selection
algorithm as an objective way of idenitifing the effects of neighbouring
pulses. The rise times, fall times, FWHM, pulse amplitudes and areas were
measured and the frequency distributions are presented here. All are consistent
with lognormal distributions. The distribution of the time intervals between
pulses is not random but consistent with a lognormal distribution. The time
intervals between pulses and pulse amplitudes are highly correlated with each
other. These results are in excellent agreement with a similar analysis that
revealed lognormal distributions for pulse properties and correlated time
intervals between pulses in bright GRBs with T90>2s. The two sub-classes of
GRBs appear to have the same emission mechanism which is probably caused by
internal shocks. They may not have the same progenitors because of the generic
nature of the fireball model.Comment: 4 pages, 7 figure
A search for rapid optical variability in radio-quiet quasars
The detection of rapid variability on a time-scale of hours in radio-quiet
quasars (RQQSOs) could be a powerful discriminator between starburst, accretion
disc and relativistic jet models of these sources. This paper contains an
account of a dedicated search for rapid optical variability in RQQSOs. The
technique used differential photometry between the RQQSO and stars in the same
field of view of the CCD. The 23 RQQSOs that were observed all have high
luminosities (-27 1.
The total amount of observation time was about 60 hours and these observations
are part of an ongoing programme, started in September 1990, to search for
rapid variability in RQQSOs. No evidence for short-term variability greater
than about 0.1 magnitudes was detected in any of the 23 sources, however
long-term variability was recorded for the radio-quiet quasar PG 2112+059. The
finding charts are included here because they identify the RQQSO and the
reference stars used in the photometry, and hence are available for use by
other observers.Comment: Accepted for publication in A&AS. 10 pages, 3 figures. Figure 1
(finding charts) available by anonymous ftp from:
bermuda.ucd.ie:/pub/outgoing/charts.eps.g
Gamma-ray bursts and X-ray melting of material as a potential source of chondrules and planets
The intense radiation from a gamma-ray burst (GRB) is shown to be capable of
melting stony material at distances up to 300 light years which subsequently
cool to form chondrules. These conditions were created in the laboratory for
the first time when millimeter sized pellets were placed in a vacuum chamber in
the white synchrotron beam at the European Synchrotron Radiation Facility
(ESRF). The pellets were rapidly heated in the X-ray and gamma-ray furnace to
above 1400 C melted and cooled. This process heats from the inside unlike
normal furnaces. The melted spherical samples were examined with a range of
techniques and found to have microstructural properties similar to the
chondrules that come from meteorites. This experiment demonstrates that GRBs
can melt precursor material to form chondrules that may subsequently influence
the formation of planets. This work extends the field of laboratory
astrophysics to include high power synchrotron sources.Comment: 8 pages, 10 figures. Proceedings of the 5th INTEGRAL Workshop, Munich
16-20 February 2004. High resolution figures available at
http://bermuda.ucd.ie/%7Esmcbreen/papers/duggan_01.pd
Timing diagrams and correlations in gamma-ray bursts signal jets from accretion into black holes
The temporal properties of a sample of 498 bright gamma-ray bursts (GRBs)
with durations between 0.05 s and 674 s were analysed. The large range in
duration (T90) is accompanied by a similarly large range in the median values
of the pulse timing properties including rise time, fall time, FWHM and
separation between the pulses. Four timing diagrams relating these pulse
properties to T90 are presented and show the power law relationships between
the median values of the 4 pulse timing properties and T90, but also that the
power laws depend in a consistent manner on the number of pulses per GRB. The
timing diagrams are caused by the correlated properties of the pulses in the
burst and can be explained by a combination of factors including the Doppler
boost factor Gamma, a viewing effect caused by a jet and different progenitors.
GRBs with similar values of T90 have a wide range in the number of pulses. GRBs
with the large number of short and spectrally hard pulses may occur either from
a homogeneous jet with a higher average value of Gamma or close to the axis of
an inhomogeneous jet with higher values of Gamma near the rotation axis. The
less luminous GRBs with fewer pulses may originate further from the axis of the
inhomogeneous jet. The pulses in GRBs have six distinctive statistical
properties including correlations between time intervals, correlations between
pulse amplitudes, an anticorrelation between pulse amplitudes and time
intervals, and a link to intermittency in GRS 1915+105. The timing diagrams and
correlated pulses suggest that GRBs are powered by accretion processes
signalling jets from the formation of black holes.Comment: 4 pages, 1 figur
Similarities in the temporal properties of gamma-ray bursts and soft gamma-ray repeaters
Magnetars are modelled as sources that derive their output from magnetic
energy that substantially exceeds their rotational energy. An implication of
the recent polarization measurement of GRB 021206 is that the emission
mechanism may be dominated by a magnetic field that originates in the central
engine. Similarities in the temporal properties of SGRs and GRBs are considered
in light of the fact that the central engine in GRBs may be magnetically
dominated. The results show that 1) the time intervals between outbursts in SRG
1806-20 and pulses in GRBs are consistent with lognormal distributions and 2)
the cumulative outputs of SGRs and GRBs increase linearly with time. This
behaviour can be successfully modelled by a relaxation system that maintains a
steady state situation.Comment: To appear in the proceedings of the 2003 GRB Conference, Santa Fe, 4
pages, 4 figure
Analysis of Temporal Features of Gamma Ray Bursts in the Internal Shock Model
In a recent paper we have calculated the power density spectrum of Gamma-Ray
Bursts arising from multiple shocks in a relativistic wind. The wind optical
thickness is one of the factors to which the power spectrum is most sensitive,
therefore we have further developed our model by taking into account the photon
down-scattering on the cold electrons in the wind. For an almost optically
thick wind we identify a combination of ejection features and wind parameters
that yield bursts with an average power spectrum in agreement with the
observations, and with an efficiency of converting the wind kinetic energy in
50-300 keV emission of order 1%. For the same set of model features the
interval time between peaks and pulse fluences have distributions consistent
with the log-normal distribution observed in real bursts.Comment: ApJ in press, 2000; with slight revisions; 12 pag, 6 fi
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The effect of Ru and Sn additions to Pt on the electrocatalysis of methanol oxidation: An in situ XAS investigation
Elements such as Ru and Sn used as ad-atoms or as alloying elements are known to enhance methanol oxidation reaction (MOR). Ru, both as alloying element as well as upd deposited on Pt/C is widely acknowledged for enhancing MOR. Sn on the other hand is more controversial, with evidence indicating enhancements for MOR when present as upd layer and marginally effective when present as an alloying element. In situ XAS is used to investigate some of these inconsistencies in the electrocatalysis of MOR. Results indicate that alloying Sn with Pt (Pt{sub 3}Sn primary phase) causes partial filling of the Pt 5 d-band vacancies and increase in the Pt-Pt bond distances which is directly opposite to a similar situation with Ru. Upd Sn however does not perturb Pt structurally or electronically. Ru and Sn (both as alloying element and as upd ad-layer) are associated with oxygenated species, the nature and strength of the Ru. and Sn - oxygen interactions are potential dependent. Hence alloying with Sn renders Pt surface unfavorable for methanol adsorption in contrast to alloying with Ru. Both Ru and Sn however promote MOR via their ability to nucleate oxygenated species on their surface at lower potentials as compared to pure Pt
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