Spectral Peak Energy Research Articles

GRB 081028 and its late-time afterglow re-brightening

Swift captured for the first time a smoothly rising X-ray re-brightening of clear non-flaring origin after the steep decay in a long gamma-ray burst (GRB): GRB081028. A rising phase is likely present in all GRBs but is usually hidden by the prompt tail emission and constitutes the first manifestation of what is later to give rise to the shallow decay phase. Contemporaneous optical observations reveal a rapid evolution of the injection frequency of a fast cooling synchrotron spectrum through the optical band, which disfavours the afterglow onset (start of the forward shock emission along our line of sight when the outflow is decelerated) as the origin of the observed re-brightening. We investigate alternative scenarios and find that the observations are consistent with the predictions for a narrow jet viewed off-axis. The high on-axis energy budget implied by this interpretation suggests different physical origins of the prompt and (late) afterglow emission. Strong spectral softening takes place from the prompt to the steep decay phase: we track the evolution of the spectral peak energy from the gamma-rays to the X-rays and highlight the problems of the high latitude and adiabatic cooling interpretations. Notably, a softening of both the high and low spectral slopes with time is also observed. We discuss the low on-axis radiative efficiency of GRB081028 comparing its properties against a sample of Swift long GRBs with secure E_gamma,iso measurements.

SWIFT GRB GRB071010B: OUTLIER OF THE E src peak – E γ AND E iso – E src peak – t src jet CORRELATIONS

We present multi-band results for GRB071010B based on \Swift, \Suzaku, and ground-based optical observations. This burst is an ideal target to evaluate the robustness of the ${\rm E^{src}_{peak}-E_{iso}}$ and ${\rm E^{src}_{peak}-E_{\gamma}}$ relations, whose studies have been in stagnation due to the lack of the combined estimation of $\rm E^{src}_{peak}$ and long term optical monitoring. The joint prompt spectral fitting using \Swift/Burst Alert Telescope and \Suzaku/Wide-band All sky Monitor data yielded the spectral peak energy as E$^{src}_{peak}$ of $86.5^{+6.4}_{-6.3}$ keV and E$_{iso}$ of $2.25^{+0.19}_{-0.16}\times10^{52}$ erg with $z=0.947$. The optical afterglow light curve is well fitted by a simple power law with temporal index $\alpha=-0.60\pm0.02$. The lower limit of temporal break in the optical light curve is 9.8 days. Our multi-wavelength analysis reveals that GRB071010B follows ${\rm E^{src}_{peak}-E_{iso}}$ but violates the ${\rm E^{src}_{peak}-E_{\gamma}}$ and ${\rm E_{iso}-E^{src}_{peak}-t^{src}_{jet}}$ at more than the 3$\sigma$ level.

A dual-channel, curved-crystal spectrograph for petawatt laser, x-ray backlighter source studies

A dual-channel, curved-crystal spectrograph was designed to measure time-integrated x-ray spectra in the approximately 1.5 to 2 keV range (6.2-8.2 A wavelength) from small-mass, thin-foil targets irradiated by the VULCAN petawatt laser focused up to 4x10(20) W/cm(2). The spectrograph consists of two cylindrically curved potassium-acid-phthalate crystals bent in the meridional plane to increase the spectral range by a factor of approximately 10 compared to a flat crystal. The device acquires single-shot x-ray spectra with good signal-to-background ratios in the hard x-ray background environment of petawatt laser-plasma interactions. The peak spectral energies of the aluminum He(alpha) and Ly(alpha) resonance lines were approximately 1.8 and approximately 1.0 mJ/eV sr (approximately 0.4 and 0.25 J/A sr), respectively, for 220 J, 10 ps laser irradiation.

GAMMA-RAY BURST LUMINOSITY FUNCTIONS BASED ON A NEWLY DISCOVERED CORRELATION BETWEEN PEAK SPECTRAL ENERGY ANDV/Vmax

We have discovered a correlation between the observed peak spectral energy E_(pk,obs) and the Euclidean value of of gamma-ray bursts (GRBs). We present the evidence for the correlation in the GUSBAD catalog and use it to derive the luminosity function of GRBs without using any redshifts. The procedure involves dividing GUSBAD GRBs into five spectral classes based on their E_(pk,obs). The overall luminosity function is derived assuming that each of the spectral classes contributes a Gaussian luminosity function. Their central luminosity is derived from the observed Euclidean . We explore various forms for the GRB rate function GR(z) in predicting redshift distributions of GRBs detected by Swift. We find that GR(z) peaks at a higher redshift than the typical star formation history currently favored in the literature. We consider two examples of GR(z) that successfully predict the observed redshift distribution of Swift GRBs. With the luminosity functions in hand, we convert the E_(pk,obs) – V/V_(max) correlation into an E_(pk,obs) – L_(iso) correlation and a rest-frame E_(pk) – L_(iso) correlation. By comparing the E_(pk) – L_(iso) correlation with a published correlation based on GRBs with known E_(pk,obs) and redshifts, we discuss the effect of Malmquist bias.

Time-Evolution of Peak Energy and Luminosity Relation within Pulses for GRB 061007: Probing Fireball Dynamics

Abstract We performed a time-resolved spectral analysis of a bright, long gamma-ray burst GRB 061007 using Suzaku/WAM and Swift/BAT. Because of the large effective area of the WAM, we can investigate the time evolution of the spectral peak energy $E^{\rm t}_{ \rm peak}$ and the luminosity $L^{\rm t}_{\rm iso}$ with 1-s time resolution, and we find that the time-resolved pulses also satisfy the $E_{\rm peak}$–$L_{\rm iso}$ relation, which was found for the time-averaged spectra of other bursts, suggesting the same physical conditions in each pulse. Furthermore, the initial rising phase of each pulse could be an outlier of this relation with higher $E^{\rm t}_{ \rm peak}$ value by a factor of 2. This difference could suggest that the fireball radius expands by a factor of 2–4 and/or bulk Lorentz factor of the fireball is decelerated by a factor of $\sim$4 during the initial phase, providing a new probe of fireball dynamics in real time.

Acoustics and perception of velar softening for unaspirated stops

This paper provides articulatory, acoustic and perceptual data in support of the hypothesis that the velar softening process through which /k/ becomes /tʃ/ is based on articulation rather than on acoustic equivalence if operating on unaspirated stops. Production data for unaspirated /k/ are analyzed for five speakers of Majorcan Catalan, where the velar stop phoneme exhibits (alveolo)palatal or velar allophones depending on vowel context and position. Data on several parameters, i.e., contact anteriority and dorsopalatal contact degree, burst spectral peak frequency, energy and duration, and F2 and F3 vowel transition endpoints and ranges, suggest that /tʃ/ may have originated from (alveolo)palatal stop realizations not only before front vocalic segments but also before low and central vowels and word finally. Perception results are consistent with the production data in indicating that the most significant /tʃ/ perception cues are burst energy before /a, u/ and burst duration word/utterance finally. They also suggest that velar softening for unaspirated /k/ before front vocalic segments is triggered by an increase in burst frication energy and duration resulting from the narrowing of an (alveolo)palatal central channel occurring at stop closure release. These findings are in agreement with the existence of contextual and positional (alveolo)palatal stop allophones of /k/, and with evidence from sound change, in the Romance languages.

GENERALIZED TESTS FOR SELECTION EFFECTS IN GAMMA-RAY BURST HIGH-ENERGY CORRELATIONS

Several correlations among parameters derived from modeling the high-energy properties of gamma-ray bursts (GRBs) have been reported. We show that well known examples of these have common features indicative of strong contamination by selection effects. We focus here on the impact of detector threshold truncation on the spectral peak versus isotropic equivalent energy release (E pk-E iso) relation, extended to a large sample of 218 Swift and 56 HETE-2 GRBs with and without measured redshift. The existence of faint Swift events missing from pre-Swift surveys calls into question inferences based on pre-Swift surveys which must be subject to complicated incompleteness effects. We demonstrate a generalized method for treating data truncation in correlation analyses and apply this method to Swift and pre-Swift data. Also, we show that the E pk-E γ (Ghirlanda) correlation is effectively independent of the GRB redshifts, which suggests that its existence has little to do with intrinsic physics. We suggest that a physically based correlation, manifest observationally, must show significantly reduced scatter in the rest frame relative to the observer frame and must not persist if the assumed redshifts are scattered. As with the E pk-E γ correlation, we find that the pre-Swift, bright GRB E pk-E iso correlation of Amati does not rigorously satisfy these conditions.

Scaling of the proton density reduction scheme for the laser acceleration of proton beams with a narrow energy spread

The laser acceleration of proton beams with quasi-monoenergetic features in the energy spectra from microdot targets is investigated by numerical simulation. The formation of these spectral peaks is strongly dependent on the interplay between different ion species in the target. The scaling of the spectral peak's energy, and number of protons in the spectral peak, with both microdot composition and laser intensity is considered. Particular attention is given to determining the proton concentration below which the number of protons in the spectral peak rapidly diminishes. It is shown that at proton concentrations of 1-5ncrit a spectral peak is produced that reaches an energy up to 70% of the maximum proton energy, whilst still containing more protons than would be produced by a conventional target in this energy range.

The X-ray transient 080109 in NGC 2770: an X-ray flash associated with a normal core-collapse supernova

Although it is generally thought that long-duration gamma-ray bursts (LGRBs) are associated with core-collapse supernovae (SNe), so far only four pairs of GRBs and SNe with firmly established connection have been found. All the four GRB-SNe belong to special class of Type Ic – called the broad-lined SNe indicative of a large explosion energy, suggesting that only a small fraction of SNe Ibc have GRBs associated with them. This scheme has been refreshed by the discovery of a bright X-ray transient in NGC 2770 on 2008 January 9, which was followed by a rather normal Type Ib SN 2008D. In this paper, I argue that the transient 080109 is an X-ray flash (XRF, the soft version of a GRB) because of the following evidence. (1) The transient cannot be interpreted as a SN shock breakout event. (2) The GRB X-ray flare interpretation is not supported by the high-energy observation. I then show that XRF 080109 satisfies the well-known relation between the isotropic-equivalent energy and the peak spectral energy for LGRBs, which highly strengthens the XRF interpretation. Finally, I point out that the peak spectral energy of XRF 080109 and the maximum bolometric luminosity of SN 2008D agree with the Eγ,peak–LSN,max relationship of Li, strengthening the validity of the relationship. I speculate that events like XRF 080109 may occur at a rate comparable to SNe Ibc, and a soft X-ray telescope devoted to surveying for nearby X-ray flares will be very fruitful in discovering them.

Broadband Spectral Properties of Bright High‐Energy Gamma‐Ray Bursts Observed with BATSE and EGRET

We present the spectral analysis of duration-integrated broadband spectra (in ~30 keV–200 MeV) of 15 bright BATSE gamma-ray bursts (GRBs). Some GRB spectra are very hard, with their spectral peak energies being above the BATSE LAD passband limit of ~2 MeV. In such cases, their high-energy spectral parameters (peak energy and high-energy power-law indices) cannot be adequately constrained by BATSE LAD data alone. A few dozen bright BATSE GRBs were also observed with EGRET's calorimeter, TASC, in multi-MeV energy band, with a large effective area and fine energy resolution. Combining the BATSE and TASC data, therefore, affords spectra that span four decades of energy (30-200 MeV), allowing for a broadband spectral analysis with good statistics. Studying such broadband high-energy spectra of GRB prompt emission is crucial, as they provide key clues to understanding its gamma-ray emission mechanism. Among the 15 GRB spectra, we found two cases with a significant high-energy excess, and another case with a extremely high peak energy (Epeak 170 MeV). There have been very limited number of GRBs observed at MeV energies and above, and only a few instruments have been capable of observing GRBs in this energy band with such high sensitivity. Thus, our analysis results presented here should also help predict GRB observations with current and future high-energy instruments such as AGILE and GLAST, as well as with ground-based very-high-energy telescopes.

Swift captures the spectrally evolving prompt emission of GRB 070616★

The origins of gamma-ray burst (GRB) prompt emission are currently not well understood and in this context long, well-observed events are particularly important to study. We present the case of GRB 070616, analysing the exceptionally long-duration multipeaked prompt emission, and later afterglow, captured by all the instruments on-board Swift and by Suzaku Wide-Band All-Sky Monitor (WAM). The high-energy light curve remained generally flat for several hundred seconds before going into a steep decline. Spectral evolution from hard to soft is clearly taking place throughout the prompt emission, beginning at 285 s after the trigger and extending to 1200 s. We track the movement of the spectral peak energy, whilst observing a softening of the low-energy spectral slope. The steep decline in flux may be caused by a combination of this strong spectral evolution and the curvature effect. We investigate origins for the spectral evolution, ruling out a superposition of two power laws and considering instead an additional component dominant during the late prompt emission. We also discuss origins for the early optical emission and the physics of the afterglow. The case of GRB 070616 clearly demonstrates that both broad-band coverage and good time resolution are crucial to pin down the origins of the complex prompt emission in GRBs.

A Complete Catalog ofSwiftGamma‐Ray Burst Spectra and Durations: Demise of a Physical Origin for Pre‐SwiftHigh‐Energy Correlations

We calculate durations and spectral parameters for 218 Swift bursts detected by the BAT instrument between and including gamma-ray bursts (GRBs) 041220 and 070509, including 77 events with measured redshifts. Incorporating prior knowledge into the spectral fits, we are able to measure the characteristic νFν spectral peak energy Epk, obs and the isotropic equivalent energy Eiso (1-104 keV) for all events. This complete and rather extensive catalog, analyzed with a unified methodology, allows us to address the persistence and origin of high-energy correlations suggested in pre-Swift observations. We find that the Epk, obs-Eiso correlation is present in the Swift sample; however, the best-fit power-law relation is inconsistent with the best-fit pre-Swift relation at >5 σ significance. It has a factor 2 larger intrinsic scatter, after accounting for large errors on Epk, obs. A large fraction of the Swift events are hard and subluminous relative to (and inconsistent with) the pre-Swift relation, in agreement with indications from BATSE GRBs without redshift. Moreover, we determine an experimental threshold for the BAT detector and show how the Epk, obs-Eiso correlation arises artificially due to partial correlation with the threshold. We show that pre-Swift correlations found by Amati et al., Yonetoku et al., and Firmani et al., and independently by others are likely unrelated to the physical properties of GRBs and are likely useless for tests of cosmology. Also, an explanation for these correlations in terms of a detector threshold provides a natural and quantitative explanation for why short-duration GRBs and events at low redshift tend to be outliers to the correlations.

GRB Energies and E γ - E peak Correlation with the Jet Expanding Laterally at the Sound Speed

A Gamma-ray burst (GRB) is generally believed to be a jet with a small opening angle, this opening angle is usually derived with the afterglow light curve break time using an analytical method. Here we show that the method is not accurate. Using the set of equations of hydrodynamic evolution with the sideways expansion at the local sound speed derived by previous authors and the observed light curve break times, we numerically derive the initial opening angles. Then the collimation-corrected energies (E-gamma) for a sample of GRBs are calculated. They are found to show a wide spread, suggesting that the previously declared clustering by some authors may not exist. Also, the E-peak - E-gamma relation, claimed by some other authors (E-peak is the spectral peak energy), is found still to hold, with a slightly stronger correlation.

AreSwiftgamma-ray bursts consistent with the Ghirlanda relation?

A few tight correlations linking several properties of gamma-ray bursts (GRBs), namely the spectral peak energy, the total radiated energy, and the afterglow break time, have been discovered with pre-Swift GRBs. They were used to constrain the cosmological parameters, together with type-Ia supernovae. However, the tightness of these correlations is a challenge to GRB models. We explore the effect of adding Swift bursts to the Ghirlanda and Liang-Zhang relations. Although they are both still valid, they become somewhat weakened mostly due to the presence of significant outliers, which otherwise are apparently normal GRBs so difficult to distinguish. The increased dispersion of the relations makes them less reliable for purposes of precision cosmology.

Quasimonochromatic emission spectra in the near field by polar semiconductor thermal sources

The near-field spectra of the electromagnetic field emitted from a planar surface are theoretically investigated for a number of polar semiconductor and dielectric materials that support surface phonon polaritons. All of the studied materials, InP, GaAs, GaN, SiC, and α-Al2O3 (sapphire), exhibit quasimonochromatic thermal emission symbolized by strong peaks of evanescent modes at well-defined frequencies in the near field that correspond to the appropriate peaks in the density of states for surface phonon polaritons. It is also found that the materials with lower polariton frequencies (e.g., InP and GaAs) generally demonstrate a higher peak spectral energy density compared to those with higher frequencies (e.g., SiC). This trend is maintained over the entire range of temperature (300–600K) and the distance from the surface (⩽10μm) considered in the calculation. Thus, the results clearly indicate that among the studied materials InP and GaAs are the best candidates to provide the quasicoherent thermal emission for potential use as a nanoscale thermal source. The energy density stored in the evanescent peaks, when close to the surface, is estimated to be many orders of magnitude larger than that in the blackbody radiation.

SwiftDiscovery of Gamma‐Ray Bursts without a Jet Break Feature in Their X‐Ray Afterglows

We analyze Swift gamma-ray bursts (GRBs) and X-ray afterglows for three GRBs with spectroscopic redshift determinations -- GRB 050401, XRF 050416a, and GRB 050525a. We find that the relation between spectral peak energy and isotropic energy of prompt emissions (the Amati relation) is consistent with that for the bursts observed in pre-Swift era. However, we find that the X-ray afterglow lightcurves, which extend up to 10-70 days, show no sign of the jet break that is expected in the standard framework of collimated outflows. We do so by showing that none of the X-ray afterglow lightcurves in our sample satisfies the relation between the spectral and temporal indices that is predicted for the phase after jet break. The jet break time can be predicted by inverting the tight empirical relation between the peak energy of the spectrum and the collimation-corrected energy of the prompt emission (the Ghirlanda relation). We find that there are no temporal breaks within the predicted time intervals in X-ray band. This requires either that the Ghirlanda relation has a larger scatter than previously thought, that the temporal break in X-rays is masked by some additional source of X-ray emission, or that it does not happen because of some unknown reason.

Redshift degeneracy in the Eiso-Epeak relation of gamma-ray bursts

Abstract In this Letter we show that there is a redshift degeneracy in the Eiso–Epeak relation of gamma-ray bursts (GRBs). If a GRB has a redshift solved from the Eiso–Epeak relation that lies in the range of 0.9 < z < 20, a GRB that has the same observed fluence and peak spectral energy but is at a different redshift in that range also satisfies the Eiso–Epeak relation within a 1σ error, implying an extremely large error in the calculated redshift. Even if the data scatter in the Eiso–Epeak relation is reduced by a factor of 2, the error in the predicted redshift is still large enough to prevent one from constraining the redshift meaningfully. Hence, the Eiso–Epeak relation is not useful for determining the GRB distance.

A comparative study of the internal quantum efficiency for the green MQWs grown on sapphire and FS‐GaN substrates

We observed the green MQWs grown on FS-GaN substrates leading to less generation of dislocation, V-defect, and stacking fault than those on sapphire by AFM and TEM. The temperature-dependent PL spectral peak energies of the green MQWs grown on FS-GaN substrates reveal more clear “S”-shaped behaviors due to stronger carrier localization than those on sapphire substrates. Also, the green MQWs grown on FS-GaN have the higher internal quantum efficiency (IQE) of 9 ∼ 12% in compared to those grown on sapphire at the same MQW thickness and wavelength of 530 ∼ 540 nm. These results will be due to the effects of the enhanced carrier localization and the reduced quantum confined stark effect (QCSE) by reason of the optical-loss reduction as decreasing the defect density and the piezoelectric field reduction as decreasing the relaxation of strain. We could identify that the green MQWs grown on FS-GaN substrates have the superior characteristics of the optics, structure, and surface to those on sapphire substrates. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlation between the peak spectral energy of gamma-ray bursts and the peak luminosity of the underlying supernovae: implication for the nature of the gamma-ray burst-supernova connection

In this paper, we present a correlation between the peak spectral energy of gamma-ray bursts (GRBs) and the peak bolometric luminosity of the underlying supernovae (SNe), based on a sample of four pairs of GRBs‐SNe with a spectroscopically confirmed connection. Combining it with the well-known relation between the peak spectral energy and the isotropic equivalent energy of GRBs, we obtain an upper limit on the isotropic energy of GRBs, which is ≈ 10 52 erg(L SN,peak/10 43 erg s −1 ) 10 , where LSN,peak is the peak bolometric luminosity of the SNe. Our results suggest that the critical parameter determining the GRB‐SN connection is the peak luminosity of SNe, rather than the feature of the SN spectra and/or the SN explosion energy as commonly hypothesized. Because it is generally believed that the peak luminosity of SNe powered by radioactive decays is related to the amount of 56 Ni produced in the SN explosion, the mass of 56 Ni may be a key physical factor for understanding the nature of GRBs and their connection with SNe. Application of our relation to Type Ibc SNe with normal peak luminosities indicates that, if those normal SNe have GRBs accompanying them, the GRBs would be extremely soft and subenergetic in gamma-rays and, hence, easier to detect with X-ray or UV detectors than with gamma-ray detectors.

Gamma-ray bursts as standard candles to constrain the cosmological parameters

Gamma-ray bursts (GRBs) are among the most powerful sources in the Universe: they emit up to 1054 erg in the hard x-ray band in a few tens of seconds. The cosmological origin of GRBs has been confirmed by several spectroscopic measurements of their redshifts, distributed in the range z∊(0.1,6.3). These two properties make GRBs very appealing for investigating the far Universe. Indeed, they can be used to constrain the geometry of the present-day Universe and the nature and evolution of dark energy by testing the cosmological models in a redshift range hardly achievable by other cosmological probes. Moreover, the use of GRBs as cosmological tools could unveil the ionization history of the Universe, the inter-galactic medium (IGM) properties and the formation of massive stars in the early Universe. The energetics implied by the observed fluences and redshifts span at least four orders of magnitudes. Therefore, at first sight, GRBs are all but standard candles. But there are correlations among some observed quantities which allow us to know the total energy or the peak luminosity emitted by a specific burst with a great accuracy. Through these correlations, GRBs become ‘known’ candles, and then a new tool to constrain the cosmological parameters. One of these correlation is between the rest frame peak spectral energy Epeak and the total energy emitted in γ-rays Eγ, properly corrected for the collimation factor. Another correlation, discovered very recently, relates the total GRB luminosity Liso, its peak spectral energy Epeak and a characteristic timescale T0.45, related to the variability of the prompt emission. This last correlation is based only on prompt emission properties, it is completely phenomenological, model independent and assumption-free. These correlations have been already used to constrain ΩM and ΩΛ, which are found to be consistent with the concordance model. The present limited sample of bursts and the lack of low redshift events, necessary to calibrate the correlations used to standardize GRBs energetics, make the cosmological constraints obtained with GRBs still large compared to those obtained with other cosmological probes (e.g. SNIa or CMB). However, the newly born field of GRB-cosmology is very promising for the future.