Photospheric Emission Research Articles

Predicting Photospheric Ultraviolet Emission from Stellar Evolutionary Models

Stellar ultraviolet (UV) emission serves as a crucial indicator for estimating magnetic activity and evaluating the habitability of exoplanets orbiting stars. In this paper, we present a straightforward method to derive stellar photospheric UV emission for F to M main-sequence stars. By using PARSEC models, we establish relations between near-UV (NUV) and far-UV (FUV) magnitudes from the Galaxy Evolution Explorer, NUV magnitudes from the China Space Station Telescope, and stellar effective temperatures and Gaia blue photometer–red photometer colors for different metallicities. Together with the observed sample, we find that for NUV emission, the photospheric contribution to the observed flux is less than 20% for M stars, around 10%–70% for K stars, and in a range from 30% to 85% for G and F stars. For FUV emission, the photospheric contribution is less than 10−6 for M stars, below 10−4 for K stars, around 10−4–10% for G stars, and between 6% and 50% for F stars. Our work enables the simple and effective determination of stellar excess UV emission and the exploration of magnetic activity.

Modeling of the Gamma Ray Burst photospheric emission: Monte Carlo simulation of the GRB prompt emission, numerical results and discussion

Modeling of the Gamma Ray Burst photospheric emission: Monte Carlo simulation of the GRB prompt emission, numerical results and discussion

Exploring Gamma-Ray Burst Diversity: Clustering Analysis of the Emission Characteristics of Fermi- and BATSE-detected Gamma-Ray Bursts

Gamma-ray bursts (GRBs), often attributed to massive star collapse or binary compact object mergers, exhibit diverse emission characteristics hinting at multiple GRB classes based on various factors like progenitors, radiation mechanisms, and central engines. This study employs unsupervised clustering using the nested Gaussian mixture model algorithm to analyze data from Fermi and BATSE, identifying four classes (A–D) based on duration, spectral peak, and spectral index of time-integrated spectra of GRBs. Class proportions are approximately 70%, 10%, 3%, and 17%, respectively, with A and B comprising mostly long GRBs, C mainly short GRBs, and D encompassing both types. The classes are further assessed based on spectral index α, indicating distinct radiation mechanisms: α > −0.67 for photospheric emission, α ≤ −1.5 for fast-cooling synchrotron, and −1.5 < α ≤ −0.67 for slow-cooling synchrotron. Classes B and C align with photospheric emission, while A and D predominantly exhibit synchrotron radiation. Short GRBs are predominantly photospheric emission, whereas long GRBs tend to favor synchrotron emission. Overall, 63% of the total bursts exhibit α profiles indicative of synchrotron emission, with the remaining 37% associated with photospheric emission. Considering the limited data of kilonova and supernova associated with GRBs, classes are examined for progenitor origins, suggesting a hybrid nature for A and D, and collapsar and merger origins for B and C, respectively. This clustering analysis results in four GRB classes, which, upon investigation, reveal the diverse and complex nature of GRBs in terms of their radiation, duration, and progenitor.

A Multiwavelength Light-curve Model of the Classical Nova V339 Del: A Mechanism for the Coexistence of Dust Dip and Supersoft X-Rays

The classical nova V339 Del 2013 is characterized by a 1.5 mag dip of the V light curve owing to a dust shell formation, with which soft X-ray emissions coexist. We present a Strömgren y-band light curve, which represents continuum emission, not influenced by strong [O iii] emission lines. The y light curve monotonically decreases in marked contrast to the V light curve that shows a 1.5 mag dip. We propose a multiwavelength light-curve model that reproduces the y and V light curves as well as the gamma-ray and X-ray light curves. In our model, a strong shock arises far outside the photosphere after optical maximum, because later ejected matter collides with earlier ejected gas. Our shocked shell model explains optical emission lines, Hα, hard X-ray, and gamma-ray fluxes. A dust shell forms behind the shock that suppresses [O iii]. This low flux of [O iii] shapes a 1.5 mag drop in the V light curve. Then, the V flux recovers with an increasing contribution from [O iii] lines, while the y flux does not. However, the optical depth of the dust shell is too small to absorb the photospheric (X-ray) emission of the white dwarf. This is the reason that a dust shell and soft X-ray radiation coexist. We determined the white dwarf mass to be M WD = 1.25 ± 0.05 M ☉ and the distance modulus in the V band to be (m − M) V = 12.2 ± 0.2; the distance is d = 2.1 ± 0.2 kpc for the reddening of E(B − V) = 0.18.

Ending the prompt phase in photospheric models of gamma-ray bursts

The early steep decay, a rapid decrease in X-ray flux as a function of time following the prompt emission, is a robust feature seen in almost all gamma-ray bursts with early enough X-ray observations. This peculiar phenomenon has often been explained as emission from high latitudes of the last flashing shell. However, in photospheric models of gamma-ray bursts, the timescale of high-latitude emission is generally short compared to the duration of the steep decay phase, and hence an alternative explanation is needed. In this paper we show that the early steep decay can directly result from the final activity of the dying central engine. We find that the corresponding photospheric emission can reproduce both the temporal and spectral evolution observed. This requires a late-time behaviour that should be common to all gamma-ray burst central engines, and we estimate the necessary evolution of the kinetic power and the Lorentz factor. If this interpretation is correct, observation of the early steep decay can give us insights into the last stages of central activity, and provide new constraints on the late evolution of the Lorentz factor and photospheric radius.

Searching for Intermediate-mass Black Holes in Globular Clusters through Tidal Disruption Events

Intermediate-mass black holes (IMBHs) may be the link between stellar mass holes and the supermassive variety in the nuclei of galaxies, and globular clusters (GCs) may be one of the most promising environments for their formation. Here, we carry out a pilot study of the observability of tidal disruption events (TDEs) from 103 M ⊙ < M • < 105 M ⊙ IMBHs embedded in stellar cusps at the center of GCs. We model the long super-Eddington accretion phase and ensuing optical flare, and derive the disruption rate of main-sequence stars as a function of black hole mass and GC properties with the help of a 1D Fokker–Planck approach. The photospheric emission of the adiabatically expanding outflow dominates the observable radiation and peaks in the near-ultraviolet/optical bands, outshining the brightness of the (old) stellar population of GCs in Virgo for a period of months to years. A search for TDE events in a sample of nearly 4000 GCs observed at multiple epochs by the Next Generation Virgo Cluster Survey yields null results. Given our model predictions, this sample is too small to set stringent constraints on the present-day occupation fraction of GCs hosting IMBHs. Naturally, better simulations of the properties of the cluster central stellar distribution, TDE light curves, and rates, together with larger surveys of GCs are all needed to gain deeper insights into the presence of IMBHs in GCs.

Numerical Simulation of Photospheric Emission in Long Gamma-Ray Bursts: Prompt Correlations, Spectral Shapes, and Polarizations

We explore the properties of photospheric emission in the context of long gamma-ray bursts (GRBs) using three numerical models that combine relativistic hydrodynamical simulations and Monte Carlo radiation transfer calculations in three dimensions. Our simulations confirm that photospheric emission gives rise to correlations between the spectral peak energy and luminosity that agree with the observed Yonetoku, Amati, and Golenetskii correlations. It is also shown that the spectral peak energy and luminosity correlate with the bulk Lorentz factor, as indicated in the literature. On the other hand, synthetic spectral shapes tend to be narrower than those of the observations. This result indicates that an additional physical process that can provide nonthermal broadening is needed to reproduce the spectral features. Furthermore, the polarization analysis finds that, while the degree of polarization is low for the emission from the jet core (Π < 4%), it tends to increase with viewing angle outside of the core and can be as high as Π ∼ 20%–40% in an extreme case. This suggests that the typical GRBs show systematically low polarization compared to softer, dimmer counterparts (X-ray-rich GRBs and X-ray flashes). Interestingly, our simulations indicate that photospheric emission exhibits large temporal variation in the polarization position angle (Δψ ∼ 90°), which may be compatible with those inferred in observations. A notable energy dependence of the polarization property is another characteristic feature found in the current study. Particularly, the difference in the position angle among different energy bands can be as large as ∼90°.

Spectral and Jet Properties of the Quasi-thermal-dominated GRB 210121A, GRB 210610B, and GRB 221022B

Some quasi-thermal (QT)-dominated gamma-ray bursts (GRBs) could be well described by a multicolor blackbody (mBB) function or a combined model of a BB plus a nonthermal (NT) component. In this analysis, two QT radiation-dominated bursts with known emission properties (GRB 210610B, likely from a hybrid jet, and GRB 210121A, with a spectrum consistent with nondissipative photospheric emission from a pure hot fireball) are used to make a comparison between these two models. To diagnose the magnetization properties of the central engine, the “top-down” approach proposed by Gao and Zhang is adopted. It is found that diagnoses based on these two models provide similar conclusions qualitatively; however, the model with mBB (or mBB+NT) may give more reasonable physical explanations. This implies that impacts from the GRB jet structure and geometrical broadening on the observed spectrum should be considered. However, conservatively, these methods may be not sensitive enough to distinguish between a pure hot fireball and a mildly magnetized hybrid jet. Some other information is necessary to provide more evidence when determining the jet properties for similar GRBs. Based on these considerations, we suggest that the photospheric emission of GRB 221022B is from a hot jet, where dissipation is caused by an internal shock mechanism due to the increasing Lorentz factor with time, which makes its prompt emission display typical evolution from thermal to NT.

Observational Characteristics of Radiation-mediated Shocks in Photospheric Gamma-Ray Burst Emission

Emission from the photosphere in gamma-ray burst jets can be substantially affected by subphotospheric energy dissipation, which is typically caused by radiation-mediated shocks. We study the observational characteristics of such emission, in particular the spectral signatures. Relevant shock initial conditions are estimated using a simple internal collision framework, which then serve as inputs for a radiation-mediated shock model that generates synthetic photospheric spectra. Within this framework, we find that if the free fireball acceleration starts at r 0 ∼ 1010 cm, in agreement with hydrodynamical simulations, then the typical spectrum consists of a broad, soft power-law segment with a cutoff at high energies and a hardening in X-rays. The synthetic spectra are generally well fitted with a standard cutoff power-law (CPL) function, as the hardening in X-rays is commonly outside the observable energy range of current detectors. The CPL-fits yield values for the low-energy index, α, and the peak energy, E peak, that are centered around ∼ −0.8 and ∼220 keV, respectively, similar to typical observed values. We also identify a nonnegligible parameter region for what we call optically shallow shocks: shocks that do not accumulate enough scatterings to reach a steady-state spectrum before decoupling and thereby produce more complex spectra. These occur for optical depths , where u u = γ u β u is the dimensionless specific momentum of the upstream as measured in the shock rest frame.

How much large dust could be present in hot exozodiacal dust systems?

Context. An infrared excess over the stellar photospheric emission of main-sequence stars has been found in interferometric surveys, commonly attributed to the presence of hot exozodiacal dust (HEZD). While submicrometer-sized grains in close vicinity to their host star have been inferred to be responsible for the found near-infrared excesses, the presence and amount of larger grains as part of the dust distributions are weakly constrained. Aims. We quantify how many larger grains (above-micrometer-sized) could be present in addition to submicrometer-sized grains, while being consistent with observational constraints. This is important in order to distinguish between various scenarios for the origin of HEZD and to better estimate its observational appearance when observed with future instruments. Methods. We extended a model suitable to reproduce current observations of HEZD to investigate a bimodal size distribution. By deriving the characteristics of dust distributions whose observables are consistent with observational limits from interferometric measurements in the K and N bands we constrained the radii of sub- and above-micrometer-sized grains as well as their mass, number, and flux density ratios. Results. In the most extreme cases of some of the investigated systems, large grains ≳10 µm might dominate the mass budget of HEZD while contributing up to 25 % of the total flux density originating from the dust at a wavelength of 2.13 µm and up to 50 % at a wavelength of 4.1 µm; at a wavelength of 11.1 µm their emission might clearly dominate over the emission of small grains. While it is not possible to detect such hot-dust distributions using ALMA, the ngVLA might allow us to detect HEZD at millimeter wavelengths. Conclusions. Large dust grains (above-micrometer-sized) might have a more important impact on the observational appearance of HEZD than previously assumed, especially at longer wavelengths.

Emission line variability of young 10–30 MJup companions

Context. Emission lines that are indicative of active accretion have been observed for a number of low-mass companions (M &lt; 30 MJup) to stars. Line variability is ubiquitous on stellar accretors, but it has never been characterized in detail for low-mass companions. Such characterizations can offer insights into the accretion mechanism at play. Aims. We aim to characterize the short-to-long-term H I Paschen β emission line variability of two 10 to 30 MJup companions on wide orbits: GQ Lup b and GSC 06214-00210 b. We also aim to clarify their accretion mechanisms. Methods. We used J-band observations at R = 1800–2360, obtained with VLT/SINFONI in 2017, to record time-series investigations of the hours-to-weeks variability of the H I Paschen β emission line (1.282 µm). Contrary to H∝, it should be less affected by chromospheric activity contamination. The photospheric emission was analyzed at each epoch and removed with the ForMoSA forward-modeling tool, using new grids of ATMO models exploring different C/O and [M/H] values. The time series of line profiles and intensities were compared to those of more massive accretors and to predictions from the latest magnetospheric accretion and shock models. To complement these results, we also re-investigated archival spectroscopic observations at near-infrared wavelengths of each target to increase the time frame up to a decade and to build a more comprehensive understanding of the variability processes at play. Results. For GQ Lup b, we find line variability on timescales of several months to decades, whereas it is within the acceptable noise levels on shorter timescales. For GSC 06214-00210 b, we find line variability on timescales of tens of minutes all the way up to a decade. The line profiles of GSC 06214-00210 b are partially resolved in at least one epoch. Both objects show H I Paschen β flux variability that is moderate (&lt;50%), on timescales that are below their rotation period, and that is more significant on longer timescales (up to ~1000% on decade-long timescales). This behavior resembles that of classical T Tauri stars. The line profiles of GQ Lup b are blue-shifted and can only be reproduced by magnetospheric accretion models, while those of GSC 06214-00210 b are fairly well reproduced by both magnetospheric accretion and shock models, except for one epoch for which the shock model is highly favored. The companions have C/O values broadly consistent with solar values. Conclusions. While magnetospheric accretion is favored for GQ Lup b, higher resolution (R &gt; 10 000) observations are required to disentangle the two (non-exclusive) emitting mechanisms. The similar variability behavior observed in these low mass companions and in classical T Tauri stars may support similar accretion mechanisms. The high amplitude of variability on timescales of over a month and longer that is found for both objects could be key to explaining the low yield of H∝ imaging campaigns.

A Broken “α–intensity” Relation Caused by the Evolving Photosphere Emission and the Nature of the Extraordinarily Bright GRB 230307A

GRB 230307A is one of the brightest gamma-ray bursts detected so far. With the excellent observation of GRB 230307A by the Fermi Gamma-ray Burst Monitor, we can reveal the details of prompt emission evolution. As found in high time-resolution spectral analysis, the early low-energy spectral indices (α) of this burst exceed the limit of synchrotron radiation (α = −2/3) and gradually decreases with the energy flux (F). A tight E p ∝ F 0.54 correlation holds within the whole duration of the burst, where E p is the spectral peak energy. Such an evolution pattern of α and E p with intensity is called “double tracking.” For the α–F relation, we find a log Bayes factor ∼210 in favor of a smoothly broken power-law function over a linear function in log-linear space. We call this particular α–F relation a broken “α–intensity” and interpret it as the evolution of the ratio of thermal and nonthermal components, which is also the evolution of the photosphere. GRB 230307A with a duration of ∼35 s, if indeed at a redshift of z = 0.065, is likely a neutron star merger event (i.e., it is intrinsically “short”). Intriguingly, different from GRB 060614 and GRB 211211A, this long event is not composed of a hard spike followed by a soft tail, suggesting that the properties of the prompt emission light curves are not a good tracer of the astrophysical origins of the bursts. The other possibility of z = 3.87 would point toward a very peculiar nature of both GRB 230307A and its late-time thermal-like emission.

Fermi-GBM Discovery of GRB 221009A: An Extraordinarily Bright GRB from Onset to Afterglow

We report the discovery of GRB 221009A, the highest flux gamma-ray burst (GRB) ever observed by the Fermi Gamma-ray Burst Monitor (Fermi-GBM). This GRB has continuous prompt emission lasting more than 600 s, which smoothly transitions to afterglow emission visible in the Fermi-GBM energy range (8 keV–40 MeV), and total energetics higher than any other burst in the Fermi-GBM sample. By using a variety of new and existing analysis techniques we probe the spectral and temporal evolution of GRB 221009A. We find no emission prior to the Fermi-GBM trigger time (t 0; 2022 October 9 at 13:16:59.99 UTC), indicating that this is the time of prompt emission onset. The triggering pulse exhibits distinct spectral and temporal properties suggestive of the thermal, photospheric emission of shock breakout, with significant emission up to ∼15 MeV. We characterize the onset of external shock at t 0 + 600 s and find evidence of a plateau region in the early-afterglow phase, which transitions to a slope consistent with Swift-XRT afterglow measurements. We place the total energetics of GRB 221009A in context with the rest of the Fermi-GBM sample and find that this GRB has the highest total isotropic-equivalent energy (E γ,iso = 1.0 × 1055 erg) and second highest isotropic-equivalent luminosity (L γ,iso = 9.9 × 1053 erg s–1) based on its redshift of z = 0.151. These extreme energetics are what allowed us to observe the continuously emitting central engine of Fermi-GBM from the beginning of the prompt emission phase through the onset of early afterglow.

High-Cadence Observations of Magnetic Field Dynamics and Photospheric Emission Sources in the Eruptive Near-the-Limb X4.9 Solar Flare on 25 February, 2014: Evidences for Two-Stage Magnetic Reconnection during the Impulsive Phase

—We present an analysis of the pre-limb eruptive X4.9 solar flare on February 25, 2014, by means of which we confirm a hypothesis of the two-stage energy release corresponding to two magnetic reconnection regimes in the flare impulsive phase. This flare is selected, firstly, because of its morphological peculiarities suggesting the presence of the two energy release stages. Secondly, the flare was very suitably located near the solar limb and it was well-observed by many instruments. We performed an analysis of multiwavelength observational data of this flare region to find a connection between changes of the photospheric magnetic field, morphology of hard and soft X-ray sources, dynamics of the photospheric optical emission sources, metric radio bursts, and kinematics of an eruptive structure. The simultaneous usage of the line-of-sight and vector Helioseismic Magnetic Imager (HMI) magnetograms allowed us to trace magnetic field changes during the flare impulsive phase with high temporal resolution. HMI filtergrams allowed to trace displacement of the photospheric emission sources, associated with the magnetic reconnection, with very high temporal resolution up to 2 s. Using all observational results, we argue that the found flare stages are characterized by the following magnetic reconnection regimes. The first stage is predominantly characterized by the three-dimensional zipping reconnection in the strong sheared magnetic field assuming the tether-cutting geometry. The second stage corresponds to the so-called “standard” model of eruptive flares with the quasi-two-dimensional reconnection below the eruptive flux-rope. All observational peculiarities of these two stages are discussed in details.

Magnetar Giant Flare Origin for GRB 210410A?

In general, giant flares (GFs) produced by magnetars have a very short-hard initial spike that is followed by a weak oscillatory phase. GFs from a nearby galaxy would appear as cosmic short-hard gamma-ray bursts (GRBs), such as GRB 200415A. In this paper, we search for GF-originated bursts in the Fermi GRB category and report GRB 210410A, which is presented with a very short-hard spike followed by an extended tail emission. In the E p,z − E iso plane, GRB 210410A with a duration of T 90 ∼ 48 s differs from long GRBs, might be classified as a short GRB with a redshift of z ∼ 0.28, and could be regarded as a GF with a distance of ∼2.7 Mpc. Here, E p,z , E iso, and L iso denote the rest-frame peak photon energy, the isotropic energy, and the isotropic luminosity of the burst, respectively. The radiation spectrum of GRB 210410A, similar to that of GRB 200415A, can be well fitted with a non-dissipative photospheric emission. However, GRB 210410A in the E p,z − L iso plane is beyond the death line of cosmic GRBs for non-dissipated photospheric emission with a general initial size of the fireball. Since the E p,z − L iso relation of GFs is far beyond the death line of cosmic GRBs, GRB 210410A may have originated from the same channel that produces GFs. We also perform the analysis and discuss both the highest photon energy event (4.2 GeV) and the extended tail emission in this burst.

Misalignment of the outer disk of DK Tau and a first look at its magnetic field using spectropolarimetry

Context. Misalignments between a forming star’s rotation axis and its outer disk axis, although not predicted by standard theories of stellar formation, have been observed in several classical T Tauri stars (cTTs). The low-mass cTTs DK Tau is suspected of being among them. In addition, it is an excellent subject to investigate the interaction between stellar magnetic fields and material accreting from the circumstellar disk, as it presents clear signatures of accretion. Aims. The goal of this paper is to study DK Tau’s average line-of-sight magnetic field in both photospheric absorption lines and emission lines linked to accretion, using spectropolarimetric observations, as well as to examine inconsistencies regarding its rotation axis. Methods. We used data collected with the ESPaDOnS spectropolarimeter, at the Canada-France-Hawaii Telescope, and the NARVAL spectropolarimeter, at the Télescope Bernard Lyot, probing two distinct epochs (December 2010 to January 2011 and November to December 2012), each set spanning a few stellar rotation cycles. We first determined the stellar parameters of DK Tau, such as effective temperature and vsini. Next, we removed the effect of veiling from the spectra, then obtained least-squares deconvolution (LSD) profiles of the photospheric absorption lines for each observation, before determining the average line-of-sight magnetic field from them. We also investigated accretion-powered emission lines, namely the 587.6 nm HeI line and the CaII infrared triplet (at 849.8 nm, 854.2 nm and 866.2 nm), as tracers of the magnetic fields present in the accretion shocks. Results. We find that DK Tau experiences accretion onto a magnetic pole at an angle of ∼30° from the pole of its rotation axis, with a positive field at the base of the accretion funnels. In 2010 we find a magnetic field of up to 0.95 kG (from the CaII infrared triplet) and 1.77 kG (from the HeI line) and in 2012 we find up to 1.15 kG (from the CaII infrared triplet) and 1.99 kG (from the HeI line). Additionally, using our derived values of period, vsini and stellar radius, we find a value of 58° (+18)(−11) for the inclination of the stellar rotation axis, which is significantly different from the outer disk axis inclination of 21° given in the literature. Conclusion. We find that DK Tau’s outer disk axis is likely misaligned compared to its rotation axis by 37°.

GRB 200829A: External-shock Origin of the Very Early Prompt Emission?

Long-duration GRB 200829A was detected by Fermi-GBM and Swift-BAT/XRT, and then rapidly observed by other ground-based telescopes. It has a weak γ-ray emission in the very early phase and is followed by a bright spiky γ-ray emission pulse. The radiation spectrum of the very early emission is best fitted by a power-law function with index ∼−1.7. However, the bright spiky γ-ray pulse, especially the time around the peak, exhibits a distinct two-component radiation spectrum, i.e., Band function combined with a blackbody radiation spectrum. We infer the photospheric properties and reveal a medium magnetization at a photospheric position by adopting the initial size of the outflow as r 0 = 109 cm. It implies that the Band component in this pulse may be formed during the dissipation of the magnetic field. The power-law radiation spectra found in the very early prompt emission may imply the external-shock origination of this phase. Then, we perform the Markov Chain Monte Carlo method fitting on the light curves of this burst, where the jet corresponding to the γ-ray pulse at around 20 s is used to refresh the external shock. It is shown that the light curves of the very early phase and X-ray afterglow after 40 s, involving the X-ray bump at around 100 s, can be well modeled in the external-shock scenario. For the obtained initial outflow, we estimate the minimum magnetization factor of the jet based on the fact that the photospheric emission of this jet is missed in the very early phase.

A Cosmological Fireball with 16% Gamma-Ray Radiative Efficiency

Gamma-ray bursts (GRBs) are the most powerful explosions in the universe. How efficiently the jet converts its energy to radiation is a long-standing problem, which is poorly constrained. The standard model invokes a relativistic fireball with a bright photosphere emission component. A definitive diagnosis of GRB radiation components and the measurement of GRB radiative efficiency require prompt emission and afterglow data, with high resolution and wide band coverage in time and energy. Here, we present a comprehensive temporal and spectral analysis of the TeV-emitting bright GRB 190114C. Its fluence is one of the highest for all the GRBs that have been detected so far, which allows us to perform a high-resolution study of the prompt emission spectral properties and their temporal evolutions, down to a timescale of about 0.1 s. We observe that each of the initial pulses has a thermal component contributing ∼20% of the total energy and that the corresponding temperature and inferred Lorentz factor of the photosphere evolve following broken power-law shapes. From the observation of the nonthermal spectra and the light curve, the onset of the afterglow corresponding to the deceleration of the fireball is considered to start at ∼6 s. By incorporating the thermal and nonthermal observations, as well as the photosphere and synchrotron radiative mechanisms, we can directly derive the fireball energy budget with little dependence on hypothetical parameters, measuring a ∼16% radiative efficiency for this GRB. With the fireball energy budget derived, the afterglow microphysics parameters can also be constrained directly from the data.

The High-resolution Soft X-Ray Spectrum of Nova Delphini 2013

We present the high-resolution soft X-ray spectrum of Nova Delphini 2013. Two spectra were taken with the Low Energy Transmission Grating Spectrometer on the Chandra X-ray Observatory, on 2013 November 9 and 2013 December 6, 87 and 114 days after the nova eruption, respectively. The spectra are of very high statistical quality, and reveal clear spectral evolution between the two observations. The source is bright enough on the two occasions that the third spectral order, with resolving power up to ∼3000, can easily be seen. We observe the photospheric emission spectrum of the hot white dwarf, which exhibits a rich absorption line spectrum from an atmosphere of effective temperature likely near T eff ∼ 640,000 K, and complex chemical abundances. Superimposed on this photospheric spectrum, we detect the absorption spectrum of a shell of highly ionized gas, comprising absorption by the K-shell ions of C and N, blueshifted (outflowing) by ∼1400 km s−1, and with a velocity width of ∼1000 km s−1. The abundance ratio C/N is clearly very nonsolar, and indicative of thermonuclear fusion by the CNO cycle. We discuss the physical properties (kinematics, ionization balance, radiative transfer, mechanical energy balance, chemical abundances) of this hot shell, in the context of the physics of nova eruptions.

Evidence of Photosphere Emission Origin for Gamma-Ray Burst Prompt Emission

The physical origins of gamma-ray burst (GRB) prompt emission (photosphere or synchrotron) are still subject to debate, after more than five decades. Here, we find that many of the observed characteristics of 15 long GRBs, which have the highest prompt emission efficiency ϵ γ (ϵ γ ≳ 80%), strongly support photosphere (thermal) emission origin, in the following ways: (1) the relation between E p and E iso is almost and the dispersion is quite small; (2) the simple power-law shape of the X-ray afterglow light curves and the presence of significant reverse shock signals in the optical afterglow light curves; (3) the best fits using the cutoff power-law model for time-integrated spectra; and (4) the consistent efficiency from observations (with E iso/E k ) and the predictions from the photosphere emission model (with η/Γ). We then further investigate the characteristics of the long GRBs for two distinguished samples (ϵ γ ≳ 50% and ϵ γ ≲ 50%). It is found that the different distributions for E p and E iso, and the similar observed efficiency (from the X-ray afterglow) and theoretically predicted efficiency (from the prompt emission or the optical afterglow), follow the predictions of the photosphere emission model well. Also, based on the same efficiency, we derive an excellent correlation of to estimate Γ. Finally, we show that different distributions for E p and E iso, and the consistent efficiency, exist for short GRBs. We also give a natural explanation of the extended emission (ϵ γ ≲ 50%) and the main pulse (ϵ γ ≳ 50%).