J. P. Halpern

The afterglow, the redshift, and the extreme energetics of the gamma-ray burst 990123

Long-lived emission, known as afterglow, has now been detected from about a dozen γ-ray bursts. Distance determinations place the bursts at cosmological distances, with redshifts, z, ranging from ∼1 to 3. The energy required to produce these bright γ-ray flashes is enormous: up to ∼10 53 erg, or 10 per cent of the rest-mass energy of a neutron star, if the emission is isotropic. Here we present optical and near-infrared observations of the afterglow of GRB990123, and we determine a redshift of z ⩾ 1.6. This is to date the brightest γ-ray burst with a well-localized position and if the γ-rays were emitted isotropically, the energy release exceeds the rest-mass energy of a neutron star, so challenging current theoretical models of the sources. We argue, however, that our data may provide evidence of beamed (rather than isotropic) radiation, thereby reducing the total energy released to a level where stellar-death models are still tenable.Afterglow, or long-lived emission, has now been detected from about a dozen well-positioned gamma-ray bursts. Distance determinations made by measuring optical emission lines from the host galaxy, or absorption lines in the afterglow spectrum, place the burst sources at significant cosmological distances, with redshifts ranging from ~1--3. The energy required to produce the bright gamma-ray flashes is enormous: up to ~10^{53} erg or 10 percent of the rest mass energy of a neutron star, if the emission is isotropic. Here we present the discovery of the optical afterglow and the redshift of GRB 990123, the brightest well-localized GRB to date. With our measured redshift of >1.6, the inferred isotropic energy release exceeds the rest mass of a neutron star thereby challenging current theoretical models for the origin of GRBs. We argue that the optical and IR afterglow measurements reported here may provide the first observational evidence of beaming in a GRB, thereby reducing the required energetics to a level where stellar death models are still tenable.

Vela Pulsar and Its Synchrotron Nebula

We present high-resolution Chandra X-ray observations of PSR B0833-45, the 89 ms pulsar associated with the Vela supernova remnant. We have acquired two observations separated by 1 month to search for changes in the pulsar and its environment following an extreme glitch in its rotation frequency. We find a well-resolved nebula with a toroidal morphology remarkably similar to that observed in the Crab Nebula, along with an axial Crab-like jet. Between the two observations, taken ~3 × 105 s and ~3 × 106 s after the glitch, the flux from the pulsar is found to be steady to within 0.75%; the 3 σ limit on the fractional increase in the pulsars X-ray flux is 10-5 of the inferred glitch energy. We use this limit to constrain parameters of glitch models and neutron star structure. We do find a significant increase in the flux of the nebulas outer arc; if associated with the glitch, the inferred propagation velocity is 0.7c, similar to that seen in the brightening of the Crab Nebula wisps. We propose an explanation for the X-ray structure of the Vela synchrotron nebula based on a model originally developed for the Crab Nebula. In this model, the bright X-ray arcs are the shocked termination of a relativistic equatorial pulsar wind that is contained within the surrounding kidney-bean shaped synchrotron nebula comprising the postshock, but still relativistic, flow. In a departure from the Crab model, the magnetization parameter σ of the Vela pulsar wind is allowed to be of order unity; this is consistent with the simplest MHD transport of magnetic field from the pulsar to the nebula, where B ≤ 4 × 10-4 G. The inclination angle of the axis of the equatorial torus with respect to the line of sight is identical to that of the rotation axis of the pulsar as previously measured from the polarization of the radio pulse. The projection of the rotation axis on the sky may also be close to the direction of proper motion of the pulsar if previous radio measurements were confused by orthogonal-mode polarized components. We review effects that may enhance the probability of alignment between the spin axis and space velocity of a pulsar, and speculate that short-period, slowly moving pulsars are just the ones best-suited to producing synchrotron nebulae with such aligned structures. Previous interpretations of the compact Vela nebula as a bow-shock in a very weakly magnetized wind suffered from data of inadequate spatial resolution and less plausible physical assumptions.

Observation of X-ray Lines from a Gamma-Ray Burst (GRB991216): Evidence of Moving Ejecta from the Progenitor

We report on the discovery of two emission features observed in the x-ray spectrum of the afterglow of the gamma-ray burst (GRB) of 16 December 1999 by the Chandra X-ray Observatory. These features are identified with the Ly(alpha) line and the narrow recombination continuum by hydrogenic ions of iron at a redshift z = 1.00 +/- 0.02, providing an unambiguous measurement of the distance of a GRB. Line width and intensity imply that the progenitor of the GRB was a massive star system that ejected, before the GRB event, a quantity of iron approximately 0.01 of the mass of the sun at a velocity approximately 0.1 of the speed of light, probably by a supernova explosion.

UV/Optical Detections of Candidate Tidal Disruption Events by GALEX and CFHTLS*

We present two luminous UV/optical flares from the nuclei of apparently inactive early-type galaxies at z = 0.37 and 0.33 that have the radiative properties of a flare from the tidal disruption of a star. In this paper we report the second candidate tidal disruption event discovery in the UV by the GALEX Deep Imaging Survey and present simultaneous optical light curves from the CFHTLS Deep Imaging Survey for both UV flares. The first few months of the UV/optical light curves are well fitted with the canonical t^(−5/3) power-law decay predicted for emission from the fallback of debris from a tidally disrupted star. Chandra ACIS X-ray observations during the flares detect soft X-ray sources with T_(bb) = (2–5) × 10^5 K or Γ > 3 and place limits on hard X-ray emission from an underlying AGN down to L_X(2–10 keV) ≾ 10^41 ergs s^−1. Blackbody fits to the UV/optical spectral energy distributions of the flares indicate peak flare luminosities of ≳ 10^44-10^45 ergs s^−1. The temperature, luminosity, and light curves of both flares are in excellent agreement with emission from a tidally disrupted main-sequence star onto a central black hole of several times 10^7 M⊙. The observed detection rate of our search over ~2.9 deg^2 of GALEX Deep Imaging Survey data spanning from 2003 to 2007 is consistent with tidal disruption rates calculated from dynamical models, and we use these models to make predictions for the detection rates of the next generation of optical synoptic surveys.

1E 1547.0-5408: a radio-emitting magnetar with a rotation period of 2 seconds

The variable X-ray source 1E 1547.0-5408 was identified by Gelfand & Gaensler as a likely magnetar in G327.24-0.13, an apparent supernova remnant. No X-ray pulsations have been detected from it. Using the Parkes radio telescope, we discovered pulsations with period P = 2.069 s. Using the Australia Telescope Compact Array, we localized these to 1E 1547.0-5408. We measure = (2.318 ? 0.005) ? 10-11, which for a magnetic dipole rotating in vacuo gives a surface field strength of 2.2 ? 1014 G, a characteristic age of 1.4 kyr, and a spin-down luminosity of 1.0 ? 1035 ergs s-1. Together with its X-ray characteristics, these rotational parameters of 1E 1547.0-5408 prove that it is a magnetar, only the second known to emit radio waves. The distance is ?9 kpc, derived from the dispersion measure of 830 cm-3 pc. The pulse profile at a frequency of 1.4 GHz is extremely broad and asymmetric due to multipath propagation in the ISM, as a result of which only ?75% of the total flux at 1.4 GHz is pulsed. At higher frequencies the profile is more symmetric and has FWHM = 0.12P. Unlike in normal radio pulsars, but in common with the other known radio-emitting magnetar, XTE J1810-197, the spectrum over 1.4-6.6 GHz is flat or rising, and we observe large, sudden changes in the pulse shape. In a contemporaneous Swift X-ray observation, 1E 1547.0-5408 was detected with record high flux, fX(1-8 keV) ? 5 ? 10-12 ergs cm-2 s-1, 16 times the historic minimum. The pulsar was undetected in archival radio observations from 1998, implying a flux <0.2 times the present level. Together with the transient behavior of XTE J1810-197, these results suggest that radio emission is triggered by X-ray outbursts of usually quiescent magnetars.

The detailed optical light curve of GRB 030329

We present densely sampled BVRI light curves of the optical transient associated with the gamma-ray burst (GRB) 030329, the result of a coordinated observing campaign conducted at five observatories. Augmented with published observations of this GRB, the compiled optical data set contains 2687 photometric measurements, obtained between 78 minutes and 79 days after the burst. This data set allows us to follow the photometric evolution of the transient with unprecedented detail. We use the data to constrain the light curve of the underlying supernova (SN) 2003dh and show that it evolved faster than and was probably somewhat fainter than the Type Ic SN 1998bw, associated with GRB 980425. We find that our data can be described by a broken power-law decay perturbed by a complex variable component. The early- and late-time decay slopes are determined to be alpha(1) approximate to 1.1 and alpha(2) approximate to 2. Assuming this single-break power-law model, we constrain the break to lie between similar to3 and similar to8 days after the burst. This simple, singly broken power-law model, derived only from the analysis of our optical observations, may also account for available multiband data, provided that the break happened similar to8 days after the burst. The more complex double-jet model of Berger et al. provides a comparable fit to the optical, X-ray, millimeter, and radio observations of this event. The unique early coverage available for this event allows us to trace the color evolution of the afterglow during the first hours after the burst. We detect a significant change in optical colors during the first day. Our color analysis is consistent with a cooling-break frequency sweeping through the optical band during the first day. The light curves of GRB 030329 reveal a rich array of variations, superposed over the mean power-law decay. We find that the early variations (less than or similar to8 days after the burst) are asymmetric, with a steep rise followed by a relatively slower ( by a factor of about 2) decline. The variations maintain a similar timescale during the first 4 days and then get significantly longer. The structure of these variations is similar to those previously detected in the afterglows of several GRBs.

Discovery of the ultra-bright type II-L supernova 2008es

We report the discovery by the Robotic Optical Transient Search Experiment (ROTSE-IIIb) telescope of SN 2008es, an overluminous supernova (SN) at z = 0.205 with a peak visual magnitude of –22.2. We present multiwavelength follow-up observations with the Swift satellite and several ground-based optical telescopes. The ROTSE-IIIb observations constrain the time of explosion to be 23 ± 1 rest-frame days before maximum. The linear decay of the optical light curve, and the combination of a symmetric, broad Hα emission line profile with broad P Cygni Hβ and Na I λ5892 profiles, are properties reminiscent of the bright Type II-L SNe 1979C and 1980K, although SN 2008es is greater than 10 times more luminous. The host galaxy is undetected in pre-supernova Sloan Digital Sky Survey images, and similar to Type II-L SN 2005ap (the most luminous SN ever observed), the host is most likely a dwarf galaxy with Mr > – 17. Swift Ultraviolet/Optical Telescope observations in combination with Palomar 60 inch photometry measure the spectral energy distribution of the SN from 200 to 800 nm to be a blackbody that cools from 14000 K at the time of the optical peak to 6400 K 65 days later. The inferred blackbody radius is in good agreement with the radius expected for the expansion speed measured from the broad lines (10000 km s^–1). The bolometric luminosity at the optical peak is 2.8 × 10^44 erg s^–1, with a total energy radiated over the next 65 days of 5.6 × 10^50 erg. The exceptional luminosity of SN 2008es requires an efficient conversion of kinetic energy produced from the core-collapse explosion into radiation. We favor a model in which the large peak luminosity is a consequence of the core collapse of a progenitor star with a low-mass extended hydrogen envelope and a stellar wind with a density close to the upper limit on the mass-loss rate measured from the lack of an X-ray detection by the Swift X-Ray Telescope.

The Faint Optical Afterglow and Host Galaxy of GRB 020124: Implications for the Nature of Dark Gamma-ray Bursts

F. A. H. acknowledges support from a Presidential Early Career award. S. R. K. and S. G. D. thank the NSF for support. R. S. is grateful for support from a NASA ATP grant. R. S. and T. J. G. acknowledge support from the Sherman Fairchild Foundation. J. C. W. acknowledges support from NASA grant NAG 59302. K. H. is grateful for Ulysses support under JPL contract 958056 and for IPN support under NASA grants FDNAG 5-11451 and NAG 5-17100. Support for Proposal HST-GO-09180.01-A was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

Models for X-Ray Emission from Isolated Pulsars

A model is proposed for the observed combination of power-law and thermal emission of keV X-rays from rotationally powered pulsars. For γ-ray pulsars with accelerators very many stellar radii above the neutron star surface, 100 MeV curvature γ-rays from e- or e+ flowing starward out of such accelerators are converted to e± pairs on closed field lines all around the star. These pairs strongly affect X-ray emission from near the star in two ways. (1) The pairs are a source of synchrotron emission immediately following their creation in regions where B ~ 1010 G. This emission, in the photon energy range 0.1 keV EX 5 MeV, has a power-law spectrum with energy index 0.5 and X-ray luminosity that depends on the backflow current and is typically ~1033 ergs s-1. (2) The pairs ultimately form a cyclotron resonance blanket surrounding the star except for two holes along the open field line bundles that pass through it. In such a blanket, the gravitational pull on e± pairs toward the star is balanced by the hugely amplified push of outflowing surface-emitted X-rays wherever cyclotron resonance occurs. Because of it, the neutron star is surrounded by a leaky hohlraum of hot blackbody radiation with two small holes, which prevents direct X-ray observation of a heated polar cap of a γ-ray pulsar. Weakly spin-modulated radiation from the blanket together with more strongly spin-modulated radiation from the holes through it would then dominate observed low-energy (0.1-10 keV) emission. For non-γ-ray pulsars, in which no such accelerators with their accompanying extreme relativistic backflow toward the star are expected, optically thick e± resonance blankets should not form (except in special cases very close to the open field line bundle). From such pulsars, blackbody radiation from both the warm stellar surface and the heated polar caps should be directly observable. In these pulsars, details of the surface magnetic field evolution, especially of polar cap areas, become relevant to observations. The models are compared to X-ray data from Geminga, PSR 1055-52, PSR 0656+14, PSR 1929+10, and PSR 0950+08.

A Redshift Determination for XRF 020903: First Spectroscopic Observations of an X-Ray Flash

We report the discovery of optical and radio afterglow emission from the extremely soft X-ray flash XRF 020903. Our spectroscopic observations provide the first redshift for an X-ray flash, thereby setting the distance scale for these events. At z = 0.251, XRF 020903 is one of the nearest cosmic explosions ever detected, second only to the recent GRB 030329 and the unusual GRB 980425/SN 1998bw. Moreover, XRF 020903 is the first X-ray flash for which we detect an optical afterglow. The luminosity of the radio afterglow of XRF 020903 is 1000 times greater than that of Ibc supernovae but similar to those of GRB afterglows. From broadband afterglow modeling we show that the explosion energy of XRF 020903 is not dissimilar to values inferred for typical gamma-ray bursts, suggesting that these cosmological explosions may derive from a similar mechanism.