F. Ryde

The cooling behavior of thermal pulses in gamma-ray bursts

We discuss gamma-ray bursts (GRBs) that have very hard spectra, consistent with blackbody radiation. Several emission components are expected, on the basis of theoretical considerations, to be visible in the gamma-ray band, mainly nonthermal emission from cooling, relativistic electrons and thermal emission from a wind photosphere. We find that the pulses we study are consistent with a thermal blackbody radiation throughout their duration and that the temperature kT can be well described by a broken power law as a function of time, with an initially constant temperature or weak decay (~100 keV). After the break, most cases are consistent with a decay with index -. A few of the pulses have a weak nonthermal component overlaying the thermal one and are better fitted with a combination of a thermal and a nonthermal component. We further demonstrate that such a two-component model can explain the whole time evolution of other bursts that are found to be thermal only initially and later become nonthermal. The relative strengths between the two components vary with time, and this is suggested, among other things, to account for the change in the modeled low-energy power-law slope that is often observed in GRBs. The secondary, nonthermal components are consistent with optically thin synchrotron emission in the cooling regime. We interpret the observations within a model of an optically thick shell (fireball) that expands adiabatically. The slow, or absent, temperature decrease is in the acceleration phase, during which the bulk Lorentz factor increases, and the faster temperature decay is reached as the flow saturates and starts to coast with a constant speed. We also discuss a Poynting-flux model, in which the saturation radius is reached close to the photosphere. Even though these observations cannot tell these models apart, the latter has several attractive advantages. The GLAST satellite will be able to clarify and further test the physical setting of similar thermal pulses.

IDENTIFICATION AND PROPERTIES OF THE PHOTOSPHERIC EMISSION IN GRB090902B

The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B, lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) cover th ...

DETECTION OF A THERMAL SPECTRAL COMPONENT IN THE PROMPT EMISSION OF GRB 100724B.

Observations of GRB 100724B with the Fermi Gamma-Ray Burst Monitor find that the spectrum is dominated by the typical Band functional form, which is usually taken to represent a non-thermal emission component, but also includes a statistically highly significant thermal spectral contribution. The simultaneous observation of the thermal and non-thermal components allows us to confidently identify the two emission components. The fact that these seem to vary independently favors the idea that the thermal component is of photospheric origin while the dominant non-thermal emission occurs at larger radii. Our results imply either a very high efficiency for the non-thermal process or a very small size of the region at the base of the flow, both quite challenging for the standard fireball model. These problems are resolved if the jet is initially highly magnetized and has a substantial Poynting flux.

Is thermal emission in gamma-ray bursts ubiquitous?

The prompt emission of gamma-ray bursts has yet defied any simple explanation, despite the presence of rich observational material and great theoretical efforts. Here we show that all the types of spectral evolution and spectral shapes that have been observed can indeed be described with one and the same model, namely, a hybrid model with a thermal and a nonthermal component. We further show that the thermal component is the key emission process determining the spectral evolution. Even though bursts appear to have a variety of sometimes complex spectral evolutions, the behaviors of the two separate components are remarkably similar for all bursts, with the temperature describing a broken power law in time. The nonthermal component is consistent with emission from a population of fast-cooling electrons emitting optically thin synchrotron emission or nonthermal Compton radiation. This indicates that these behaviors are the fundamental and characteristic ones for gamma-ray bursts.

A New Method of Determining the Initial Size and Lorentz Factor of Gamma-Ray Burst Fireballs Using a Thermal Emission Component

In recent years, increasing evidence has emerged for a thermal component in the gamma- and X- ray spectrum of the prompt emission phase in gamma- ray bursts. The temperature and flux of the thermal ...

Search for Relativistic Curvature Effects in Gamma-Ray Burst Pulses

We analyze the time profiles of individual gamma-ray burst (GRB) pulses that are longer than 2 s by modeling them with analytical functions that are based on physical first principles and well-established empirical descriptions of GRB spectral evolution. These analytical profiles are independent of the emission mechanism and can be used to model both the rise and decay profiles, allowing for the study of the entire pulse light curve. Using this method, we have studied a sample of 77 individual GRB pulses, allowing us to examine the fluence, pulse width, asymmetry, and rise and decay power-law distributions. We find that the rise phase is best modeled with a power law of average index r = 1.48 ± 0.07 and that the average decay phase has an index of d = 2.44 ± 0.12. We also find that the ratio between the rise and decay times (the pulse asymmetry) exhibited by the GRB pulse shape has an average value of 0.47, which varies little from pulse to pulse and is independent of pulse duration or intensity. Although this asymmetry is largely uncorrelated to other pulse properties, a statistically significant trend is observed between the pulse asymmetry and the decay power-law index, possibly hinting at the underlying physics. We compare these parameters with those predicted to occur if individual pulse shapes are created purely by relativistic curvature effects in the context of the fireball model, a process that makes specific predictions about the shape of GRB pulses. The decay index distribution obtained from our sample shows that the average GRB pulse fades faster than the value predicted by curvature effects, with only 39% of our sample being consistent with the curvature model. We discuss several refinements of the relativistic curvature scenario that could naturally account for these observed deviations, such as symmetry breaking and varying relative timescales within individual pulses.

A THEORY OF MULTICOLOR BLACKBODY EMISSION FROM RELATIVISTICALLY EXPANDING PLASMAS

We consider the emission of photons from the inner parts of a relativistically expanding plasma outflow, characterized by a constant Lorentz factor, Γ. Photons that are injected in regions of high optical depth are advected with the flow until they escape at the photosphere. Due to multiple scattering below the photosphere, the locally emerging comoving photon distribution is thermal. However, as an observer simultaneously sees photons emitted from different angles, hence with different Doppler boosting, the observed spectrum is a multicolor blackbody. We calculate here the properties of the observed spectrum at different observed times. Due to the strong dependence of the photospheric radius on the angle to the line of sight, for parameters characterizing gamma-ray bursts (GRBs) thermal photons are seen up to tens of seconds following the termination of the inner engine. At late times, following the inner engine termination, both the number flux and energy flux of the thermal spectrum decay as Ft –2. At these times, the multicolor blackbody emission results in a power law at low energies (below the thermal peak), with power-law index F νν0. We discuss the implications and limitations of this result in the study of GRBs.

XIPE: the X-ray imaging polarimetry explorer

Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2–10 keV band in 105 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15–35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time.

A theory of photospheric emission from relativistic, collimated outflows

Relativistic outflows in the form of jets are common in many astrophysical objects. By their very nature, jets have angle-dependent velocity profiles, Γ=Γ(r, Θ, φ), where Γ is the outflow Lorentz factor. In this work we consider photospheric emission from non-dissipative jets with various Lorentz factor profiles, of the approximate form Γ ≈ Γ0/[(Θ/Θj)p + 1], where Θj is the characteristic jet opening angle. In collimated jets, the observed spectrum depends on the viewing angle, Θv. We show that for narrow jets (ΘjΓ0 l few), the obtained low-energy photon index is α ≈-1 (dN/dE / Eα), independent of viewing angle, and weakly dependent on the Lorentz factor gradient (p). A similar result is obtained for wider jets observed at Θv ≈ Θj. This result is surprisingly similar to the average low-energy photon index seen in gamma-ray bursts. For wide jets (ΘjΓ0 g few) observed at Θv g Θj, a multicolour blackbody spectrum is obtained. We discuss the consequences of this theory on our understanding of the prompt emission in gamma-ray bursts.

Gamma-Ray Burst Spectra and Light Curves as Signatures of a Relativistically Expanding Plasma

Temporal and spectral characteristics of prompt emission of gamma-ray burst (GRB) pulses are the primary observations for constraining the energizing and emission mechanisms. In spite of very complex temporal behavior of the GRBs, several patterns have been discovered in how some spectral characteristics change during the decaying phase of individual, well-defined long (greater than a few seconds) pulses. In this paper we compare these observed signatures with those expected from a relativistically expanding, shock-heated, and radiation-emitting plasma shell. Within the internal shock model and assuming a short cooling time, we show that the angular dependence in arrival time from a spherical expanding shell can explain the general characteristics of some well-defined long GRB pulses. This includes the pulse shape, with a fast rise and a slower decay, ∝(1 + t/τ)2, where τ is a time constant, and the spectral evolution, which can be described by the hardness-intensity correlation (HIC), with the intensity being proportional to the square of the hardness measured by the value of the peak, e.g., Ep of the νFν spectrum. A variation of the relevant timescales involved (the angular spreading and the dynamic) can explain the broad, observed dispersion of the HIC index. Reasonable estimates of physical parameters lead to situations where the HIC relation deviates from a pure power law, features that are indeed present in the observations. Depending on the relative values of the rise and decay times of the intrinsic light curve, the spectral/temporal behavior, as seen by an observer, will produce the hard-to-soft evolution and the so-called tracking pulses. In our model the observed spectrum is a superposition of many intrinsic spectra arriving from different parts of the fireball shell with varying spectral shifts. Therefore, it will be broader than the emitted spectrum and its spectral parameters could have complex relations with the intrinsic ones. Furthermore, we show that the softening of the low-energy power-law index, that has been observed in some pulses, can be explained by geometric effects and does not need to be an intrinsic behavior.