Hot dense matter and stellar collapse
We calculate the equation of state for hot dense matter and discuss implications for stellar collapse.

title: High-energy observations of XRF 030723: Evidence for an off-axis gamma-ray burst?
abstract: We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-ray Monitor (WXM) and French Gamma Telescope observations of XRF 030723 along with observations of the XRF afterglow made using the 6.5 m Magellan Clay telescope and Chandra. The observed peak energy E of the νFν burst spectrum is found to lie within (or below) the WXM 2-25 keV passband at 98.5% confidence, and no counts are detected above 30 keV. Our best-fit value is E = 8.4 keV. The ratio of X-ray to γ-ray flux for the burst follows a correlation found for GRBs observed with HETE-2, and the duration of the burst is similar to that typical of long-duration GRBs. If we require that the burst isotropic equivalent energy Eiso and Epk satisfy the relation discovered by Amati et al. (2002), a redshift of z = 0.38 can be determined, in agreement with constraints determined from optical observations. We are able to fit the X-ray afterglow spectrum and to measure its temporal fade. Although the best-fit fade is shallower than the concurrent fade in the optical, the spectral similarity between the two bands indicates that the X-ray fade may actually trace the optical fade. If this is the case, the late-time rebrightening observed in the optical cannot be due to a supernova bump. We interpret the prompt and afterglow X-ray emission as arising from a jetted GRB observed off-axis and possibly viewed through a complex circumburst medium that is due to a progenitor wind.

title: The Extinction toward the GRB 970228 Field
abstract: We determine the local Galactic extinction toward the field of gamma-ray burst GRB 970228 using a variety of methods. We develop a maximum-likelihood method for measuring the extinction that compares galaxy counts in different bands in the field of interest to those in a field of known extinction, and we apply this method to the GRB 970228 field. We also measure the extinction by comparing the observed broadband colors of stars in the GRB 970228 field to the colors of library spectra of the same spectral type. Finally, we estimate the extinction using the neutral hydrogen column density and the amount of infrared dust emission toward this field. Combining the results of these methods, we find a best-fit Galactic extinction in the optical of AV = 1.09, which implies that the intrinsic afterglow of GRB 970228 is significantly brighter and bluer than it appears.

title: Evidence that gamma-ray bursts repeat and are Galactic in origin
abstract: We describe work which has provided evidence that there are two distinct classes of γ-ray bursts, that the bursts repeat, and that they are Galactic in origin.

title: Likelihood methods and classical burster repetition
abstract: We develop a likelihood methodology which can be used to search for evidence of burst repetition in the BATSE catalog, and to study the properties of the repetition signal. We use a simplified model of burst repetition in which a number Nr of sources which repeat a fixed number of times Nrep are superposed upon a number Nnr of non‐repeating sources. The instrument exposure is explicitly taken into account. By computing the likelihood for the data, we construct a probability distribution in parameter space that may be used to infer the probability that a repetition signal is present, and to estimate the values of the repetition parameters. The likelihood function contains contributions from all the bursts, irrespective of the size of their positional errors—the more uncertain a burst’s position is, the less constraining is its contribution. Thus this approach makes maximal use of the data, and avoids the ambiguities of sample selection associated with data cuts on error circle size. We present the results ...

title: Evidence for reionization at z ∼ 6: Detection of a gunn-peterson trough in a z = 6.28 quasar
abstract: We present moderate-resolution Keck spectroscopy of quasars at z = 5.82, 5.99, and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Ly? absorption in the spectra of these quasars evolves strongly with redshift. To z ~ 5.7, the Ly? absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest-redshift object, SDSSp J103027.10+052455.0 (z = 6.28), the average transmitted flux is 0.0038 ? 0.0026 times that of the continuum level over 8450 ? 20, on the optical depth to Ly? absorption at z = 6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Ly? forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z ~ 5 to 6, and the universe is approaching the reionization epoch at z ~ 6.

title: Elastic Electroproduction of $\rho$ Mesons at HERA
abstract: The elastic electroproduction of rho mesons is studied at HERA with the H1 detector for a photon virtuality in the range 1 Q^2 60 GeV^2 and for a hadronic centre of mass energy in the range 30 W 140 GeV. The shape of the pipi mass distribution in the rho resonance region is measured as a function of Q^2. The full set of rho spin density matrix elements is determined, and evidence is found for a helicity flip amplitude at the level of 8 +- 3 % of the non-flip amplitudes. Measurements are presented of the dependence of the cross section on Q^2, W and t (the four-momentum transfer squared to the proton). They suggest that, especially at large Q^2, the gamma^*p cross section develops a stronger W dependence than that expected from the behaviour of elastic and total hadron-hadron cross sections.

title: Search for leptoquark bosons in ep collisions at HERA
abstract: A search for scalar and vector leptoquarks coupling to first generation fermions is performed using the e(+) p and e(-) p scattering data collected by the H1 experiment between 1994 and 2000. The data correspond to a total integrated luminosity of 117 pb(-1). No evidence for the direct or indirect production of such particles is found in data samples with a large transverse momentum final state electron or with large missing transverse momentum. Constraints on leptoquark models are established. For leptoquark couplings of electromagnetic strength, leptoquarks with masses up to 275-325 GeV are ruled out at 95% confidence level. These limits improve and supercede earlier H1 limits based on subsamples of the data used here.

title: Deep inelastic inclusive e p scattering at low x and a determination of alpha(s)
abstract: A precise measurement of the inclusive deep-inelastic \(e^+ p\) scattering cross section is reported in the kinematic range \(1.5 \le Q^2 \le 150\) GeV\(^2\) and \( 3 \cdot 10^{-5} \le x \le ). The data were recorded with the H1 detector at HERA in 1996 and 1997, and correspond to an integrated luminosity of 20 pb\(^{-1}\). The double differential cross section, from which the proton structure function \(\) and the longitudinal structure function \(F_L(x,Q^2)\) are extracted, is measured with typically 1% statistical and 3% systematic uncertainties. The measured derivative \((\partial F_2(x,Q^2) / \partial \ln Q^2)_x \) is observed to rise continuously towards small x for fixed \(Q^2\). The cross section data are combined with published H1 measurements at high \(Q^2\) for a next-to-leading order DGLAP QCD analysis. The H1 data determine the gluon momentum distribution in the range \(3 \cdot 10^{-4} \le x \le 0.1\) to within an experimental accuracy of about 3% for \(Q^2 =20\) GeV\(^2\). A fit of the H1 measurements and the \(\mu p \) data of the BCDMS collaboration allows the strong coupling constant \(\alpha_s\) and the gluon distribution to be simultaneously determined. A value of \(\alpha_s (M_Z^2) =0.1150 \pm 0.0017 (exp)^{+0.0009}_{-0.0005} (model)\) is obtained in NLO, with an additional theoretical uncertainty of about \(\pm 0.005\), mainly due to the uncertainty of the renormalisation scale.

title: Three-jet production in deep-inelastic scattering at HERA
abstract: Three-jet production is studied for the first time in deep-inelastic positron–proton scattering. The measurement carried out with the H1 detector at HERA covers a large range of four-momentum transfer squared 5 2 000 GeV 2 and invariant three-jet masses 25 3 jet ≲140 GeV . Jets are defined by the inclusive k ⊥ algorithm in the Breit frame. The size of the three-jet cross section and the ratio of the three-jet to the dijet cross section R 3/2 are described over the whole phase space by the predictions of perturbative QCD in next-to-leading order. The shapes of angular jet distributions deviate significantly from a uniform population of the available phase space but are well described by the QCD calculation.

title: Measurement of deeply virtual Compton scattering at HERA
abstract: The cross-section for deeply virtual Compton scattering in the reaction ep → egammap has been measured with the ZEUS detector at HERA using integrated luminosities of 95.0 pb(-1) of e(+) p and 16.7 pb(-1) of e(-) p collisions. Differential cross-sections are presented as a function of the exchanged- photon virtuality, Q(2), and the centre-of-mass energy, W, of the gamma*p system in the region 5 lt Q(2) lt 100 GeV2 and 40 lt W lt 140 GeV. The measured cross-sections rise steeply with increasing W. The measurements are compared to QCD-based calculations.