Michael R. Goad

Bio: Michael R. Goad is an academic researcher from University of Leicester. The author has contributed to research in topics: Gamma-ray burst & Planet. The author has an hindex of 33, co-authored 117 publications receiving 4541 citations. Previous affiliations of Michael R. Goad include University of Southampton & Space Telescope Science Institute.

A short γ-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225

Abstract: Gamma-ray bursts (GRBs) are either ‘long and soft’, or ‘short and hard’. The long-duration type leave a strong afterglow and have been extensively studied. So we have a good idea of what causes them: explosions of massive stars in distant star-forming galaxies. Short GRBs, with no strong afterglow, were harder to pin down. The Swift satellite, launched last November, is designed to study bursts as soon as they happen. Having shown its worth with long GRBs (reported in the 18 August issue of Nature), Swift has now bagged a short burst, GRB 050509B, precisely measured its location and detected the X-ray afterglow. Four papers this week report on this and another recent short burst. Now, over 20 years after they were first recognized, the likely origin of the short GRBs is revealed as a merger between neutron stars of a binary system and the instantaneous production of a black hole. Gamma-ray bursts (GRBs) come in two classes1: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z ≈ 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars2. In contrast, no short GRB had been accurately (< 10″) and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from—and the localization of—the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect3,4 if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.

Bright X-ray Flares in Gamma-Ray Burst Afterglows

Abstract: Gamma-ray burst (GRB) afterglows have provided important clues to the nature of these massive explosive events, providing direct information on the nearby environment and indirect information on the central engine that powers the burst. We report the discovery of two bright x-ray flares in GRB afterglows, including a giant flare comparable in total energy to the burst itself, each peaking minutes after the burst. These strong, rapid x-ray flares imply that the central engines of the bursts have long periods of activity, with strong internal shocks continuing for hundreds of seconds after the gamma-ray emission has ended.

Panchromatic study of GRB 060124: From precursor to afterglow

Abstract: We present observations of GRB 060124, the first event for which both the prompt and the afterglow emission could be observed simultaneously and in their entirety by the three Swift instruments. Indeed, Swift-BAT triggered on a precursor " 570 s before the main burst peak, and this allowed Swift to repoint the narrow field instruments to the burst position " 350 s before the main burst occurred. GRB 060124 also triggered Konus-Wind, which observed the prompt emission in a harder gamma-ray band (up to 2 MeV). Thanks to these exceptional circumstances, the temporal and spectral properties of the prompt emission can be studied in the optical, X-ray and gamma-ray ranges. While the X-ray emission (0.2-10 keV) clearly tracks the gamma-ray burst, the optical component follows a di! erent pattern, likely indicating a di! erent origin, possibly the onset of external shocks. The prompt GRB spectrum shows significant spectral evolution, with both the peak energy and the spectral index varying. As observed in several long GRBs, significant lags are measured between the hard- and low-energy components, showing that this behaviour extends over 3 decades in energy. The GRB peaks are also much broader at soft energies. This is related to the temporal evolution of the spectrum, and can be accounted for by assuming that the electron spectral index softened with time. The burst energy (Eiso " 5 # 10 53 erg) and average peak energy (Ep " 300 keV) make GRB060124 consistent with theAmati relation. TheX-ray afterglow ischaracterized by adecay which presents a break at tb " 10 5 s.

What the Optical Recombination Lines Can Tell Us about the Broad-Line Regions of Active Galactic Nuclei

Abstract: We investigate the effect of a global change in the ionizing continuum level on the behavior of the strong optical broad emission lines seen in spectra of the nuclear emission-line regions of active galactic nuclei (AGNs), including the Balmer lines, He I λ5876, and He II λ4686. Unlike most of the prominent heavy-element lines found in the UV, the optical hydrogen and helium recombination lines' emissivities are strongly dependent on the incident continuum flux, since these lines arise out of excited states whose optical depths depend on the incident flux of photons. Using photoionization calculations, we determine the luminosity-dependent responsivities, η(r, L(t)) = Δ log Lline/Δ log Lcont, of these lines for a general model of the broad emission line region (BLR), with the purpose of establishing them as important probes of the physical conditions within the BLRs of AGNs. The dependence of these lines' emissivities on the incident photon flux invokes a dependence in their responsivities on distance from the central continuum source. In particular, the responsivities of these lines are generally anticorrelated with the incident photon flux. Thus, their responsivities vary with distance within the BLR for a fixed continuum luminosity and change with time as the continuum source varies. Consequently, after correcting for light-travel-time effects, the response of the Balmer and optical helium lines should generally be strongest during low continuum luminosity states. Responsivity that depends on photon flux and continuum state may explain a number of outstanding problems currently under investigation in broad-line variability studies of these and other emission lines. These include the origin of the intrinsic Baldwin effect, measurements of luminosity-dependent lags (a "breathing" BLR), and luminosity-dependent variations in integrated broad emission line flux ratios (including He II λ4686/Hβ), broad-line profile shapes, and radial velocity-dependent intensity ratios. The broad Hα/Hβ and He I/Hβ flux ratios and the Balmer emission line responsivity are observed to decrease from the line center to the line wings. These, along with our findings, lead to the conclusion that the BLR velocity field diminishes with increasing distance from the central continuum source. This is consistent with recent reverberation studies that find a relationship between the emission-line lag and rms profile width for multiple lines in individual AGNs, which implies that the velocity field is dominated by a central massive object. Finally, the responsivity of ionization-bounded clouds can account for much of the observed behavior of the optical recombination lines (e.g., the weak response of the Balmer line wings) previously attributed to a substantial contribution from matter-bounded clouds at small BLR radii.

The Next Generation Transit Survey (NGTS)

Abstract: We describe the Next Generation Transit Survey (NGTS), which is a ground-based project searching for transiting exoplanets orbiting bright stars. NGTS builds on the legacy of previous surveys, most notably WASP, and is designed to achieve higher photometric precision and hence find smaller planets than have previously been detected from the ground. It also operates in red light, maximizing sensitivity to late K and early M dwarf stars. The survey specifications call for photometric precision of 0.1 per cent in red light over an instantaneous field of view of 100 deg2, enabling the detection of Neptune-sized exoplanets around Sun-like stars and super-Earths around M dwarfs. The survey is carried out with a purpose-built facility at Cerro Paranal, Chile, which is the premier site of the European Southern Observatory (ESO). An array of twelve 20 cm f/2.8 telescopes fitted with back-illuminated deep-depletion CCD cameras is used to survey fields intensively at intermediate Galactic latitudes. The instrument is also ideally suited to ground-based photometric follow-up of exoplanet candidates from space telescopes such as TESS, Gaia and PLATO. We present observations that combine precise autoguiding and the superb observing conditions at Paranal to provide routine photometric precision of 0.1 per cent in 1 h for stars with I-band magnitudes brighter than 13. We describe the instrument and data analysis methods as well as the status of the survey, which achieved first light in 2015 and began full-survey operations in 2016. NGTS data will be made publicly available through the ESO archive.

Multi-messenger observations of a binary neutron star merger

Abstract: 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.

Monthly Notices of the Royal Astronomical Society

Abstract: Monthly Notices is one of the three largest general primary astronomical research publications. It is an international journal, published by the Royal Astronomical Society. This article 1 describes its publication policy and practice.

Parameter estimation in astronomy through application of the likelihood ratio. [satellite data analysis techniques

Abstract: Many problems in the experimental estimation of parameters for models can be solved through use of the likelihood ratio test. Applications of the likelihood ratio, with particular attention to photon counting experiments, are discussed. The procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply. The procedures are proved analytically, and examples from current problems in astronomy are discussed.

Methods and results of an automatic analysis of a complete sample of Swift-XRT observations of GRBs

Abstract: We present a homogeneous X-ray analysis of all 318 gamma-ray bursts detected by the X-ray telescope (XRT) on the Swift satellite up to 2008 July 23; this represents the largest sample of X-ray GRB data published to date. In Sections 2-3, we detail the methods which the Swift-XRT team has developed to produce the enhanced positions, light curves, hardness ratios and spectra presented in this paper. Software using these methods continues to create such products for all new GRBs observed by the Swift-XRT. We also detail web-based tools allowing users to create these products for any object observed by the XRT, not just GRBs. In Sections 4-6, we present the results of our analysis of GRBs, including probability distribution functions of the temporal and spectral properties of the sample. We demonstrate evidence for a consistent underlying behaviour which can produce a range of light-curve morphologies, and attempt to interpret this behaviour in the framework of external forward shock emission. We find several difficulties, in particular that reconciliation of our data with the forward shock model requires energy injection to continue for days to weeks.

The Supernova Gamma-Ray Burst Connection

Abstract: Observations show that at least some gamma-ray bursts (GRBs) happen simultaneously with core-collapse supernovae (SNe), thus linking by a common thread nature's two grandest explosions. We review here the growing evidence for and theoretical implications of this association, and conclude that most long-duration soft-spectrum GRBs are accompanied by massive stellar explosions (GRB-SNe). The kinetic energy and luminosity of well-studied GRB-SNe appear to be greater than those of ordinary SNe, but evidence exists, even in a limited sample, for considerable diversity. The existing sample also suggests that most of the energy in the explosion is contained in nonrelativistic ejecta (producing the supernova) rather than in the relativistic jets responsible for making the burst and its afterglow. Neither all SNe, nor even all SNe of Type Ibc produce GRBs. The degree of differential rotation in the collapsing iron core of massive stars when they die may be what makes the difference.