The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr � 1 and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z >10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a newgeneration wide-field gamma-ray (15‐150 keV) detector that will detect bursts, calculate 1 0 ‐4 0 positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5 00 positions and perform spectroscopy in the 0.2‐10 keV band; and a narrow-field UV/optical telescope that will operate in the 170‐ 600 nm band and provide 0B3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of � 1m crab (� 2;10 � 11 ergs cm � 2 s � 1 in the 15‐150 keV band), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of

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The Swift Gamma-Ray Burst Mission

Supermassive black holes (BHs) have been found in 85 galaxies by dynamical modeling of spatially resolved kinematics. The Hubble Space Telescope revolutionized BH research by advancing the subject from its proof-of-concept phase into quantitative studies of BH demographics. Most influential was the discovery of a tight correlation between BH mass and the velocity dispersion σ of the bulge component of the host galaxy. Together with similar correlations with bulge luminosity and mass, this led to the widespread belief that BHs and bulges coevolve by regulating each other's growth. Conclusions based on one set of correlations from in brightest cluster ellipticals to in the smallest galaxies dominated BH work for more than a decade. New results are now replacing this simple story with a richer and more plausible picture in which BHs correlate differently with different galaxy components. A reasonable aim is to use this progress to refine our understanding of BH-galaxy coevolution. BHs with masses of 105−106M...

https://www.annualreviews.org/doi/suppl/10.1146/annurev-astro-082708-101811/suppl_file/aa51_kormendy_supmat.pdf

Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of $\sim 1.7\,{\rm{s}}$ with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of ${40}_{-8}^{+8}$ Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 $\,{M}_{\odot }$. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at $\sim 40\,{\rm{Mpc}}$) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient's position $\sim 9$ and $\sim 16$ days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

/pdf/multi-messenger-observations-of-a-binary-neutron-star-merger-3g2lxan5gn.pdf

Multi-messenger observations of a binary neutron star merger

On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB 170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory. The probability of the near-simultaneous temporal and spatial observation of GRB 170817A and GW170817 occurring by chance is $5.0\times {10}^{-8}$. We therefore confirm binary neutron star mergers as a progenitor of short GRBs. The association of GW170817 and GRB 170817A provides new insight into fundamental physics and the origin of short GRBs. We use the observed time delay of $(+1.74\pm 0.05)\,{\rm{s}}$ between GRB 170817A and GW170817 to: (i) constrain the difference between the speed of gravity and the speed of light to be between $-3\times {10}^{-15}$ and $+7\times {10}^{-16}$ times the speed of light, (ii) place new bounds on the violation of Lorentz invariance, (iii) present a new test of the equivalence principle by constraining the Shapiro delay between gravitational and electromagnetic radiation. We also use the time delay to constrain the size and bulk Lorentz factor of the region emitting the gamma-rays. GRB 170817A is the closest short GRB with a known distance, but is between 2 and 6 orders of magnitude less energetic than other bursts with measured redshift. A new generation of gamma-ray detectors, and subthreshold searches in existing detectors, will be essential to detect similar short bursts at greater distances. Finally, we predict a joint detection rate for the Fermi Gamma-ray Burst Monitor and the Advanced LIGO and Virgo detectors of 0.1–1.4 per year during the 2018–2019 observing run and 0.3–1.7 per year at design sensitivity.

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Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

New large-scale CO surveys of the first and second Galactic quadrants and the nearby molecular cloud complexes in Orion and Taurus, obtained with the CfA 1.2 m telescope, have been combined with 31 other surveys obtained over the past two decades with that instrument and a similar telescope on Cerro Tololo in Chile, to produce a new composite CO survey of the entire Milky Way. The survey consists of 488,000 spectra that Nyquist or beamwidth ( °) sample the entire Galactic plane over a strip 4°-10° wide in latitude, and beamwidth or ° sample nearly all large local clouds at higher latitudes. Compared with the previous composite CO survey of Dame et al. (1987), the new survey has 16 times more spectra, up to 3.4 times higher angular resolution, and up to 10 times higher sensitivity per unit solid angle. Each of the component surveys was integrated individually using clipping or moment masking to produce composite spatial and longitude-velocity maps of the Galaxy that display nearly all of the statistically significant emission in each survey but little noise. The composite maps provide detailed information on individual molecular clouds, suggest relationships between clouds and regions widely separated on the sky, and clearly display the main structural features of the molecular Galaxy. In addition, since the gas, dust, and Population I objects associated with molecular clouds contribute to the Galactic emission in every major wavelength band, the precise kinematic information provided by the present survey will form the foundation for many large-scale Galactic studies. A map of molecular column density predicted from complete and unbiased far-infrared and 21 cm surveys of the Galaxy was used both to determine the completeness of the present survey and to extrapolate it to the entire sky at |b| 5°), X shows little systematic variation with latitude from a mean value of (1.8 ± 0.3) × 1020 cm-2 K-1 km-1 s. Given the large sky area and large quantity of CO data analyzed, we conclude that this is the most reliable measurement to date of the mean X value in the solar neighborhood.

https://iopscience.iop.org/article/10.1086/318388/pdf

The Milky Way in Molecular Clouds: A New Complete CO Survey

Many previous studies have determined that the long lasting emission at X-ray, optical and radio wavelengths from gamma-ray bursts (GRBs), called the afterglow, is likely produced by the external forward shock model. In this model, the GRB jet interacts with the circum-stellar medium and drives a shock that heats the medium, which radiates via synchrotron emission. In this work, we carried out a detailed analysis of the late time afterglow data of GRB 090902B using a very careful accounting of the Inverse Compton losses. We find that in the context of the external forward shock model, the only viable option to explain the X-ray and optical data of GRB 090920B is to have the electron energy distribution deviate from a power-law shape and exhibit some slight curvature immediately downstream of the shock front (we explored other models that rely on a single power-law assumption, but they all fail to explain the observations). We find the fraction of the energy of shocked plasma in magnetic field to be ~10^{-6} using late time afterglow data, which is consistent with the value obtained using early gamma-ray data. Studies like the present one might be able to provide a link between GRB afterglow modeling and numerical simulations of particle acceleration in collisionless shocks. We also provide detailed calculations for the early ( 100 MeV) emission and confirm that it is consistent with origin in the external forward shock. We investigated the possibility that the ~10 keV excess observed in the spectrum during the prompt phase also has its origin in the external shock and found the answer to be negative.

Evidence for mild deviation from power-law distribution of electrons in relativistic shocks: GRB 090902B

Under the assumption that fast radio bursts (FRBs) are from coherent curvature emission powered by the dissipation of magnetic energy in the magnetosphere of neutron stars, we predict a maximum isotropic equivalent luminosity of (~2x10^{47} erg/s) min({\rho}^2, B{\rho}^{4/3}{\nu}^{-2/3}), where {\rho} is the curvature radius of the magnetic field lines near the source region in units of 10 km, B is the local magnetic field strength in units of 10^{16} G, and {\nu} is the FRB wave frequency in units of GHz. This is because the electric field responsible for accelerating the radiating particles becomes close to the quantum critical field strength and is then quickly shielded by Schwinger pairs within a nano-second. Future observations should be able to confirm this cut-off luminosity and hence provide a unique probe of the source location and magnetic field strength. We discuss how the maximum luminosity affects the observed flux distributions for repeating bursts from the same object and for the entire FRB population.

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The maximum luminosity of fast radio bursts

X-ray lines have been recently detected in the afterglows of a few gamma-ray bursts. We derive general constraints on the physical conditions in the line-emitting gas and illustrate our results using as an example the multiple Kα lines detected by Reeves et al. in GRB 011211. We argue that photoionization models previously discussed in the literature require either a very extreme geometry or too much mass in the line-emitting region. Shock-heated models also have a serious problem since they require the emitting region to have a large optical depth unless electrons in this region are shock-heated multiple times. We propose a new model in which gamma rays from the burst and hard X-rays from the early afterglow are backscattered by an electron-positron pair screen at a distance of about 1014-1015 cm from the source and irradiate the expanding outer layers of the supernova ejecta, thereby producing X-ray lines. The model suffers from fewer problems compared to previous models. It also has the advantage of requiring only a single explosion to produce both the gamma-ray burst (GRB) and the supernova ejecta, in contrast to most other models for the lines that require the supernova to go off days or weeks prior to the GRB. The model, however, has difficulty explaining the greater than 1048 ergs of energy emitted in the X-ray lines, which requires somewhat extreme choices of model parameters. The difficulties associated with the various models are not particular to GRB 011211. They are likely to pose a problem for any GRB with X-ray lines.

/pdf/x-ray-lines-from-gamma-ray-bursts-52wd9eqcv4.pdf

X‐Ray Lines from Gamma‐Ray Bursts

We introduce a dynamical Smart Monte Carlo algorithm and assess its applicability for simulating the rare‐event dynamics of adsorbatediffusion. Using the dynamical Smart Monte Carlo method, we simulate the self‐diffusion of an adatom in the Cu/Cu(001) and Rh/Rh(111) systems and we compare the simulated diffusion coefficients to values arising from molecular dynamics and transition‐state theory. We find that the accuracy of Smart Monte Carlo is sensitive to details of the potential‐energy surface. For Cu/Cu(001), the agreement between dynamical Smart Monte Carlo,molecular dynamics, and transition‐state theory is excellent. A similar comparison for the Rh/Rh(111) systems shows discrepancies between these three techniques. We find that the origins of the discrepancies in the Rh/Rh(111) system are transition‐state recrossings, for small simulation time steps, and low escape rates of the adatom from the binding sites, at large time steps. We examine the sampling and dynamics in trajectories using a smaller time step for motion perpendicular to the surface than that for parallel motion. These studies show that low Smart Monte Carlo escape rates in the Rh/Rh(111) system can be correlated to excessive sampling, beyond the configurational space of the potential‐energy minimum, at large time steps. Recrossings can be understood to arise from the absence of velocity correlations in the low‐friction, transition‐state region and can be minimized through the use of a large time step for parallel motion. With the appropriate choice of simulation time steps it is possible to improve the agreement between dynamical Smart Monte Carlo and more rigorous dynamical techniques.

Smart Monte Carlo for accurate simulation of rare‐event dynamics: Diffusion of adsorbed species on solid surfaces

Observational evidence suggests a link between long-duration gamma-ray bursts (LGRBs) and Type Ic supernovae. Here, we propose a potential mechanism for Type Ic supernovae in LGRB progenitors powered solely by accretion energy. We present spherically symmetric hydrodynamic simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a 'collapsar', onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in one spatial dimension and with rotation, an explicit shear viscosity, and convection in the mixing length theory approximation. Once the accretion flow becomes rotationally supported outside of the black hole, an accretion shock forms and traverses the stellar envelope. Energy is carried from the central geometrically thick accretion disk to the stellar envelope by convection. Energy losses through neutrino emission and nuclear photodisintegration are calculated but do not seem important following the rapid early drop of the accretion rate following circularization. We find that the shock velocity, energy, and unbound mass are sensitive to convective efficiency, effective viscosity, and initial stellar angular momentum. Our simulations show that given the appropriate combinations of stellar and physical parameters, explosions with energies {approx}5 Multiplication-Sign 10{sup 50}more » erg, velocities {approx}3000 km s{sup -1}, and unbound material masses {approx}> 6 M{sub Sun} are possible in a rapidly rotating 16 M{sub Sun} main-sequence progenitor star. Further work is needed to constrain the values of these parameters, to identify the likely outcomes in more plausible and massive LRGB progenitors, and to explore nucleosynthetic implications.« less

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Pawan Kumar

Papers

Evidence for mild deviation from power-law distribution of electrons in relativistic shocks: GRB 090902B

The maximum luminosity of fast radio bursts

X‐Ray Lines from Gamma‐Ray Bursts

Smart Monte Carlo for accurate simulation of rare‐event dynamics: Diffusion of adsorbed species on solid surfaces

Simulations of Accretion Powered Supernovae in the Progenitors of Gamma Ray Bursts