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

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