Time-resolved Spectral Analysis Research Articles

Probing the Interaction between Human Serum Albumin and 9-Hydroxyphenanthrene: A Spectroscopic and Molecular Docking Study.

9-Hydroxyphenanthrene (9-OHPhe), the representative hydroxyl metabolite of phenanthrene, has generated increasing concern as it is potentially hazardous to organisms. Herein, multispectroscopic and molecular docking techniques were applied to investigate the molecular interaction of human serum albumin (HSA) with 9-hydroxyphenanthrene (9-OHPhe) under simulated physiological conditions. Steady-state fluorescence and time-resolved fluorescence spectral analysis showed that 9-OHPhe quenched HSA fluorescence through a mixed static and dynamic process. HSA can bind with 9-OHPhe to form a 1:1 complex, with binding constants of 1.28 × 105, 1.36 × 105, and 1.26 × 105 L·mol–1 at 298.15, 303.15, and 308.15 K, respectively. The strong binding between HSA and 9-OHPhe is spontaneous and entropy-driven. Molecular docking indicated that the optimal binding site of 9-OHPhe with HSA was located in the IA subdomain of HSA. Thermodynamic analysis and molecular docking results suggested that hydrophobic interactions and hydrogen bond force dominated the binding process of HSA with 9-OHPhe. Specifically, 9-OHPhe formed hydrophobic interactions with LEU134, LEU139, ILE142, LEU154, PHE157, ALA158, and TYR161 and formed a 1.86 Å hydrogen bond with LEU135. Circular dichroism spectral analysis showed that the α-helical content of HSA decreased from 52.3 to 50.9% after adding 9-OHPhe with a ratio of 1:1. The obtained results are hoped to provide basic data for understanding the potential effects of the hydroxyl metabolites of PAHs on functional biomacromolecules.

Classification of Photospheric Emission in Short GRBs

Abstract In order to better understand the physical origin of short-duration gamma-ray bursts (GRBs), we perform a time-resolved spectral analysis on a sample of 70 pulses in 68 short GRBs with burst durations T 90 ≲ 2 s detected by the Fermi/Gamma-ray Burst Monitor. We apply a Bayesian analysis to all spectra that have statistical significance S ≥ 15 within each pulse and apply a cutoff power-law model. We then select in each pulse the time bin that has the maximum value of the low-energy spectral index for further analysis. Under the assumption that the main emission mechanism is the same throughout each pulse, this analysis is indicative of pulse emission. We find that about 1/3 of the short GRBs are consistent with a pure, nondissipative photospheric model, at least around the peak of the pulse. This fraction is larger than the corresponding fraction (1/4) obtained for long GRBs. For these bursts, we find (i) a bimodal distribution in the values of the Lorentz factors and the hardness ratios and (ii) an anticorrelation between T 90 and the peak energy, E pk: . This correlation disappears when we consider the entire sample. Our results thus imply that the short GRB population may in fact be composed of two separate populations: one that is a continuation of the long GRB population to shorter durations, and another that is distinctly separate with different physical properties. Furthermore, thermal emission is initially ubiquitous, but is accompanied at longer times by additional radiation (likely synchrotron).

Flow Evolution and Unsteady Spectra of Dynamic Stall at Transitional Reynolds Numbers

Dynamic stall experiments were performed on a NACA 0012 airfoil subjected to a constant pitch rate maneuver at three transitional Reynolds numbers of , , and . The primary objectives of this investigation were to describe the evolution of the flow and to quantify the characteristic timescales of dominant fluctuations in the off-body flowfield. These objectives were achieved through a combination of time-resolved particle image velocimetry and spectral analysis using empirical mode decomposition. Using spectral analysis, the velocity fluctuations near the leading edge were observed to be in an amplified state during the initial ejection of vortical structures. During this amplified phase, the most dominant velocity fluctuation modes were associated with displacement-thickness-based Strouhal numbers between and . The variation in the frequency of the dominant fluctuation modes was also analyzed as a function of the angle of attack. A numerical implementation of the Orr–Sommerfeld equation was used to extract the spatially unstable modes associated with the velocity profiles at the airfoil leading edge. The amplified frequencies from linear stability analysis were found to be consistent with those determined from the spectral analysis of the acquired velocity field.

A Spectral Analysis of Fermi-LLE Gamma-Ray Bursts

Abstract The prompt emission of gamma-ray bursts remains mysterious since the mechanism is difficult to understand even though there are many more observations with the development of detection technology. Most of the gamma-ray bursts spectra show the Band shape, which consists of the low-energy spectral index α, the high-energy spectral index β, the peak energy E p , and the normalization of the spectrum. We present a systematic analysis of the spectral properties of 36 gamma-ray bursts (GRBs), which were detected by the Gamma-ray Burst Monitor (GBM) and simultaneously were also observed by the Large Area Telescope (LAT) and the LAT Low Energy (LLE) detector on the Fermi satellite. We performed a detailed time-resolved spectral analysis for all of the bursts in our sample. We found that the time-resolved spectrum at peak flux can be well fitted by the empirical Band function for each burst in our sample. Moreover, the evolution patterns of α and E p have been carried for statistical analysis and the parameter correlations have been obtained such as E p –F, α–F, and E p –α, all of them are presented by performing a detailed time-resolved spectral analysis. We also demonstrated that the two strong positive correlations α–F and E p –α for some bursts originate from nonphysical selection effects through simulation.

Origin for the Prompt Spectral Evolution Characteristics and High Energy Emission during the X-Ray Flare in GRB 180720B

Abstract The gamma-ray burst GRB 180720B is very peculiar. On one hand, some interesting features have been found by performing the detailed time-resolved spectral analysis in the prompt phase. First, the “flux-tracking” pattern is exhibited both for the low energy spectral index α and the peak energy E p in the Band function. Second, some parameter relations show strong monotonous positive correlations, include E p –F, α–F, E p –α, and E p –L γ,iso for all time-resolved spectra. Lastly, it should be noted that the values of α do not exceed the synchrotron limits . On the other hand, the photons with the energy of ≳100 MeV were detected by LAT both in the prompt phase and afterglow. Notably, the 5 GeV photon was observed at 142 s after the GBM trigger. The spectrum of this burst in the LAT range can be described as F ν ∝ ν −1.3 t −1.54±0.02 in the afterglow phase. And there are six GeV photons during the X-ray flare when the lower energy emission is fading to a weaker level. We try to give reasonable interpretations of the mechanism for prompt emission and the high energy emission (100 MeV to GeV) in the afterglow. The interpretations suggesting that synchrotron origin can account for the prompt emission and synchrotron self-Compton radiation can account for both the spectrum and temporal behavior of the 100 MeV to GeV afterglow emission that have been accepted by us.

NUSTAR and Swift observations of AMXP Swift J1756.9−2508 during its 2018 outburst

ABSTRACT We present here the timing and spectral analysis of the accreting millisecond pulsar (AMXP) SwiftJ1756.9−2508 during its recent outburst in 2018 using Swift and NUSTAR observations. The simultaneous fitting of the Swift and NUSTAR spectra indicates that the source was in the hard state with a cut-off energy of about 74.58 keV. We also study in detail the pulse profile of the AMXP and its dependence on energy. The colour–colour diagram of the source is different from those previously reported. We performed phase- and time-resolved spectral analysis using NUSTAR data. Pulse phase-resolved spectra were fitted with a power-law model and significant changes in the spectral parameters with pulse phase were observed. The orbital phase and time-resolved spectra were fitted with a cut-off power-law model. The column density and photon index obtained from orbital phase spectral analysis were found to show some anticorrelation with the flux. Through time-resolved spectral analysis, we observed that the spectral parameters show positive correlation with each other and with the flux. We do not observe a softening of the spectrum with time. No emission lines or Compton bump were observed in the spectrum of the AMXP.

The Time-resolved Spectra of Photospheric Emission from a Structured Jet for Gamma-Ray Bursts

Abstract The quasi-thermal components found in many Fermi gamma-ray bursts (GRBs) imply that the photosphere emission indeed contributes to the prompt emission of many GRBs. But whether the observed spectra empirically fitted by the Band function or cutoff power law, especially the spectral and peak energy (E p ) evolutions can be explained by the photosphere emission model alone needs further discussion. In this work, we investigate in detail the time-resolved spectra and E p evolutions of photospheric emission from a structured jet, with an inner-constant and outer-decreasing angular Lorentz factor profile. Also, a continuous wind with a time-dependent wind luminosity has been considered. We show that the photosphere spectrum near the peak luminosity is similar to the cutoff power-law spectrum. The spectrum can have the observed average low-energy spectral index α ∼ −1, and the distribution of the low-energy spectral index in our photosphere model is similar to that observed (−2 ≲ α ≲ 0). Furthermore, the two kinds of spectral evolutions during the decay phase, separated by the width of the core (θ c ), are consistent with the time-resolved spectral analysis results of several Fermi multi-pulse GRBs and single-pulse GRBs, respectively. Also, for this photosphere model we can reproduce the two kinds of observed E p evolution patterns rather well. Thus, by considering the photospheric emission from a structured jet, we reproduce the observations well for the GRBs best fitted by the cutoff power-law model for the peak-flux spectrum or the time-integrated spectrum.

Signature of Stochastic Acceleration and Cooling Processes in an Outburst Phase of the TeV Blazar ON 231

Abstract We present a detailed spectral and temporal study of the intermediate-type blazar ON 231 during the TeV outburst phase in 2008 June with observations performed by Swift and XMM-Newton. The X-ray flux of the source, which was significantly dominated by the soft photons (below 3–4 keV), varies between 27% and 38% on day timescales, while mild variations were observed in the optical/UV emissions. We found a maximum soft lag of ∼1 hr between the UV and soft X-ray band, which can be understood if the magnetic field of the emitting region is ∼5.6 δ −1/3 G. The 0.6–10 keV spectra can be well represented by a broken power-law model, which indicates the presence of both synchrotron and inverse Compton components in the studied X-ray regime. The synchrotron part of the spectral energy distributions (SEDs) constructed with simultaneous optical/UV and X-ray data follows a log-parabolic shape. A time-resolved spectral analysis shows that the break energy varies significantly between 2.4 and 7.3 keV with the changing flux state of the source, and the similar variations of the spectral slopes of the two components support the synchrotron self-Compton scenario. The synchrotron tail, following a log-parabolic function, shows that the peak frequency (ν p ) varies by two orders of magnitude (∼1014–1016 Hz) during the event. A significantly positive E p –β relation is observed from both SED and time-resolved spectral analyses. The most feasible scenario for the observed trend during the flaring event could be associated with a magnetic-field-driven stochastic process evolving toward an equilibrium energy level.

Multipulse Fermi Gamma-Ray Bursts. I. Evidence of the Transition from Fireball to Poynting-flux-dominated Outflow

Abstract The composition of a jet is still an unsolved problem in gamma-ray bursts (GRBs). Several previous studies have suggested that the prompt emission spectrum of GRBs is likely to consist of a few components that may arise from different jet compositions. Here we present a systematic analysis to search for the GRBs that seem to show the transition from a fireball to the Poynting-flux-dominated outflow between well-separated pulses within a single burst, like GRB 160626B, using the Gamma-ray Burst Monitor data of the Fermi satellite. We obtain 43 GRBs with clear multiple pulses and find that 9/43 (21%) bursts may exhibit such a transition based on the time-integrated spectral analysis. We then select a further four bursts with data of adequate quality to perform a detailed time-resolved spectral analysis, and we find that in three bursts the thermal-like pulse is a precursor. Furthermore, based on the time-resolved spectra, we constrain the outflow properties for those thermal pulses and find them consistent with the typical properties of the photosphere emission. Also, the later pulses with the softer low-energy spectral index are compatible with the optically thin synchrotron emission model. Our analysis indicates that a good fraction of the multipulse Fermi bursts may obtain the transition from fireball to the Poynting-flux-dominated outflow.

X-ray analysis of the accreting supermassive black hole in the radio galaxy PKS 2251+11

Context. We have investigated the dichotomy between jetted and non-jetted active galactic nuclei (AGNs), focusing on the fundamental differences of these two classes in the accretion physics onto the central supermassive black hole (SMBH). We tested the validity of the unification model of AGNs through the characterization of the mutual interaction between accreting and outflowing matter in radio galaxies. Aims. Our aim is to study and constrain the structure, kinematics and physical state of the nuclear environment in the broad line radio galaxy (BLRG) PKS 2251+11. The high X-ray luminosity and the relative proximity make such AGN an ideal candidate for a detailed analysis of the accretion regions in radio galaxies. The investigation will help to shed light on the analogies and differences between the BLRGs and the larger class of radio-quiet Seyfert galaxies and hence on the processes that trigger the launch of a relativistic jet. Methods. We performed a spectral and timing analysis of a ∼64 ks observation of PKS 2251+11 in the X-ray band with XMM-Newton. We modeled the spectrum considering an absorbed power law superimposed to a reflection component. We performed a time-resolved spectral analysis to search for variability of the X-ray flux and of the individual spectral components. Results. We find that the power law has a photon index Γ = 1.8 ± 0.1, absorbed by an ionized partial covering medium with a column density NH = (10.1 ± 0.8) × 1023 cm−2, a ionization parameter log ξ = 1.3 ± 0.1 erg s−1 cm and a covering factor f ≃ 90%. Considering a density of the absorber typical of the broad line region (BLR), its distance from the central SMBH is of the order of r ∼ 0.1 pc. An Fe Kα emission line is found at 6.4 keV, whose intensity shows variability on timescales of hours. We derive that the reflecting material is located at a distance r ≳ 600rs, where rs is the Schwarzschild radius. Conclusions. Concerning the X-ray properties, we found that PKS 2251+11 does not differ significantly from the non-jetted AGNs, confirming the validity of the unified model in describing the inner regions around the central SMBH, but the lack of information regarding the state of the very innermost disk and SMBH spin still leaves unconstrained the origin of the jet.

Simultaneous Kepler/K2 and XMM-Newton observations of superflares in the Pleiades

Context. Flares are powerful events ignited by a sudden release of magnetic energy which triggers a cascade of interconnected phenomena, each resulting in emission in different electromagnetic bands. In fact, in the Sun flares are observed across the whole electromagnetic spectrum. Multi-band observations of stellar flares are instead rare. This limits our ability to extend what we learn from solar flares to the case of flares occurring in stars with different properties. Aims. With the aim of studying flares in the 125-Myr-old stars in the Pleiades observed simultaneously in optical and X-ray light, we obtained new XMM-Newton observations of this cluster during the observations of Kepler K2 Campaign 4. The objective of this paper is to characterize the most powerful flares observed in both bands and to constrain the energy released in the optical and X-ray, the geometry of the loops, and their time evolution. We also aim to compare our results to existing studies of flares occurring in the Sun and stars at different ages. Methods. We selected bright X-ray/optical flares that occurred in 12 known members of the Pleiades from their K2 and XMM-Newton light curves. The sample includes ten K-M stars, one F9 star, and one G8 star. Flare average properties were obtained from integrated analysis of the light curves during the flares. The time evolution of the plasma in the magnetic loops is constrained with time-resolved X-ray spectral analysis. Results. Most of the flares studied in this work emitted more energy in optical than in X-rays, as in most solar flares, even if the Pleiades flares output a larger fraction of their total energy in X-rays than typical solar flares do. Additionally, the energy budget in the two bands is weakly correlated. We also found comparable flare duration in optical and X-rays and observed that rapidly rotating stars (e.g., with rotation period shorter than 0.5 days) preferentially host short flares. We estimated the slope of the cooling path of the flares in the log(EM)-vs.-log(T) plane. The values we obtained are affected by large uncertainties, but their nominal values suggest that the flares analyzed in this paper are mainly due to single loops with no sustained heating occurring during the cooling phase. We also observed and analyzed oscillations with a period of 500 s during one of the flares. Conclusions. The flares observed in the Pleiades can be classified as “superflares” based on their energy budget in the optical, and share some of the properties of the flares observed in the Sun, despite being more energetic. For instance, as in most solar flares, more energy is typically released in the optical than in X-rays and the duration of the flares in the two bands is correlated. We have attempted a comparison between the X-ray flares observed in the Pleiades and those observed in clusters with different ages, but to firmly address any evolutionary pattern of flare characteristics, similar and uniform multi-wavelength analyses on more complete samples are necessary.

Time-resolved photoluminescence spectral analysis of phonon-assisted DAP and e-A recombination in N+B-doped n-type 4H-SiC epilayers

It is crucial to clarify the roles of phonon-assisted donor-acceptor pairs (DAPs) and free-to-acceptor (e-A) emissions in n-type 4H-SiC doped with nitrogen (N) and boron (B), where N and B induce the shallow donor and the D center (deep B) acceptor levels, respectively, in order to understand the complicated carrier recombination mechanism, as well as developing fluorescent SiC with a high color rendering index by controlling the ratio of the two overlapped emissions. Here, time-resolved photoluminescence (TRPL) spectral analyses were performed, in which phonon-assisted DAPs and e-A components were individually recognized. The D center-related green luminescence (1.6–2.8 eV) shows a non-exponential decay followed by a very slow decay in TRPL measurements at room temperature (RT). It emerges that most of the DAP emission intensities decay much faster than e-A emissions and contribute to what is initially fast and non-exponential decay at low temperatures, while the slow decay at RT is mainly from e-A emissions. At a much higher temperature, such as 473 K, only the e-A emission remains and the decay transforms from non-exponential to exponential behavior. High-temperature thermal quenching of e-A emissions exhibits different behaviors for samples with differing B doping concentrations. An activation energy of 0.6 eV was estimated from the Arrhenius plot of e-A emission intensity from a B-doped sample, which matches the D center level. This indicates that the hole thermal emission rate is greatly enhanced at a high temperature, which accelerates the decay of e-A emission intensity at high temperatures.

TOWARD AN UNDERSTANDING OF GRB PROMPT EMISSION MECHANISM: II. PATTERNS OF PEAK ENERGY EVOLUTION AND THEIR CONNECTION TO SPECTRAL LAGS.

The prompt emission phase of gamma-ray bursts (GRBs) exhibits two distinct patterns of the peak-energy (E p ) evolution; i.e., time-resolved spectral analyses of νF ν spectra of broad pulses reveal (1) "hard-to-soft" and (2) "flux-tracking" patterns of E p evolution in time, the physical origin of which still remains not well understood. We show here that these two patterns can be successfully reproduced within a simple physical model invoking synchrotron radiation in a bulk-accelerating emission region. We show further that the evolution patterns of the peak energy have, in fact, direct connections to the existence of two different (positive or negative) types of spectral lags, seen in the broad pulses. In particular, we predict that (1) only the positive type of spectral lags is possible for the hard-to-soft evolution of the peak energy, (2) both the positive and negative type of spectral lags can occur in the case of flux-tracking pattern of the peak energy, (3) for the flux-tracking pattern, the peak location of the flux light curve slightly lags behind the peak of the E p evolution with time if the spectral lags are positive, and (4) in the case of flux-tracking pattern, double-peaked broad pulses can appear in the light curves, the shape of which is energy-dependent.

Structure of the Prompt Emission of GRB 151027A Within the Fireshell Model

Long gamma-ray burst GRB 151027A was observed by all three detectors onboard the Swift spacecraft, and many more, including MAXI, Konus-Wind and Fermi GBM/LAT instruments. This revealed a complex structure of the prompt and afterglow emission, consisting of a double-peak gammaray prompt with a quiescent period and a HRF/SXF within the X-ray afterglow, together with multiple BB components seen within the time-resolved spectral analysis. These features, within the fireshell model, are interpreted as the manifestation of the same physical process viewed at different angles with respect to the HN ejecta. Here we present the time-resolved and time-integrated spectral analysis used to determine the energy of the e−e+ plasma Etot and the baryon load B. These quantities describe the dynamics of the fireshell up to the transparency point. We proceed with the light-curve simulation from which CBM density values and its inhomogeneities are deduced. We also investigate the properties of GRB 140206A, whose prompt emission exhibits a similar structure.

The December 2015 re-brightening of V404 Cygni

In December 2015 the black hole binary V404 Cyg underwent a secondary outburst after the main June 2015 event. We monitored this re-brightening with the INTEGRAL and Swift satellites, and in this paper we report the results of the time-resolved spectral analysis of these data. The December outburst shared several characteristics with the June event. The well-sampled INTEGRAL light curve shows up to ten Crab flares, which are separated by relatively weak non-flaring emission phases when compared to the June outburst. The spectra are nicely described by absorbed Comptonization models, with hard photon indices, Γ ≲ 2, and significant detections of a high-energy cut-off only during the bright flares. This is in contrast to the June outburst, where the Comptonization models gave electron temperatures mostly in the 30–50 keV range, while some spectra were soft (Γ ~ 2.5) without signs of any spectral cut-off. Similarly to the June outburst, we see clear signs of a variable local absorber in the soft energy band covered by Swift/XRT and INTEGRAL/JEM-X, which causes rapid spectral variations observed during the flares. During one flare, both Swift and INTEGRAL captured V404 Cyg in a state where the absorber was nearly Compton thick, N H ≈ 1024 cm−2, and the broad-band spectrum was similar to obscured AGN spectra, as seen during the X-ray plateaus in the June outburst. We conclude that the spectral behaviour of V404 Cyg during the December outburst was analogous with the first few days of the June outburst, both having hard X-ray flares that were intermittently influenced by obscuration due to nearly Compton-thick outflows launched from the accretion disc.

Consistency with synchrotron emission in the bright GRB 160625B observed by Fermi

We present time-resolved spectral analysis of prompt emission from GRB 160625B, one of the brightest bursts ever detected by Fermi in its nine years of operations. Standard empirical functions fail to provide an acceptable fit to the GBM spectral data, which instead require the addition of a low-energy break to the fitting function. We introduce a new fitting function, called 2SBPL, consisting of three smoothly connected power laws. Fitting this model to the data, the goodness of the fits significantly improves and the spectral parameters are well constrained. We also test a spectral model that combines non-thermal and thermal (black body) components, but find that the 2SBPL model is systematically favoured. The spectral evolution shows that the spectral break is located around Ebreak ~100 keV, while the usual νFν peak energy feature Epeak evolves in the 0.5–6 MeV energy range. The slopes below and above Ebreak are consistent with the values –0.67 and –1.5, respectively, expected from synchrotron emission produced by a relativistic electron population with a low-energy cut-off. If Ebreak is interpreted as the synchrotron cooling frequency, the implied magnetic field in the emitting region is ~10 Gauss, i.e. orders of magnitudes smaller than the value expected for a dissipation region located at ~1013−14 cm from the central engine. The low ratio between Epeak and Ebreak implies that the radiative cooling is incomplete, contrary to what is expected in strongly magnetized and compact emitting regions.

Transition from fireball to Poynting-flux-dominated outflow in the three-episode GRB 160625B

The ejecta composition is an open question in gamma-ray burst (GRB) physics 1 . Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis 2 , suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the Band function3–5, consistent with a synchrotron radiation origin5,6 and suggesting that the jet is Poynting-flux dominated at the central engine and probably in the emission region as well7,8. There are also bursts showing a sub-dominant thermal component and a dominant synchrotron component 9 , suggesting a probable hybrid jet composition 10 . Here, we report an extraordinarily bright GRB 160625B, simultaneously observed in gamma-ray and optical wavelengths, whose prompt emission consists of three isolated episodes separated by long quiescent intervals, with the durations of each sub-burst being approximately 0.8 s, 35 s and 212 s, respectively. Its high brightness (with isotropic peak luminosity Lp,iso ≈ 4 × 1053 erg s−1) allows us to conduct detailed time-resolved spectral analysis in each episode, from precursor to main burst and to extended emission. The spectral properties of the first two sub-bursts are distinctly different, allowing us to observe the transition from thermal to non-thermal radiation between well-separated emission episodes within a single GRB. Such a transition is a clear indication of the change of jet composition from a fireball to a Poynting-flux-dominated jet. The extremely bright GRB 160625B, consisting of three sub-bursts separated by quiescent intervals, shows a transition from thermal to non-thermal radiation that indicates a change of jet composition from a fireball to a Poynting-flux-dominated jet.

Thermal components in the early X-ray afterglows of GRBs: likely cocoon emission and constraints on the progenitors

The early X-ray afterglows of gamma-ray bursts (GRBs) are usually well described by absorbed power laws. However, in some cases, additional thermal components have been identified. The origin of this emission is debated, with proposed explanations including supernova shock breakout, emission from a cocoon surrounding the jet, as well as emission from the jet itself. A larger sample of detections is needed in order to place constraints on these different models. Here, we present a time-resolved spectral analysis of 74 GRBs observed by Swift X-ray Telescope in a search for thermal components. We report six detections in our sample, and also confirm an additional three cases that were previously reported in the literature. The majority of these bursts have a narrow range of blackbody radii around ∼2 × 1012 cm, despite having a large range of luminosities (Lpeak ∼ 1047–1051 erg s−1). This points to an origin connected to the progenitor stars, and we suggest that emission from a cocoon breaking out from a thick wind may explain the observations. For two of the bursts in the sample, an explanation in terms of late prompt emission from the jet is instead more likely. We also find that these thermal components are preferentially detected when the X-ray luminosity is low, which suggests that they may be hidden by bright afterglows in the majority of GRBs.

Decomposition and Reaction of Polyvinyl Nitrate under Shock and Thermal Loading: A ReaxFF Reactive Molecular Dynamics Study

We use molecular dynamics (MD) simulations with the reactive force field ReaxFF to investigate the response of polyvinyl nitrate (PVN), a high-energy polymer, to shock loading using the Hugoniostat technique. We compare predictions from three widely used ReaxFF versions, and in all cases, we observe shock-induced, volume-increasing exothermic reactions following a short induction time for strong enough insults. The three models predict NO2 dissociation to be the first chemical, and relatively similar final product populations; however, we find significant differences in intermediate populations indicating different reaction mechanisms due to discrepancies in the relative stability of various intermediate fragments. A time-resolved spectral analysis of the reactive MD trajectories enables the first direct comparison of shock-induced chemistry between atomistic simulations and experiments; specifically, ultrafast spectroscopy on laser shocked samples. The results from one of the ReaxFF versions are in excel...

Photospheric Emission in the Joint GBM and Konus Prompt Spectra of GRB 120323A

Abstract GRB 120323A is a very intense short gamma -ray burst (GRB) detected simultaneously during its prompt γ-ray emission phase with the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope and the Konus experiment on board the Wind satellite. GBM and Konus operate in the keV–MeV regime; however, the GBM range is broader toward both the low and the high parts of the γ-ray spectrum. Analyses of such bright events provide a unique opportunity to check the consistency of the data analysis as well as cross-calibrate the two instruments. We performed time-integrated and coarse time-resolved spectral analysis of GRB 120323A prompt emission. We conclude that the analyses of GBM and Konus data are only consistent when using a double-hump spectral shape for both data sets; in contrast, the single hump of the empirical Band function, traditionally used to fit GRB prompt emission spectra, leads to significant discrepancies between GBM and Konus analysis results. Our two-hump model is a combination of a thermal-like and a non-thermal component. We interpret the first component as a natural manifestation of the jet photospheric emission.