Properties Of X-ray Flashes Research Articles

Rise and fall of the X-ray flash 080330: an off-axis jet?

Context. X-ray flashes (XRFs) are a class of gamma-ray bursts (GRBs) with a peak energy of the time-integrated νFν spectrum, Ep, typically below 30 keV, whereas classical GRBs have Ep of a few hundreds of keV. Apart from Ep and the systematically lower luminosity, the properties of XRFs, such as their duration or spectral indices, are typical of the classical GRBs. Yet, the nature of XRFs and their differences from GRBs are not understood. In addition, there is no consensus on the interpretation of the shallow decay phase observed in most X-ray afterglows of both XRFs and GRBs. Aims. We examine in detail the case of XRF 080330 discovered by Swift at redshift 1.51. This burst is representative of the XRF class and exhibits an X-ray shallow decay. The rich broadband (from NIR to UV) photometric data set we collected during this phase makes it an ideal candidate for testing the off-axis jet interpretation proposed to explain both the softness of XRFs and the shallow decay phase. Methods. We present prompt γ-ray, early and late NIR/visible/UV and X-ray observations of the XRF 080330. We derive a spectral energy distribution from NIR to X-ray bands across the shallow/plateau phase and describe the temporal evolution of the multiwavelength afterglow within the context of the standard afterglow model. Results. The multiwavelength evolution of the afterglow is achromatic from ∼10 2 st o∼8 × 10 4 s. The energy spectrum from NIR to X-ray is reproduced well by a simple power-law, Fν ∝ ν −βox , with βox = 0.79 ± 0.01 and negligible rest-frame dust extinction. The light curve can be modelled by either a piecewise power-law or the combination of a smoothly broken power law with an initial rise up to ∼600 s, a plateau lasting up to ∼2 ks, followed by a gradual steepening to a power-law decay index of ∼2 until 82 ks. At this point, a bump appears to be modelled well with a second component, while the corresponding optical energy spectrum, Fν ∝ ν −βo , reddens by Δβo = 0.26 ± 0.06. Conclusions. A single-component jet viewed off-axis can explain the light curve of XRF 080330, the late-time reddening being due to the reverse shock of an energy injection episode and its being an XRF. Other possibilities, such as the optical rise marking the pre-deceleration of the fireball within a wind environment, cannot be excluded definitely, but appear to be contrived. We exclude the possibility of a dust decreasing column density being swept up by the fireball as explaining the rise of the afterglow.

Global Characteristics of X‐Ray Flashes and X‐Ray–Rich Gamma‐Ray Bursts Observed byHETE‐2

We describe and discuss the global properties of 45 gamma-ray bursts (GRBs) observed by HETE-2 during the first 3 years of its mission, focusing on the properties of X-ray flashes (XRFs) and X-ray-rich GRBs (XRRs). We find that the numbers of XRFs, XRRs, and GRBs are comparable, and that the durations and the sky distributions of XRFs and XRRs are similar to those of GRBs. We also find that the spectral properties of XRFs and XRRs are similar to those of GRBs, except that the values of the peak energy E of the burst spectrum in νFν, the peak energy flux Fpeak, and the energy fluence SE of XRFs are much smaller (and those of XRRs are smaller) than those of GRBs. Finally, we find that the distributions of all three kinds of bursts form a continuum in the [SE(2-30 keV), SE(30-400) keV] plane, the [SE(2-400 keV), Epeak] plane, and the [Fpeak(50-300 keV), Epeak] plane. These results provide strong evidence that all three kinds of bursts arise from the same phenomenon.

A Unified Jet Model of X‐Ray Flashes, X‐Ray–rich Gamma‐Ray Bursts, and Gamma‐Ray Bursts. I. Power‐Law–shaped Universal and Top‐Hat–shaped Variable Opening Angle Jet Models

HETE-2 has provided strong evidence that the properties of X-ray flashes (XRFs), X-ray-rich gamma-ray bursts (GRBs), and GRBs form a continuum, and therefore that these three kinds of bursts are the same phenomenon. A key feature found by HETE-2 is that the density of bursts is roughly constant per logarithmic interval in burst fluence SE and observed spectral peak energy E, and in isotropic-equivalent energy Eiso and spectral peak energy Epeak in the rest frame of the burst. In this paper, we explore a unified jet model of all three kinds of bursts, using population synthesis simulations of the bursts and detailed modeling of the instruments that detect them. We show that both a variable jet opening angle model in which the emissivity is a constant independent of the angle relative to the jet axis and a universal jet model in which the emissivity is a power-law function of the angle relative to the jet axis can explain the observed properties of GRBs reasonably well. However, if one tries to account for the properties of XRFs, X-ray-rich GRBs, and GRBs in a unified picture, the extra degree of freedom available in the variable jet opening angle model enables it to explain the observations reasonably well while the power-law universal jet model cannot. The variable jet opening angle model of XRFs, X-ray-rich GRBs, and GRBs implies that the energy Eγ radiated in gamma rays is ~100 times less than has been thought. The model also implies that most GRBs have very small jet opening angles (~half a degree). This suggests that magnetic fields may play an important role in GRB jets. It also implies that there are ~104-105 more bursts with very small jet opening angles for every burst that is observable. If this is the case, the rate of GRBs could be comparable to the rate of Type Ic core-collapse supernovae. These results show that XRFs may provide unique information about the structure of GRB jets, the rate of GRBs, and the nature of Type Ic supernovae.

A unified jet model of X-ray flashes and γ-ray bursts

HETE-2 has provided strong evidence that the properties of X-ray flashes (XRFs) and GRBs form a continuum, and therefore that these two types of bursts are the same phenomenon. We show that both the structured jet and the uniform jet models can explain the observed properties of GRBs reasonably well. However, if one tries to account for the properties of both XRFs and GRBs in a unified picture, the uniform jet model works reasonably well while the structured jet model does not. The uniform jet model of XRFs and GRBs implies that most GRBs have very small jet opening angles (∼half a degree). This suggests that magnetic fields play a crucial role in GRB jets. The model also implies that the energy radiated in γ rays is ∼100 times smaller than has been thought. Most importantly, the model implies that there are ∼10 4–10 5 more bursts with very small jet opening angles for every such burst we see. Thus the rate of GRBs could be comparable to the rate of Type Ic core collapse supernovae. Determination of the spectral parameters and redshifts of many more XRFs will be required in order to confirm or rule out the uniform jet model and its implications. HETE-2 is ideally suited to do this (it has localized 16 XRFs in ∼2 years), whereas Swift is less so. The unique insights into the structure of GRBs jets, the rate of GRBs, and the nature of Type Ic supernovae that XRFs may provide therefore constitute a compelling scientific case for continuing HETE-2 during the Swift mission.