A deceleration search for magnetar pulsations in the X-ray plateaus of short GRBs

A newly formed magnetar has been proposed as the central engine of short GRBs to explain ongoing energy injection giving observed plateau phases in the X-ray light curves. These rapidly spinning magnetars may be capable of emitting pulsed emission comparable to known pulsars and magnetars. In this paper we show that, if present, a periodic signal would be detectable during the plateau phases observed using the Swift/X-Ray Telescope recording data in Window Timing mode. We conduct a targeted deceleration search for a periodic signal from a newly formed magnetar in 2 Swift short GRBs and rule out any periodic signals in the frequency band 10–285 Hz to ≈15–30 per cent rms. These results demonstrate that we would be able to detect pulsations from the magnetar central engine of short GRBs if they contribute to 15–30 per cent of the total emission. We consider these constraints in the context of the potential emission mechanisms. The non-detection is consistent with the emission being reprocessed in the surrounding environment or with the rotation axis being highly aligned with the observing angle. As the emission may be reprocessed, the expected periodic emission may only constitute a few per cent of the total emission and be undetectable in our observations. Applying this strategy to future observations of the plateau phases with more sensitive X-ray telescopes may lead to the detection of the periodic signal

The Pulse Luminosity Function of Swift Gamma-ray Bursts

The complete Swift Burst Alert Telescope and X-Ray Telescope light curves of 118 gamma-ray bursts (GRBs) with known redshifts were fitted using the physical model of GRB pulses by Willingale et al. to produce a total of 607 pulses. We compute the pulse luminosity function utilizing three GRB formation rate models: a progenitor that traces the cosmic star formation rate density (CSFRD) with either a single population of GRBs, coupled to various evolutionary parameters, or a bimodal population of high- and low-luminosity GRBs; and a direct fit to the GRB formation rate excluding any a priori assumptions. We find that a single population of GRB pulses with an evolving luminosity function is preferred over all other univariate evolving GRB models, or bimodal luminosity functions in reproducing the observed GRB pulse L-z distribution and that the magnitude of the evolution in brightness is consistent with studies that utilize only the brightest GRB pulses. We determine that the appearance of a GRB formation rate density evolution component is an artefact of poor parametrization of the CSFRD at high redshifts rather than indicating evolution in the formation rate of early epoch GRBs. We conclude that the single brightest region of a GRB light curve holds no special property; by incorporating pulse data from the totality of GRB emission we boost the GRB population statistics by a factor of 5, rule out some models utilized to explain deficiencies in GRB formation rate modelling, and constrain more tightly some of the observed parameters of GRB behaviour

The spectral-temporal properties of the prompt pulses and rapid decay phase of gamma-ray bursts

The prompt emission from gamma-ray burst is the brightest electromagnetic emission known, yet its origin is not understood. The flux density of individual prompt pulses of a GRB can be represented by an analytical expression derived assuming the emission is from a thin, ultrarelativistically expanding, uniform, spherical shell over a finite range of radii. We present the results of fitting this analytical expression to the light curves from the four standard Swift Burst Alert Telescope energy bands and two standard Swift X-ray Telescope energy bands of 12 bursts. The expression includes the high latitude emission (HLE) component and the fits provide a rigorous demonstration that the HLE can explain the rapid decay phase of the prompt emission. The model also accommodates some aspects of energy-dependent lag and energy-dependent pulse width, but there are features in the data which are not well represented. Some pulses have a hard, narrow peak which is not well fitted or a rise and decay which are faster than expected using the standard indices derived assuming synchrotron emission from internal shocks, although it might be possible to accommodate these features using a different emission mechanism within the same overall framework. The luminosity of pulses is correlated with the peak energy of the pulse spectrum, Lf∝[Epeak(1 +z)]1.8, and anticorrelated with the time since ejection of the pulse, Lf∝[Tf/(1 +z)]−2.0

The dust scattering model cannot explain the shallow X-ray decay in GRB afterglows

A dust scattering model was recently proposed to explain the shallow X-ray decay (plateau) observed prevalently in Gamma-Ray Burst (GRB) early afterglows. In this model, the plateau is the scattered prompt X-ray emission by the dust located close (about 10 to a few hundred pc) to the GRB site. In this paper, we carefully investigate the model and find that the scattered emission undergoes strong spectral softening with time, due to the model's essential ingredient that harder X-ray photons have smaller scattering angle thus arrive earlier, while softer photons suffer larger angle scattering and arrive later. The model predicts a significant change, that is Δβ∼ 2–3, in the X-ray spectral index from the beginning of the plateau towards the end of the plateau, while the observed data show close to zero softening during the plateau and the plateau-to-normal transition phase. The scattering model predicts a big difference between the harder X-ray light curve and the softer X-ray light curve, i.e. the plateau in harder X-rays ends much earlier than in softer X-rays. This feature is not seen in the data. The large scattering optical depths of the dust required by the model imply strong extinction in optical, A[subscript: V]≳ 10, which contradicts current findings of A[subscript: V]= 0.1–0.7 from optical and X-ray afterglow observations. We conclude that the dust scattering model cannot explain the X-ray plateaus

Different progenitors of short hard gamma-ray bursts

We consider the spatial offsets of short hard gamma-ray bursts (SHBs) from their host galaxies. We show that all SHBs with extended-duration soft emission components lie very close to their hosts. We suggest that neutron star–black hole binary mergers offer a natural explanation for the properties of this extended-duration/low-offset group. SHBs with large offsets have no observed extended emission components and are less likely to have an optically detected afterglow, properties consistent with neutron star–neutron star binary mergers occurring in low-density environments

Soft X-ray emission lines in the afterglow spectrum of GRB 011211: A detailed XMM-Newton analysis

We report on an XMM-Newton observation of the X-ray afterglow of the Gamma Ray Burst GRB 011211, originally detected by Beppo-SAX on 11th December 2001. The early afterglow spectrum obtained by XMM-Newton, observed 11 hours after the initial burst, appeared to reveal decaying H-like K $\alpha$ emission lines of Mg, Si, S, Ar and Ca, arising in enriched material with an outflow velocity of order 0.1c (Reeves et al. 2002). This was attributed to matter ejected from a massive stellar progenitor occurring shortly before the burst itself. Here, we present a detailed re-analysis of the XMM-Newton EPIC observations of GRB 011211. In particular, we show that the detection of the soft X-ray line emission appears robust, regardless of detector background, calibration, spectral binning, or the spectral model that is assumed. We demonstrate that thermal emission, from an optically thin plasma, is the most plausible model that can account for the soft X-ray emission, which appears to be the case for at least two burst afterglow spectra observed by XMM-Newton. The X-ray spectrum of GRB 011211 appears to evolve with time after the first 10 ks of the XMM-Newton observation as the Si and S emission lines are only detected during the first 10 ks of observation. The observations suggest that thermal emission is present during the early afterglow spectrum, whilst a power-law component dominates the latter stages. Finally we estimate the mass of the ejected material in GRB 011211 to be of the order 4-20 solar masses

Can X-ray emission powered by a spinning-down magnetar explain some gamma-ray burst light-curve features?

Long-duration gamma-ray bursts (GRBs) are thought to be produced by the core-collapse of a rapidly rotating massive star. This event generates a highly relativistic jet and prompt gamma-ray and X-ray emission arises from internal shocks in the jet or magnetized outflows. If the stellar core does not immediately collapse to a black hole, it may form an unstable, highly magnetized millisecond pulsar or magnetar. As it spins down, the magnetar would inject energy into the jet causing a distinctive bump in the GRB light curve where the emission becomes fairly constant followed by a steep decay when the magnetar collapses. We assume that the collapse of a massive star to a magnetar can launch the initial jet. By automatically fitting the X-ray light curves of all GRBs observed by the Swift satellite, we identified a subset of bursts which have a feature in their light curves which we call an internal plateau – unusually constant emission followed by a steep decay – which may be powered by a magnetar. We use the duration and luminosity of this internal plateau to place limits on the magnetar spin period and magnetic field strength, and find that they are consistent with the most extreme predicted values for magnetars

XMM-Newton observation of the Seyfert 1 ESO 198-G24

We present the results from an XMM-Newton observation (January 24, 2001) of the bright Seyfert 1 galaxy ESO 198-G24 ( z=0.045). We found that this Seyfert has an intrinsic 2-10 keV luminosity of about 10 44 erg s -1. This source shows no intrinsic absorption in addition to the Galactic absorption ( ${\cal N}_{\rm H}\sim3\times10^{20}$ cm -2). We found both with EPIC and RGS that this source possesses significantly steeper spectra below ~1.5-2 keV than observed at higher X-ray energies, the so-called soft X-ray excess. The RGS spectra reveal no significant narrow absorption lines suggesting that if there is a warm absorber, it either has a relatively low column density, or a very high ionization parameter. The RGS data are well described by the combination of a power-law, a modified black body continuum, and weak relativistic lines of \ion{O}{viii}, and \ion{C}{vi} Ly $_{\alpha}$. However other interpretations are not definitely excluded. The 2-10 keV energy band is well fitted by an absorbed power-law with a photon spectral index of $\Gamma=1.77\pm0.04$ (consistent with the typical $\Gamma \sim1.7$ found in Seyfert 1 galaxies). We found the presence of a narrow Gaussian emission line at 6.41 keV (i.e. < \ion{Fe}{xvii}) with a moderate equivalent width of about 60-70 eV, and we found an upper limit for a broad component, if any, of 75 eV. We also found a weak absorption edge associated with cold iron with an optical depth of about 0.2

The X-ray afterglow of GRB 020322

The spectrum of the afterglow of GRB 020322 is the highest-quality X-ray spectrum of a GRB afterglow available to date. It was detected by XMM-Newton in an observation starting fifteen hours after the GRB with a mean 0.2-10.0 keV observed flux of $3.5\pm0.2\times10^{-13}$ erg cm -2 s -1, making it the brightest X-ray afterglow observed so far with XMM-Newton. The source faded; its lightcurve was well fit by a power-law with a decay index of $1.26\pm0.23$. The spectrum is adequately fit with a power-law absorbed with neutral or ionised gas significantly in excess of the foreground Galactic column, at redshift 1.8-1.1+1.0 or with low metal abundances. No spectral line or edge features are detected at high significance, in particular, a thermal emission model fits the data poorly, the upper limit on its contribution to the spectrum is $3.7\times10^{-14}$ erg cm -2 s -1, or ~10% of the total flux. No spectral variability is observed

On the nature of late X-ray flares in Swift gamma-ray bursts

Context. Previously detected in only a few gamma-ray bursts (GRBs), X-ray flares are now observed in $\sim$50% of Swift GRBs, though their origins remain unclear. Most flares are seen early on in the afterglow decay, while some bursts exhibit flares at late times of 104 to 105 s, which may have implications for flare models. Aims. We investigate whether a sample of late time ($\gtrsim$ $1 \times 10^4$ s) flares are different from previous samples of early time flares, or whether they are merely examples on the tail of the early flare distribution. Methods. We examine the X-ray light curves of Swift bursts for late flares and compare the flare and underlying temporal power-law properties with those of early flares, and the values of these properties predicted by the blast wave model. Results. The burst sample shows late flare properties consistent with those of early flares, where the majority of the flares can be explained by either internal or external shock, though in a few cases one origin is favoured over the other. The underlying power-laws are mainly consistent with the normal decay phases of the afterglow. Conclusions. If confirmed by the ever growing sample of late time flares, this would imply that, in some cases, prolonged activity out to a day or a restarting of the central engine is required