Christoffer Lundman

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 ...

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.

Observational evidence of dissipative photospheres in gamma-ray bursts

The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a Planck function or it can be broadened and have the shape ...

Variable jet properties in GRB 110721A: time resolved observations of the jet photosphere

Fermi Gamma-ray Space Telescope observations of GRB110721A have revealed two emission components from the relativistic jet: emission from the photosphere, peaking at 100 keV and a non-thermal component, which peaks at 1000 keV. We use the photospheric component to calculate the properties of the relativistic outow. We nd a strong evolution in the ow properties: the Lorentz factor decreases with time during the bursts from 1000 to 150 (assuming a redshift z = 2; the values are only weakly dependent on unknown eciency parameters). Such a decrease is contrary to the expectations from the internal shocks and the isolated magnetar birth models. Moreover, the position of the ow nozzle measured from the central engine, r0, increases by more than two orders of magnitude. Assuming a moderately magnetised outow we estimate that r0 varies from 10 6 cm to 10 9 cm during the burst. We suggest that the maximal value reects the size of the progenitor core. Finally, we show that these jet properties naturally explain the observed broken power-law decay of the temperature which has been reported as a characteristic for GRB pulses.

Polarization properties of photospheric emission from relativistic, collimated outflows

We consider the polarization properties of photospheric emission originating in jets consisting of a highly relativistic core of opening angle theta(j) and Lorentz factor Gamma(0), and a surrounding shear layer where the Lorentz factor is decreasing as a power law of index p with angle from the jet axis. We find significant degrees of linear polarization for observers located at viewing angles theta(v) greater than or similar to theta(j). In particular, the polarization degree of emission from narrow jets (theta(j) approximate to 1/Gamma(0)) with steep Lorentz factor gradients (p greater than or similar to 4) reaches similar to 40 per cent. The angle of polarization may shift by pi/2 for time-variable jets. The spectrum below the thermal peak of the polarized emission appears non-thermal due to aberration of light, without the need for additional radiative processes or energy dissipation. Furthermore, above the thermal peak a power law of photons forms due to Comptonization of photons that repeatedly scatter between regions of different Lorentz factor before escaping. We show that polarization degrees of a few tens of per cent and broken power-law spectra are natural in the context of photospheric emission from structured jets. Applying the model to gamma-ray bursts, we discuss expected correlations between the spectral shape and the polarization degree of the prompt emission.

Spectral components in the bright, long GRB 061007: properties of the photosphere and the nature of the outflow

We present a time-resolved spectral analysis of the bright, long GRB 061007 (z = 1.261) using Swift and Suzaku data. We find that the prompt emission of the burst can be equally well explained by a photospheric component together with a power law as by a Band function, and we explore the implications of the former model. The photospheric component, which we model with a multicolour blackbody, dominates the spectra and has a very stable shape throughout the burst. This component provides a natural explanation for the hardness-intensity correlation seen within the burst and also allows us to estimate the bulk Lorentz factor and the radius of the photosphere. The power-law component dominates the fit at high energies and has a nearly constant slope of -1.5. We discuss the possibility that this component is of the same origin as the high-energy power laws recently observed in some Fermi bursts.