Gamma-ray Burst Remnants: How can we find them?

By now there is substantial evidence that Gamma-Ray Bursts (GRBs) originate at cosmological distances from very powerful explosions. The interaction between a GRB and its surrounding environment has dramatic consequences on the environment itself. At early times, the strong X-ray UV afterglow flux photoionizes the medium on distance scales on the order of 100 pc or more. Here I discuss the long-term effects resulting from the interaction between a GRB and its environment, and in particular the signatures of the emission spectrum produced while the heated and ionized gas slowly cools and recombines. Besides photoionizing the medium with its afterglow, a GRB explosion drives a blast wave which is expected to have a very long lifetime. I discuss possible candidates for such GRB remnants in our own and in nearby galaxies, and ways to distinguish them from remnants due to other phenomena, such as multiple supernova (SN) explosions.Comment: 3 pages, to appear in the proceedings of the 2nd Workshop "Gamma-Ray Bursts in the Afterglow Era",Rome,Oct.17-20,200

Accretion flows in early-type galaxies and CMB experiments

We investigate the possible contribution from the emission of accretion flows around supermassive black holes in early type galaxies to current measurements of the Cosmic Microwave Background (CMB) at radio frequencies. We consider a range of luminosities suggested by targeted radio observations and accretion models and compute the residual contribution of these sources to the spectrum and bispectrum of the observed CMB maps. As for high-resolution CMB experiments, we find that the unresolved component of these sources could make up to ~40-50% of the observed CBI and BIMA power spectrum at l > 2000. As a consequence, the inferred sigma_8^{SZ} value could be biased high by up to 6-7%. As for all sky experiments, we find that the contribution of accretion-flow sources to the WMAP bispectrum is at the 2-3 per cent level at most. At the flux limit that Planck will achieve, however, these sources may contribute up to 15 per cent of the bispectrum in the 60-100 GHz frequency range. Moreover, Planck should detect hundreds of these sources in the 30-300 GHz frequency window. These detections, possibly coupled with galaxy type confirmation from optical surveys, will allow number counts to put tighter constraints on early-type galaxies radio luminosity and accretion flows properties. These sources may also contribute up to the 30 per cent level to the residual radio sources power spectrum in future high-resolution SZ surveys (like ACT or APEX) reaching mJy flux limits.Comment: 7 pages, 3 figures, accepted to MNRA

Broadband Modeling of GRB 021004

We present a broadband modeling of the afterglow of GRB 021004. The optical transient of this burst has been detected very early and densely sampled in several bands. Its light curve shows significant deviations from a simple power law. We use the data from the X-ray to the $I-$band gathered in the first month of observations, and examine three models. Two involve variations in the energy of the shock. The first (energy injection) allows only increases to the shock energy, while the second (patchy shell) allows the energy to increase or decrease. In the final model (clumpy medium) the energy of the shock is constant while the density varies. While all three models reproduce well the optical bands, the variable density model can best account for the X-ray data, and the energy-injection model has the poorest fit. None of the models can account for the modest color variations observed during the first few days of the burst.Comment: 13 pages, 5 figures, accepted to Astrophys. J. Letters, added analysis of X-ray lightcurv

Constraints on the Emission and Viewing Geometry of the Transient Anomalous X-ray Pulsar XTE J1810-197

The temporal decay of the flux components of Transient Anomalous X-ray Pulsar XTE J1810-197 following its 2002 outburst presents a unique opportunity to probe the emission geometry of a magnetar. Toward this goal, we model the magnitude of the pulsar's modulation in narrow spectral bands over time. Following previous work, we assume that the post-outburst flux is produced in two distinct thermal components arising from a hot spot and a warm concentric ring. We include general relativistic effects on the blackbody spectra due to gravitational redshift and light bending near the stellar surface, which strongly depend on radius. This affects the model fits for the temperature and size of the emission regions. For the hot spot, the observed temporal and energy-dependent pulse modulation is found to require an anisotropic, pencil-beamed radiation pattern. We are able to constrain an allowed range for the angles that the line-of-sight (psi) and the hot spot pole (xi) make with respect to the spin-axis. Within errors, this is defined by the locus of points in the xi-psi plane that lie along the line (xi+beta(R))(psi+beta(R)) ~ constant, where beta(R) is a function of the radius R of the star. For a canonical value of R=12 km, the viewing parameters range from psi=xi=37 deg to (psi,xi)=(85 deg,15 deg). We discuss our results in the context of magnetar emission models.Comment: 8 pages, accepted to Ap

NuSTAR J095551+6940.8: a highly magnetised neutron star with super-Eddington mass accretion

The identification of the Ultraluminous X-ray source (ULX) X-2 in M82 as an accreting pulsar has shed new light on the nature of a subset of ULXs, while rising new questions on the nature of the super-Eddington accretion. Here, by numerically solving the torque equation of the accreting pulsar within the framework of the magnetically threaded-disk scenario, we show that three classes of solutions, corresponding to different values of the magnetic field, are mathematically allowed. We argue that the highest magnetic field one, corresponding to B $\sim 10^{13}$ G, is favoured based on physical considerations and the observed properties of the source. In particular, that is the only solution which can account for the observed variations in $\dot{P}$ (over four time intervals) without requiring major changes in $\dot{M}$, which would be at odds with the approximately constant X-ray emission of the source during the same time. For this solution, we find that the source can only accomodate a moderate amount of beaming, 0.5 $\lesssim b < 1$. Last, we show that the upper limit on the luminosity, L$_X < 2.5 \times 10^{38}$ erg s$^{-1}$ from archival observations, is consistent with a highly-magnetized neutron star being in the propeller phase at that time.Comment: 8 pages, 3 figures, accepted for publication on MNRA