arXiv subjects

Z. Lucas Uhm

Publications and source records attributed to Z. Lucas Uhm.

At least 19 recordsLinked to original sources

arXiv2026

Evidence of High-latitude Emission in the Prompt Phase of GRBs: How Far from the Central Engine are the GRBs Produced?

One of the difficulties in nailing down the physical mechanism of gamma-ray bursts (GRBs) comes from the fact that there has been no clear observational evidence on how far from the central engine the prompt gamma-rays of GRBs are emitted. Here we present a simple study addressing this question by making use of the "high-latitude emission" (HLE). We show that our detailed numerical modeling exhibits a clear signature of HLE in the decaying phase of "broad pulses" of GRBs. We show that the HLE can emerge as a prominent spectral break in $F_ν$ spectra and dominate the peak of $νF_ν$ spectra even while the "line-of-sight emission" (LoSE) is still ongoing. This finding provides a new view of HLE emergence since it has been believed so far that the HLE can show up and dominate the spectra only after the LoSE is turned off. We remark, however, that this "HLE break" can be hidden in some broad pulses, depending on the proximity between the peak energies of the LoSE and the HLE. Therefore, this new picture on HLE emergence explains both detection and non-detection of HLE signature in observations of broad pulses. Also, we present three examples of Fermi-GBM GRBs with broad pulses that exhibit the HLE signature. We show that their gamma-ray emitting region should be located at $\sim 10^{16}$ cm from the central engine, which places a constraint on the GRB models.

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arXiv2026

A Universal Physics Defining the Radiation Spectra of Blazars and Gamma-Ray Bursts

Blazars and gamma-ray bursts (GRBs) are both cosmic beacons of extreme energy release powered by relativistic jets. However, they originate from tremendously different environments. Blazars are the sustained powerhouses driven by supermassive black holes at galactic centers, whereas GRBs are the transient death signals of massive stars or merging compact objects. Here we show that, despite the enormous differences, a universal physics defines the radiation spectra of blazars and GRBs. The blazar spectrum is well described by a "log-parabola" function. Employing a simple toy model with a single optically-thin region of a decreasing magnetic field, we produce the log-parabola spectrum very naturally for blazars. We find that the blazar spectrum is shaped by the "cooling physics" of relativistic electrons in the fast-cooling regime, which we identify as the universal physics since we previously showed that the fast-cooling physics of electrons with a decreasing magnetic field also explains the mysterious low-energy spectral index of the gamma-ray spectrum for a majority of GRBs. This fast-cooling physics of electrons likely nails down the physical origin underlying the universal scaling of the jet energetics between blazars and GRBs, which was observationally suggested more than a decade ago. We highlight that the spectrum shaper in both blazars and GRBs is the cooling physics, not the acceleration mechanism. This finding is conventional-belief-defying and may open up new avenues in a wide range of astrophysics.

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arXiv2025

Multiwavelength Afterglow Analysis of GRB 221009A: Unveiling the Evolution of a Cooling Break in a Wind-like Medium

Gamma-ray bursts (GRBs) are the most energetic explosions in the universe, and their afterglow emission provides an opportunity to probe the physics of relativistic shock waves in an extreme environment. Several key pieces for completing the picture of the GRB afterglow physics are still missing, including jet properties, emission mechanism, and particle acceleration. Here we present a study of the afterglow emission of GRB 221009A, the most energetic GRB ever observed. Using optical, X-ray, and gamma-ray data up to approximately two days after the trigger, we trace the evolution of the multi-wavelength spectrum and the physical parameters behind the emission process. The broadband spectrum is consistent with the synchrotron emission emitted by relativistic electrons with its index of $p = 2.29\pm 0.02$. We identify a break energy at keV and an exponential cutoff at GeV in the observed multi-wavelength spectrum. The break energy increases in time from $16.0_{-4.9}^{+7.1}$ keV at 0.65 days to $46.8_{-15.5}^{+25.0}$ keV at 1.68 days, favoring a stellar wind-like profile of the circumburst medium with $k=2.4\pm0.1$ as in $ρ(r) \propto r^{-k}$. The high-energy attenuation at around 0.4 to 4 GeV is attributed to the maximum of the particle acceleration in the relativistic shock wave. This study confirms that the synchrotron process can explain the multi-wavelength afterglow emission and its evolution.

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arXiv2024

Impact of Ejecta Temperature and Mass on the Strength of Heavy Element Signatures in Kilonovae

A kilonova, the electromagnetic emission produced by compact binary mergers, is formed through a delicate interplay of physical processes, involving r-process nucleosynthesis and interactions between heavy elements and photons through radiative transfer. This complexity makes it difficult to achieve a comprehensive understanding of kilonova spectra. In this study, we aim to enhance our understanding and establish connections between physical parameters and observables through radiative-transfer simulations. Specifically, we investigate how ejecta temperature and element mass influence the resulting kilonova spectrum. For each species, the strength of its line features depends on these parameters, leading to the formation of a distinct region in the parameter space, dubbed the Resonance Island, where the line signature of that species is notably evident in the kilonova spectrum. We explore its origin and applications. Among explored r-process elements (31$\leq$Z$\leq$92), we find that four species -- Sr$_{\rm II}$, Y$_{\rm II}$, Ba$_{\rm II}$, and Ce$_{\rm II}$ -- exhibit large and strong resonance islands, suggesting their significant contributions to kilonova spectra at specific wavelengths. In addition, we discuss potential challenges and future perspectives in observable heavy elements and their masses in the context of the resonance island.

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arXiv2023

Exploring the Impact of Ejecta Velocity Profile on Kilonova Evolution: Diversity of the Kilonova Lightcurves

A kilonova is a short-lived explosive event in the universe, resulting from the merger of two compact objects. Despite its importance as a primary source of heavy elements through r-process nucleosynthesis, its nature is not well understood, due to its rarity. In this work, we introduce a model that determines the density of a radially-stratified relativistic ejecta. We apply the model to kilonova ejecta and explore several hypothesized velocity profiles as a function of the merger's ejection time. These velocity profiles result in diverse density profiles of the ejecta, for which we conduct radiative transfer simulations using TARDIS with the solar r-process composition. Consequently, we investigate the impact of the ejecta velocity profile on the resulting lightcurve and spectral evolution through the line transitions of heavy elements. The change in the rate at which these elements accumulate in the line-forming region leaves its imprint on the kilonova lightcurve at specific wavelengths, causing the lightcurves to decay at different rates. Furthermore, in several profiles, plateau-like behaviors (slow and/or flat decline) are also observed. In conclusion, this work proposes potential scenarios of the kilonova evolution due to the ejecta velocity profile.

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arXiv2023

Temporal and Spectral Evolution of Gamma-ray Burst Broad Pulses: Identification of High Latitude Emission in the Prompt Emission

We perform a detailed analysis on broad pulses in bright Gamma-ray bursts (GRBs) to understand the evolution of GRB broad pulses. Using the temporal and spectral properties, we test the high latitude emission (HLE) scenario in the decaying phase of broad pulses. The HLE originates from the curvature effect of a relativistic spherical jet, where higher latitude photons are delayed and softer than the observer's line-of-sight emission. The signature of HLE has not yet been identified undisputedly during the prompt emission of GRBs. The HLE theory predicts a specific relation, F$_{ν, E_{p}}$ $\propto$ E$_{p}\!^{2}$, between the peak energy $E_{p}$ in $ν$F$_ν$ spectra and the spectral flux F$_ν$ measured at $E_{p}$, F$_{ν, E_{p}}$. We search for evidence of this relation in 2157 GRBs detected by the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope (Fermi) from the years 2008 to 2017. After imposing unbiased selection criteria in order to minimize contamination in a signal by background and overlaps of pulses, we build a sample of 32 broad pulses in 32 GRBs. We perform a time-resolved spectral analysis on each of these 32 broad pulses and find that the evolution of 18 pulses (56%) is clearly consistent with the HLE relation. For the 18 broad pulses, the exponent $δ$ in the relation of F$_{ν, E_{p}}$ $\propto$ E$_{p}\!^δ$ is distributed as a Gaussian function with median and width of 1.99 and 0.34, respectively. This result provides constraint on the emission radius of GRBs with the HLE signature.

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arXiv2022

Bright gamma-ray flares observed in GRB131108A

GRB 131108A is a bright long Gamma-Ray Burst (GRB) detected by the Large Area Telescope and the Gamma-ray Burst Monitor on board the Fermi Gamma-ray Space Telescope. Dedicated temporal and spectral analyses reveal three $γ$-ray flares dominating above 100 MeV, which are not directly related to the prompt emission in the GBM band (10 keV - 10 MeV). The high-energy light curve of GRB 131108A (100 MeV - 10 GeV) shows an unusual evolution: a steep decay, followed by three flares with an underlying emission, and then a long-lasting decay phase. The detailed analysis of the $γ$-ray flares finds that the three flares are 6 - 20 times brighter than the underlying emission and are similar to each other. The fluence of each flare, (1.6 $\sim$ 2.0) $\times$ 10$^{-6}$ erg cm$^{-2}$, is comparable to that of emission during the steep decay phase, 1.7 $\times$ 10$^{-6}$ erg cm$^{-2}$. The total fluence from three $γ$-ray flares is 5.3 $\times$ 10$^{-6}$ erg cm$^{-2}$. The three $γ$-ray flares show properties similar to the usual X-ray flares that are sharp flux increases, occurring in $\sim$ 50\% of afterglows, in some cases well after the prompt emission. Also, the temporal and spectral indices during the early steep decay phase and the decaying phase of each flare show the consistency with a relation of the curvature effect ($α$ = 2 + $β$), which is the first observational evidence of the high-latitude emission in the GeV energy band.

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arXiv2019

"Double-tracking" Characteristic of the Spectral Evolution of GRB 131231A: Synchrotron Origin?

The characteristics of the spectral evolution of the prompt emission of gamma-ray bursts (GRBs), which are closely related to the radiation mechanism (synchrotron or photosphere), are still an unsolved subject. Here, by performing the detailed time-resolved spectral fitting of GRB 131231A, which has a very bright and well-defined single pulse, some interesting spectral evolution features have been found. (i) Both the low-energy spectral index $α$ and the peak energy $E_{\rm p}$ exhibit the "flux-tracking" pattern ("double-tracking" characteristics). (ii) The parameter relations, i.e., $F$ (the energy flux)-$α$, $F$-$E_{\rm p}$, and $E_{\rm p}$-$α$, along with the analogous Yonetoku $E_{\rm p}$-$L_{γ,\rm iso}$ relation for the different time-resolved spectra, show strong monotonous (positive) correlations, both in the rising and the decaying phases. (iii) The values of $α$ do not exceed the synchrotron limit ($α$= -2/3) in all slices across the pulse, favoring the synchrotron origin. We argue that the one-zone synchrotron emission model with the emitter streaming away at a large distance from the central engine can explain all of these special spectral evolution characteristics.

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arXiv2019

Multiple Components in the Broadband $γ$-ray Emission of the Short GRB 160709A

GRB 160709A is one of the few bright short gamma-ray bursts detected by both the Gamma-ray Burst Monitor and the Large Area Telescope on board the $Fermi$ $Gamma$-$ray$ $Space$ $Telescope$. The $γ$-ray prompt emission of GRB 160709A is adequately fitted by combinations of three distinct components: (i) a nonthermal component described by a power law (PL) with a high-energy exponential cutoff, (ii) a thermal component modeled with a Planck function, and (iii) a second nonthermal component shaped by an additional PL crossing the whole $γ$-ray spectrum. While the thermal component dominates during $\sim$ 0.12 s of the main emission episode of GRB 160709A with an unusually high temperature of $\sim$ 340 keV, the nonthermal components dominate in the early and late time. The thermal component is consistent with the photospheric emission resulting in the following parameters: the size of the central engine, $R_{0}$ = $3.8 \substack{+5.9 \\ -1.8}$ $\times 10^{8}$ cm, the size of the photosphere, R$_{ph}$ = $7.4 \substack{+0.8 \\ -1.2}$ $\times 10^{10}$ cm, and a bulk Lorentz factor, $Γ$ = $728 \substack{+75 \\ -93}$ assuming a redshift of 1. The slope of the additional PL spectrum stays unchanged throughout the burst duration; however, its flux decreases continuously as a function of time. A standard external shock model has been tested for the additional PL component using the relation between the temporal and spectral indices (the closure relation). Each set of spectral and temporal indices from two energy bands (200 keV--40 MeV and 100 MeV--10 GeV) satisfies a distinct closure relation. From the closure relation test we derived the index for the electron spectral distribution, $p$ = 2.5 $\pm$ 0.1. The interaction of the jet with the interstellar environment is preferred over the interaction with the wind medium.

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arXiv2019

Closure relations of Gamma Ray Bursts in high energy emission

The synchrotron external shock model predicts the evolution of the spectral ($β$) and temporal ($α$) indices during the gamma-ray burst (GRB) afterglow for different environmental density profiles, electron spectral indices, electron cooling regimes, and regions of the spectrum. We study the relationship between $α$ and $β$, the so-called "closure relations" with GRBs detected by \textit{Fermi} Large Area Telescope (\textit{Fermi}-LAT) from 2008 August to 2018 August. The spectral and temporal indices for the > 100 MeV emission from the \textit{Fermi}-LAT as determined in the Second Fermi-LAT Gamma-ray Burst Catalog (2FLGC) are used in this work. We select GRBs whose spectral and temporal indices are well constrained (58 long-duration GRBs and 1 short-duration GRBs) and classify each GRB into the best-matched relation. As a result, we found that a number of GRBs require a very small fraction of the total energy density contained in the magnetic field ($ε_{B}$ $\lesssim$ 10$^{-7}$). The estimated mean and standard deviation of electron spectral index $\mathit{p}$ are 2.40 and 0.44, respectively. The GRBs satisfying a closure relation of the slow cooling tend to have a softer $\mathit{p}$ value compared to those of the fast cooling. Moreover, the Kolmogorov--Smirnov test of the two $\mathit{p}$ distributions from the fast and slow coolings rejects a hypothesis that the two distributions are drawn from the single reference distribution with a significance of 3.2 $σ$. Lastly, the uniform density medium is preferred over the medium that decreases like the inverse of distance squared for long-duration GRBs.

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arXiv2018

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.

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arXiv2016

Evidence of Bulk Acceleration of the GRB X-ray Flare Emission Region

Applying our recently-developed generalized version of the high-latitude emission theory to the observations of X-ray flares in gamma-ray bursts (GRBs), we present here clear observational evidence that the X-ray flare emission region is undergoing rapid bulk acceleration as the photons are emitted. We show that both the observed X-ray flare light curves and the photon index evolution curves can be simultaneously reproduced within a simple physical model invoking synchrotron radiation in an accelerating emission region far from the GRB central engine. Such an acceleration process demands an additional energy dissipation source other than kinetic energy, which points towards a significant Poynting-flux in the emission region of X-ray flares. As the X-ray flares are believed to share a similar physical mechanism as the GRB prompt emission, our finding here hints that the GRB prompt emission jets may also carry a significant Poynting-flux in their emitting region.

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arXiv2016

A Statistical Study of GRB X-ray Flares: Evidence of Ubiquitous Bulk Acceleration in the Emission Region

When emission in a conical relativistic jet ceases abruptly (or decays sharply), the observed decay light curve is controlled by the high-latitude "curvature effect". Recently, Uhm & Zhang found that the decay slopes of three GRB X-ray flares are steeper than what the standard model predicts. This requires bulk acceleration of the emission region, which is consistent with a Poynting-flux-dominated outflow. In this paper, we systematically analyze a sample of 85 bright X-ray flares detected in 63 Swift GRBs, and investigate the relationship between the temporal decay index $α$ and spectral index $β$ during the steep decay phase of these flares. The $α$ value depends on the choice of the zero time point $t_0$. We adopt two methods. "Method I" takes $t_0^I$ as the first rising data point of each flare, and is the most conservative approach. We find that at 99.9% condifence level 56/85 flares have decay slopes steeper than the simplest curvature effect prediction, and therefore, are in the acceleration regime. "Method II" extrapolates the rising light curve of each flare backwards until the flux density is three orders of magnitude lower than the peak flux density, and defines the corresponding time as the time zero point (t_0^II). We find that 74/85 flares fall into the acceleration regime at 99.9% condifence level. This suggests that bulk acceleration is common, may be even ubiquitous among X-ray flares, pointing towards a Poynting-flux-dominated jet composition for these events.

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arXiv2016

Towards an understanding of GRB prompt emission mechanism: I. The origin of spectral lags

Despite decades of investigations, the physical mechanism that powers the bright prompt $γ$-ray emission from gamma-ray bursts (GRBs) is still not identified. One important observational clue that remains not properly interpreted so far is the existence of time lags of broad light curve pulses in different energy bands, named "spectral lags". Here we show that the traditional view invoking the high-latitude emission "curvature effect" of a relativistic jet cannot account for spectral lags. Rather, the observed spectral lags demand the sweep of a spectral peak across the observing energy band in a specific manner. The duration of the broad pulses and inferred typical Lorentz factor of GRBs require that the emission region is in an optically thin emission region far from the GRB central engine. We construct a simple physical model invoking synchrotron radiation from a rapidly expanding outflow. We show that the observed spectral lags appear naturally in our model light-curves given that (1) the gamma-ray photon spectrum is curved (as observed), (2) the magnetic field strength in the emitting region decreases with radius as the region expands in space, and (3) the emission region itself undergoes rapid bulk acceleration as the prompt $γ$-rays are produced. These requirements are consistent with a Poynting-flux-dominated jet abruptly dissipating magnetic energy at a large distance from the engine.

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arXiv2015

On the Curvature Effect of a Relativistic Spherical Shell

We consider a relativistic spherical shell and calculate its spectral flux as received by a distant observer. Using two different methods, we derive a simple analytical expression of the observed spectral flux and show that the well-known relation $\hat α= 2+\hat β$ (between temporal index $\hat α$ and spectral index $\hat β$) of the high-latitude emission is achieved naturally in our derivation but holds only when the shell moves with a constant Lorentz factor $Γ$. Presenting numerical models where the shell is under acceleration or deceleration, we show that the simple $\hat α= 2+\hat β$ relation is indeed deviated as long as $Γ$ is not constant. For the models under acceleration, we find that the light curves produced purely by the high-latitude emission decay initially much steeper than the constant $Γ$ case and gradually resume the $\hat α= 2+\hat β$ relation in about one and half orders of magnitude in observer time. For the models under deceleration, the trend is opposite. The light curves made purely by the high-latitude emission decay initially shallower than the constant $Γ$ case and gradually resume the relation $\hat α= 2+\hat β$ in a similar order of magnitude in observer time. We also show that how fast the Lorentz factor $Γ$ of the shell increases or decreases is the main ingredient determining the initial steepness or shallowness of the light curves.

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arXiv2015

Synchrotron Origin of the Typical GRB Band Function - A Case Study of GRB 130606B

We perform a time-resolved spectral analysis of GRB 130606B within the framework of a fast-cooling synchrotron radiation model with magnetic field strength in the emission region decaying with time, as proposed by Uhm & Zhang. The data from all time intervals can be successfully fit by the model. The same data can be equally well fit by the empirical Band function with typical parameter values. Our results, which involve only minimal physical assumptions, offer one natural solution to the origin of the observed GRB spectra and imply that, at least some, if not all, Band-like GRB spectra with typical Band parameter values can indeed be explained by synchrotron radiation.

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arXiv2014

Dynamics and Afterglow Light Curves of GRB Blast Waves Encountering a Density Bump or Void

We investigate the dynamics and afterglow light curves of gamma-ray burst (GRB) blast waves that encounter various density structures (such as bumps, voids, or steps) in the surrounding ambient medium. We present and explain the characteristic response features that each type of density structures in the medium leaves on the forward shock (FS) and reverse shock (RS) dynamics, for blast waves with either a long-lived or short-lived RS. We show that, when the ambient medium density drops, the blast waves exhibit in some cases a period of an actual acceleration (even during their deceleration stage), due to adiabatic cooling of blast waves. Comparing numerical examples that have different shapes of bumps or voids, we propose a number of consistency tests that correct modeling of blast waves needs to satisfy. Our model results successfully pass these tests. Employing a Lagrangian description of blast waves, we perform a sophisticated calculation of afterglow emission. We show that, as a response to density structures in the ambient medium, the RS light curves produce more significant variations than the FS light curves. Some observed features (such as re-brightenings, dips, or slow wiggles) can be more easily explained within the RS model. We also discuss on the origin of these different features imprinted on the FS and RS light curves.

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arXiv2014

Fast cooling synchrotron radiation in a decaying magnetic field and $γ$-ray burst emission mechanism

Synchrotron radiation of relativistic electrons is an important radiation mechanism in many astrophysical sources. In the sources where the synchrotron cooling time scale $t_c$ is shorter than the dynamical time scale $t_{dyn}$, electrons are cooled down below the minimum injection energy. It has been believed that such "fast cooling" electrons have an energy distribution $dN_e /dγ_e \propto γ_e^{-2}$, and their synchrotron radiation flux density has a spectral shape $F_ν\propto ν^{-1/2}$. On the other hand, in a transient expanding astrophysical source, such as a gamma-ray burst (GRB), the magnetic field strength in the emission region continuously decreases with radius. Here we study such a system, and find that in a certain parameter regime, the fast cooling electrons can have a harder energy spectrum, and the standard $d N_e / d γ_e \propto γ_e^{-2}$ spectrum is achieved only in the deep fast cooling regime when $t_c \ll t_{dyn}$. We apply this new physical regime to GRBs, and suggest that the GRB prompt emission spectra whose low-energy photon index $α$ has a typical value -1 could be due to synchrotron radiation in this moderately fast cooling regime.

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