Long-lived emission, known as afterglow, has now been detected from about a dozen g-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 g-ray¯ashes 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 g-ray burst with a well-localized position and if the g-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.1 Palomar Observatory 105-24,

We present the detection of an absorpton feature at E = 8.77 +0.05 −0.06 keV in the combined X-ray spectrum of the ultraluminous X-ray source NGC 1313 X-1 observed with XMM-Newton and NuSTAR, significant at the 3σ level. If associated with blueshifted ionized iron, the implied outflow velocity is ∼0.2c for Fe XXVI, or ∼0.25c for Fe XXV. These velocities are similar to the ultrafast outflow seen in absorption recently discovered in this source at lower energies by XMM-Newton, and we therefore conclude that this is an iron component to the same outflow. Photoionization modeling marginally prefers the Fe XXV solution, but in either case the outflow properties appear to be extreme, potentially supporting a super-Eddington hypothesis for NGC 1313 X-1. Subject headings: Black hole physics -X-rays: binaries -X-rays: individual (NGC 1313 X-1)

We present the first fully simultaneous fits to the NIR and X-ray spectral slope (and its evolution) during a very bright flare from Sgr A ⋆ , the supermassive black hole at the Milky Way's center. Our study arises from ambitious multi-wavelength monitoring campaigns with XMMNewton, NuSTAR and SINFONI. The average multi-wavelength spectrum is well reproduced by a broken power-law with Γ N IR = 1.7 ± 0.1 and Γ X = 2.27 ± 0.12. The difference in spectral slopes (∆Γ = 0.57 ± 0.09) strongly supports synchrotron emission with a cooling break. The flare starts first in the NIR with a flat and bright NIR spectrum, while X-ray radiation is detected only after about 10 3 s, when a very steep X-ray spectrum (∆Γ = 1.8 ± 0.4) is observed. These measurements are consistent with synchrotron emission with a cooling break and they suggest that the high energy cut-off in the electron distribution (γ max ) induces an initial cut-off in the optical-UV band that evolves slowly into the X-ray band. The temporal and spectral evolution observed in all bright X-ray flares are also in line with a slow evolution of γ max . We also observe hints for a variation of the cooling break that might be induced by an evolution of the magnetic field (from B ∼ 30 ± 8 G to B ∼ 4.8 ± 1.7 G at the X-ray peak). Such drop of the magnetic field at the flare peak would be expected if the acceleration mechanism is tapping energy from the magnetic field, such as in magnetic reconnection. We conclude that synchrotron emission with a cooling break is a viable process for Sgr A ⋆ 's flaring emission.

The binary system, LS I ]61¡303, is unusual both because of the dramatic, periodic, radio outbursts, and because of its possible association with the 100 MeV gamma-ray source, 2CG 135]01. We have performed simultaneous radio and Rossi X-Ray T iming Explorer X-ray observations at 11 intervals over the 26.5 day orbit, and in addition searched for variability on timescales ranging from milliseconds to hours. We conÐrm the modulation of the X-ray emission on orbital timescales originally reported by Taylor et al., and in addition we Ðnd a signiÐcant o †set between the peak of the X-ray and radio Ñux. We argue that based on these results, the most likely X-ray emission mechanism is inverse Compton scattering of stellar photons o † of electrons accelerated at the shock boundary between the relativistic wind of a young pulsar and the Be star wind. In these observations we also detected 2È150 keV Ñux from the nearby low-redshift quasar QSO 0241]622. Comparing these measurements to previous hard X-ray and gamma-ray observations of the region containing both LS I ]61¡303 and QSO 0241]622, it is clear that emission from the QSO dominates.

We report late-time observations of the radio afterglow of GRB 030329. The light curves show a clear achromatic flattening at 50 days after the explosion. We interpret this flattening as resulting from the blast wave becoming transrelativistic. Modeling of this transition enables us to make estimates of the energy content of the burst, regardless of the initial jet structure or the distribution of initial Lorentz factors of the ejecta. We find, in accordance with other events, that GRB 030329 is well described by an explosion with total energy of a few ; 10 51 ergs expanding into a circumburst medium with a density of order unity.