Starting in 2013 February, Swift has been performing short daily monitoring observations of the G2 gas cloud near Sgr A* with the X-Ray Telescope to determine whether the cloud interaction leads to an increase in the flux from the Galactic center. On 2013 April 24 Swift detected an order of magnitude rise in the X-ray flux from the region near Sgr A*. Initially thought to be a flare from Sgr A*, detection of a short hard X-ray burst from the same region by the Burst Alert Telescope suggested that the flare was from an unresolved new Soft Gamma Repeater, SGR J1745-29. Here we present the discovery of SGR J1745-29 by Swift, including analysis of data before, during, and after the burst. We find that the spectrum in the 0.3-10 keV range is well fit by an absorbed blackbody model with kTBB ~ 1 keV and absorption consistent with previously measured values from the quiescent emission from Sgr A*, strongly suggesting that this source is at a similar distance. Only one SGR burst has been detected so far from the new source, and the persistent light curve shows little evidence of decay in approximately 2 weeks of monitoring after outburst. We discuss this light curve trend and compare it with those of other well covered SGR outbursts. We suggest that SGR J1745-29 belongs to an emerging subclass of magnetars characterized by low burst rates and prolonged steady X-ray emission 1-2 weeks after outburst onset.
We discuss the optimization of gamma-ray burst (GRB) detectors with a goal of maximizing the detected number of bright high-redshift GRBs, in the context of design studies conducted for the X-ray transient detector on the JANUS mission. We conclude that the optimal energy band for detection of high-z GRBs is below about 30 keV. We considered both lobster-eye and coded aperture designs operating in this energy band. Within the available mass and power constraints, we found that the coded aperture mask was preferred for the detection of high-z bursts with bright enough afterglows to probe galaxies in the era of the Cosmic Dawn. This initial conclusion was confirmed through detailed mission simulations that found that the selected design (an X-ray Coded Aperture Telescope) would detect four times as many bright, high-z GRBs as the lobster-eye design we considered. The JANUS XCAT instrument will detect 48 GRBs with z > 5 and fluence Sx > 3 {times} 10-7 erg cm-2 in a two year mission.
Massive black holes are believed to reside at the centres of most galaxies. They can be- come detectable by accretion of matter, either continuously from a large gas reservoir or impulsively from the tidal disruption of a passing star, and conversion of the gravitational energy of the infalling matter to light. Continuous accretion drives Active Galactic Nuclei (AGN), which are known to be variable but have never been observed to turn on or off. Tidal disruption of stars by dormant massive black holes has been inferred indirectly but the on- set of a tidal disruption event has never been observed. Here we report the first discovery of the onset of a relativistic accretion-powered jet in the new extragalactic transient, Swift J164449.3+573451. The behaviour of this new source differs from both theoretical models of tidal disruption events and observations of the jet-dominated AGN known as blazars. These differences may stem from transient effects associated with the onset of a powerful jet. Such an event in the massive black hole at the centre of our Milky Way galaxy could strongly ionize the upper atmosphere of the Earth, if beamed towards us.
JANUS is a NASA small explorer class mission which just completed phase A and was intended for a 2013 launch date. The primary science goals of JANUS are to use high redshift (6<z<12) gamma ray bursts and quasars to explore the formation history of the first stars in the early universe and to study contributions to reionization. The X-Ray Flash Monitor (XRFM) and the Near-IR Telescope (NIRT) are the two primary instruments on JANUS. XRFM has been designed to detect bright X-ray flashes (XRFs) and gamma ray bursts (GRBs) in the 1-20 keV energy band over a wide field of view (4 steradians), thus facilitating the detection of z>6 XRFs/GRBs, which can be further studied by other instruments. XRFM would use a coded mask aperture design with hybrid CMOS Si detectors. It would be sensitive to XRFs/GRBs with flux in excess of approximately 240 mCrab. The spacecraft is designed to rapidly slew to source positions following a GRB trigger from XRFM. XRFM instrument design parameters and science goals are presented in this paper.
We present a systematic temporal and spectral study of all Swift-XRT observations of GRB afterglows discovered between 2005 January and 2007 December. After constructing and fitting all light curves and spectra to power-law models, we classify the components of each afterglow in terms of the canonical X-ray afterglow and test them against the closure relations of the forward shock models for a variety of parameter combinations. The closure relations are used to identify potential jet breaks with characteristics including the uniform jet model with and without lateral spreading and energy injection, and a power-law structured jet model, all with a range of parameters. With this technique, we survey the X-ray afterglows with strong evidence for jet breaks (~12% of our sample), and reveal cases of potential jet breaks that do not appear plainly from the light curve alone (another ~30%), leading to insight into the missing jet break problem. Those X-ray light curves that do not show breaks or have breaks that are not consistent with one of the jet models are explored to place limits on the times of unseen jet breaks. The distribution of jet break times ranges from a few hours to a few weeks with a median of ~1 day, similar to what was found pre-Swift. On average Swift GRBs have lower isotropic equivalent gamma-ray energies, which in turn results in lower collimation corrected gamma-ray energies than those of pre-Swift GRBs. Finally, we explore the implications for GRB jet geometry and energetics.
Correlation studies of prompt and afterglow emissions from gamma-ray bursts (GRBs) between different spectral bands has been difficult to do in the past because few bursts had comprehensive and intercomparable afterglow measurements. In this paper we present a large and uniform data set for correlation analysis based on bursts detected by the Swift mission. For the first time, short and long bursts can be analyzed and compared. It is found for both classes that the optical, X-ray and gamma-ray emissions are linearly correlated, but with a large spread about the correlation line; stronger bursts tend to have brighter afterglows, and bursts with brighter X-ray afterglow tend to have brighter optical afterglow. Short bursts are, on average, weaker in both prompt and afterglow emissions. No short bursts are seen with extremely low optical to X-ray ratio as occurs for dark long bursts. Although statistics are still poor for short bursts, there is no evidence yet for a subgroup of short bursts with high extinction as there is for long bursts. Long bursts are detected in the dark category at the same fraction as for pre-Swift bursts. Interesting cases are discovered of long bursts that are detected in the optical, and yet have low enough optical to X-ray ratio to be classified as dark. For the prompt emission, short and long bursts have different average tracks on flux vs fluence plots. In Swift, GRB detections tend to be fluence limited for short bursts and flux limited for long events.
