Wang 2007 XRFs的逆康普顿产生高能射线

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astro-ph/0702617 [abs, ps, pdf, other] :

Title: High-Energy Gamma-Rays from GRB X-ray Flares
Authors: Xiang-Yu Wang, Zhuo Li, Peter Meszaros
Comments: 4 pages, Contributed talk presented at "The First GLAST Symposium", Feb.5-8 2007, Stanford University

The recent detection of X-ray flares during the afterglow phase of gamma-ray bursts (GRBs) suggests an inner-engine origin, at radii inside the forward shock. There must be inverse Compton (IC) emission arising from such flare photons scattered by forward shock afterglow electrons when they are passing through the forward shock. We find that this IC emission produces high energy gamma-ray flares, which may be detected by AGILE, GLAST and ground-based TeV telescopes. The anisotropic IC scattering between flare photons and forward shock electrons does not affect the total IC component intensity, but cause a time delay of the IC component peak relative to the flare peak. The anisotropic scattering effect may also weaken, to some extent, the suppression effect of the afterglow intensity induced by the enhanced electron cooling due to flare photons. We speculate that this IC component may already have been detected by EGRET from a very strong burst--GRB940217. Future observations by GLAST may help to distinguish whether X-ray flares originate from late central engine activity or from external shocks.

Krimm 2007 GRB060714的暴本身和XRFs并没有严格分界

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（其实所有XRF，我觉得都个GRB一样）

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astro-ph/0702603 [abs, ps, pdf, other] :

Title: GRB 060714: No Clear Dividing Line Between Prompt Emission and X-ray Flares
Authors: H. A. Krimm, J. Granot, F. Marshal, M. Perri, S. D. Barthelmy, D. N. Burrows, N. Gehrels, P. Mészáros, D. Morris
Comments: Submitted to ApJ

The long gamma-ray burst GRB 060714 was observed to exhibit a series of five X-ray flares beginning ~70 s after the burst trigger T0 and continuing until T0 + ~200 s. The first two flares were detected by the Burst Alert Telescope (BAT) on the Swift satellite, before Swift had slewed to the burst location, while the last three flares were strongly detected by the X-Ray Telescope (XRT) but only weakly detected by the BAT. This burst provides an unusual opportunity to track a complete sequence of flares over a wide energy range. The flares were very similar in their light curve morphology, showing power-law rise and fall components, and in most cases significant sub-structure. The flares also showed strong evolution with time, both spectrally and temporally. The small time scale and large amplitude variability observed are incompatible with an external shock origin for the flares, and support instead late time sporadic activity either of the central source or of localized dissipation events within the outflow. We show that the flares in GRB 060714 cannot be the result of internal shocks in which the contrast in the Lorentz factor of the colliding shells is very small, and that this mechanism faces serious difficulties in most Swift GRBs. The morphological similarity of the flares and the prompt emission and the gradual and continual evolution of the flares with time makes it difficult and arbitrary to draw a dividing line between the prompt emission and the flares.

Vietri 2007 SGR1806-20和QPO

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astro-ph/0702598 [abs, ps, pdf, other] :

Title: SGR1806-20: evidence for a superstrong Magnetic Field from Quasi Periodic Oscillations
Authors: M. Vietri, L. Stella, G. Israel
Comments: Accepted for publication in the Astrophysical Journal, Part I

Fast Quasi-Periodic Oscillations (QPOs, frequencies of $\sim 20 - 1840$ Hz) have been recently discovered in the ringing tail of giant flares from Soft Gamma Repeaters (SGRs), when the luminosity was of order $10^{41}-10^{41.5}$ erg/s. These oscillations persisted for many tens of seconds, remained coherent for up to hundreds of cycles and were observed over a wide range of rotational phases of the neutron stars believed to host SGRs. Therefore these QPOs must have originated from a compact, virtually non-expanding region inside the star's magnetosphere, emitting with a very moderate degree of beaming (if at all). The fastest QPOs imply a luminosity variation of $\Delta L/\Delta t \simeq 6 \times 10^{43}$ erg s$^{-2}$, the largest luminosity variation ever observed from a compact source. It exceeds by over an order of magnitude the usual Cavallo-Fabian-Rees (CFR) luminosity variability limit for a matter-to-radiation conversion efficiency of 100%. We show that such an extreme variability can be reconciled with the CFR limit if the emitting region is immersed in a magnetic field $\gtrsim 10^{15}$ G at the star surface, providing independent evidence for the superstrong magnetic fields of magnetars.

Cantiello 2007 长暴的双星并合模型

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astro-ph/0702540 [abs, ps, pdf, other] :

Title: Binary star progenitors of long gamma-ray bursts
Authors: M. Cantiello, S.-C. Yoon, N. Langer, M. Livio
Comments: Accepted for publication in A&A Letters

We present a binary channel for the progenitors of long gamma-ray bursts. We test the idea of producing rapidly rotating Wolf-Rayet stars in massive close binaries through mass accretion and consecutive quasi-chemically homogeneous evolution. The binary channel presented here may provide a means for massive stars to obtain the required high rotation rates. Moreover, it suggests that a possibly large fraction of long gamma-ray bursts occurs in runaway stars. This can have important observational consequences for both the positions of GRBs, and their afterglow properties.

Watson 2007 伽玛暴的光学和X射线余辉被消光于不同地方

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从X射线和光学所得到的介质柱密度不符，可能影响光学的主要是ISM。

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astro-ph/0702537 [abs, ps, pdf, other] :

Title: The optical and X-ray afterglows of gamma-ray bursts do not probe the same gas: ionisation in GRB environments
Authors: D. Watson, J. Hjorth, J. P. U. Fynbo (DARK, NBI, U. Copenhagen), P. Jakobsson (U. Herts.), S. Foley (UCD), R. A. M. J. Wijers (U. Amsterdam)
Comments: 5 pages, emulateapj, 2 figures, submitted to ApJL

We compare the HI column densities from Ly-alpha absorption to the metal column densities from soft X-ray absorption in gamma-ray burst (GRB) afterglows. Of the eight bursts for which useful measures are obtained we find a range in metallicities from sub-solar to a few hundred times the solar value. The lack of correlation in the column densities, and the large range and extreme values of these `metallicities', suggest that the column densities derived by one or both methods are not a reliable indication of the true total column densities towards GRBs. Ionisation of the GRB's gas cloud to large distances along the line of sight seems the most likely interpretation of these results. From the lower limit on the total column density and the UV luminosity of the GRBs we derive a maximum distance to the majority of the gas surrounding GRBs of ~3 pc, suggesting that the gas probed by optical afterglow spectra is not the cloud in which the burst occurs. This is an encouraging result for the use of GRB optical afterglows as probes of the interstellar medium (ISM) in their host galaxies, as the ISM observed is less likely to be strongly affected by the GRB or its progenitor.

Nomoto 2007 伽玛暴－超新星成协的分类

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分为四类：GRB-SN,XRF-SN,noGRB-SN,GRB-nonSN。准备用统一的jet模型解释。

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astro-ph/0702472 [abs, ps, pdf, other] :

Title: Diversity of the Supernova - Gamma-Ray Burst Connection
Authors: K. Nomoto, N. Tominaga, M. Tanaka, K. Maeda, T. Suzuki, J.S. Deng, P.A. Mazzali
Comments: 16 pages, 12 figures. To be published in the proceedings of the conference "SWIFT and GRBs: Unveiling the Relativistic Universe", Venice, June 5-9, 2006. To appear in "Il Nuovo Cimento"

The connection between the long Gamma Ray Bursts (GRBs) and Type Ic Supernovae (SNe) has revealed interesting diversity. We review the following types of the GRB-SN connection. (1) GRB-SNe: The three SNe all explode with energies much larger than those of typical SNe, thus being called Hypernovae (HNe). They are massive enough for forming black holes. (2) Non-GRB HNe/SNe: Some HNe are not associated with GRBs. (3) XRF-SN: SN 2006aj associated with X-Ray Flash 060218 is dimmer than GRB-SNe and has very weak oxygen lines. Its progenitor mass is estimated to be small enough to form a neutron star rather than a black hole. (4) Non-SN GRB: Two nearby long GRBs were not associated SNe. Such ``dark HNe'' have been predicted in this talk (i.e., just before the discoveries) in order to explain the origin of C-rich (hyper) metal-poor stars. This would be an important confirmation of the Hypernova-First Star connection. We will show our attempt to explain the diversity in a unified manner with the jet-induced explosion model.

Tominaga 2007 伽玛暴和极端贫金属大质量恒星的成协

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以期解释无超新星成协的长暴如GRB060614等。

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astro-ph/0702471 [abs, ps, pdf, other] :

Title: The Connection between Gamma-Ray Bursts and Extremely Metal-Poor Stars: Black Hole-forming Supernovae with Relativistic Jets
Authors: Nozomu Tominaga, Keiichi Maeda, Hideyuki Umeda, Ken'ichi Nomoto, Masaomi Tanaka, Nobuyuki Iwamoto, Tomoharu Suzuki, Paolo A. Mazzali
Comments: 7 pages, 3 figures. Accepted for publication in the Astrophysical Journal Letters (10 March 2007, v657n2 issue)

Long-duration gamma-ray bursts (GRBs) are thought to be connected to luminous and energetic supernovae (SNe), called hypernovae (HNe), resulting from the black-hole (BH) forming collapse of massive stars. For recent nearby GRBs~060505 and 060614, however, the expected SNe have not been detected. The upper limits to the SN brightness are about 100 times fainter than GRB-associated HNe (GRB-HNe), corresponding to the upper limits to the ejected $^{56}$Ni masses of $M({\rm ^{56}Ni})\sim 10^{-3}M_\odot$. SNe with a small amount of $^{56}$Ni ejection are observed as faint Type II SNe. HNe and faint SNe are thought to be responsible for the formation of extremely metal-poor (EMP) stars. In this Letter, a relativistic jet-induced BH forming explosion of the 40 $M_\odot$ star is investigated and hydrodynamic and nucleosynthetic models are presented. These models can explain both GRB-HNe and GRBs without bright SNe in a unified manner. Their connection to EMP stars is also discussed. We suggest that GRBs without bright SNe are likely to synthesize $\Mni\sim 10^{-4}$ to $10^{-3}M_\odot$ or $\sim 10^{-6}M_\odot$.

Kocevski 2007 XRFs脉冲宽度的内激波解释

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astro-ph/0702452 [abs, ps, pdf, other] :

Title: Pulse Width Evolution of Late Time X-rays Flares in GRBs: Evidence For Internal Shocks
Authors: Daniel Kocevski, Nathaniel Butler, Joshua S. Bloom
Comments: 26 pages, 5 figures, 1 table. Submitted to ApJ. This work expands upon and formalizes our previous report at the October 2006 AAS HEAD Meeting of the discovery of pulse width evolution

We study the duration and variability of late time X-ray flares following gamma-ray bursts (GRBs) observed by the narrow field X-ray telescope (XRT) aboard the {\it Swift} spacecraft. These flares are thought to be indicative of late time activity by the central engine that powers the GRB and produced by means similar to those which produce the prompt emission. We use a non-parametric procedure to study the overall temporal properties of the flares and a structure function analysis to look for an evolution of the fundamental variability time-scale between the prompt and late time emission. We find a strong correlation in 28 individual x-ray flares in 18 separate GRBs between the flare duration and their time of peak flux since the GRB trigger. We also find a qualitative trend of decreasing variability as a function of time since trigger, with a characteristic minimum variability timescale $\Delta t/t=0.1$ for most flares. We interpret these results as evidence of internal shocks at collision radii that are larger than those that produced the prompt emission. Contemporaneous detections of high energy emission by GLAST could be a crucial test in determining if indeed these X-ray flares originate as internal shocks behind the afterglow, as any X-ray emission originating from behind the afterglow is expected to undergo inverse Compton scattering as it passes through the external shock.

Waxman 2007 GRB060218的余辉的解释

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astro-ph/0702450 [abs, ps, pdf, other] :

Title: GRB060218: A Relativistic Supernova Shock Breakout
Authors: E. Waxman, P. Meszaros, S. Campana

We show that the prompt and afterglow X-ray emission of GRB060218, as well as its early (t<=1 d) optical-UV emission, can be explained by a model in which a radiation- mediated shock propagates through a compact progenitor star into a dense wind. The prompt thermal X-ray emission is produced in this model as the mildly relativistic shock, v/c=0.85 carrying few x 10^49 erg, reaches the wind (Thomson) photosphere, where the post-shock thermal radiation is released and the shock becomes collisionless. Adopting this interpretation of the thermal X-ray emission, a subsequent X-ray afterglow is predicted, due to synchrotron emission and inverse-Compton scattering of SN UV photons by electrons accelerated in the collisionless shock. Early optical-UV emission is also predicted, due to the cooling of the outer \delta M ~10^{-3} M_sun envelope of the star, which was heated to high temperature during shock passage. The observed X-ray afterglow and the early optical-UV emission are both consistent with those expected in this model. Detailed analysis of the early optical-UV emission may provide detailed constraints on the density distribution near the stellar surface.

Wang 2007 回落盘模型下的前兆时标

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astro-ph/0702441 [abs, ps, pdf, other] :

Title: GRB Precursors in the Fallback Collapsar Scenario
Authors: Xiang-Yu Wang, Peter Meszaros
Comments: 6 pages, submitted to ApJ

Precursor emission has been observed in a non-negligible fraction of gamma-ray bursts, the time gap between the precursor and the main burst extending up to hundreds of seconds in some cases, such as in GRB041219A, GRB050820A and GRB060124. We have investigated the maximum possible time gaps arising from the jet propagation inside the progenitor star, in models which assume that the precursor is produced by the jet bow shock or the cocoon breaking out of the progenitor. Due to the pressure drop ahead of the jet head after it reaches the stellar surface, a rarefaction wave propagates back into the jet at the sound speed, which re-accelerates the jet to a relativistic velocity and therefore limits the gap period to within about ten seconds. This scenario therefore cannot explain gaps which are hundreds of seconds long. Instead, we ascribe such long time gaps to the behavior of the central engine, and suggest a fallback collapsar scenario for these bursts. In this scenario, the precursor is produced by a weak jet formed during the initial core collapse, possibly related to MHD process associated with a short-lived proto-neutron star, while the main burst is produced by a stronger jet fed by the fallback disk accreting onto the black hole resulting from the collapse of the neutron star. We have examined the propagation times of the weak precursor jet through the stellar progenitor. We find that the initial weak jet can break out of the progenitor in a time less than ten seconds provided that it has a moderately high relativistic Lorentz factor (Gamma>10). The longer time gap following this is dictated by the fall-back timescale, which is long enough for the exit channel to close after the precursor activity ceases, allowing for the collimation of the main jet by the cocoon pressure as it propagates outward.