arXiv subjects

E. Nakar

Publications and source records attributed to E. Nakar.

43 records · Page 3Linked to original sources

arXiv2006

Opening angles, Lorentz factors and confinement of X-ray binary jets

We present a collation of the available data on the opening angles of jets in X-ray binaries, which in most cases are small (less than 10 degrees). Under the assumption of no confinement, we calculate the Lorentz factors required to produce such small opening angles via the transverse relativistic Doppler effect. The derived Lorentz factors, which are in most cases lower limits, are found to be large, with a mean greater than 10, comparable to those estimated for AGN and much higher than the commonly-assumed values for X-ray binaries of 2 to 5. Jet power constraints do not in most cases rule out such high Lorentz factors. The upper limits on the opening angles show no evidence for smaller Lorentz factors in the steady jets of Cygnus X-1 and GRS 1915+105. In those sources in which deceleration has been observed (notably XTE J1550-564 and Cygnus X-3), some confinement of the jets must be occurring, and we briefly discuss possible confinement mechanisms. It is however possible that all the jets could be confined, in which case the requirement for high bulk Lorentz factors can be relaxed.

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arXiv2005

Late-time Radio Observations of 68 Type Ibc Supernovae: Strong Constraints on Off-Axis Gamma-ray Bursts

We present late-time radio observations of 68 local Type Ibc supernovae, including six events with broad optical absorption lines (``hypernovae''). None of these objects exhibit radio emission attributable to off-axis gamma-ray burst jets spreading into our line-of-sight. Comparison with our afterglow models reveals the following conclusions: (1) Less than $\sim 10%$ of Type Ibc supernovae are associated with typical gamma-ray bursts initially directed away from our line-of-sight; this places an empirical constraint on the GRB beaming factor of $ \lesssim 10^4$ corresponding to an average jet opening angle, $θ_j \gtrsim 0.8$ degrees. (2) This holds in particular for the broad-lined supernovae (SNe 1997dq, 1997ef, 1998ey, 2002ap, 2002bl and 2003jd) which have been argued to host GRB jets. Our observations reveal no evidence for typical (or even sub-energetic) GRBs and rule out the scenario in which every broad-lined SN harbors a GRB at the 84% confidence level. Their large photospheric velocities and asymmetric ejecta (inferred from spectropolarimetry and nebular spectroscopy) appear to be characteristic of the non-relativistic supernova explosion and do not necessarily imply the existence of associated GRB jets.

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arXiv2005

The afterglow of GRB050709 and the nature of the short-hard gamma-ray bursts

The final chapter in the long-standing mystery of the gamma-ray bursts (GRBs) centres on the origin of the short-hard class, suspected on theoretical grounds to result from the coalescence of neutron star or black hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical, and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst whose accurate position allows us to unambiguously associate it with a star-forming galaxy at redshift z=0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.

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arXiv2005

The Afterglows, Redshifts, and Properties of Swift Gamma-Ray Bursts

We present optical, near-IR, and radio follow up of sixteen Swift bursts, including our discovery of nine afterglows and a redshift determination for three. These observations, supplemented by data from the literature, provide an afterglow recovery rate of 60% in the optical/near-IR, much higher than in previous missions (BeppoSAX, HETE-2, INTEGRAL, and IPN). The optical/near-IR afterglows of Swift events are on average 1.7 mag fainter at t=12 hr than those of previous missions. The X-ray afterglows are similarly fainter compared to those of pre-Swift bursts. In the radio the limiting factor is the VLA threshold and the detection rate for Swift bursts is similar to that for past missions. The redshift distribution of pre-Swift bursts peaked at z~1, whereas the five Swift bursts with measured redshifts are distributed evenly between 1.3 and 3.2. From these results we conclude that (i) the pre-Swift distributions were biased in favor of bright events and low redshift events, (ii) the higher sensitivity and accurate positions of Swift result in a better representation of the true burst redshift and brightness distributions (which are higher and dimmer, respectively), and (iii) as many as 1/3 of the bursts can be optically dark, as a result of a high redshift and/or dust extinction. We remark that the apparent lack of low redshift, low luminosity Swift bursts, and the lower event rate compared to pre-launch estimates (90 vs. 150 per year), are the result of a threshold that is similar to that of BATSE. In view of these inferences, afterglow observers may find it advisable to make significant changes in follow up strategies of Swift events. [abridged]

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arXiv2005

Discovery of a Radio Source following the 27 December 2004 Giant Flare from SGR 1806-20

Over a decade ago it was established that the remarkable high energy transients, known as soft gamma-ray repeaters (SGRs), are a Galactic population and originate from neutron stars with intense (<~ 10^15 G) magnetic fields ("magnetars"). On 27 December 2004 a giant flare (fluence >~ 0.3 erg/cm^2) was detected from SGR 1806-20. Here we report the discovery of a fading radio counterpart. We began a monitoring program from 0.2GHz to 250GHz and obtained a high resolution 21-cm radio spectrum which traces the intervening interstellar neutral Hydrogen clouds. Analysis of the spectrum yields the first direct distance measurement of SGR 1806-20. The source is located at a distance greater than 6.4 kpc and we argue that it is nearer than 9.8 kpc. If true, our distance estimate lowers the total energy of the explosion and relaxes the demands on theoretical models. The energetics and the rapid decay of the radio source are not compatible with the afterglow model that is usually invoked for gamma-ray bursts. Instead we suggest that the rapidly decaying radio emission arises from the debris ejected during the explosion.

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arXiv2002

GRBs Light Curves - Another Clue on the Inner Engine

The nature of the `inner engine' that accelerate and collimate the relativistic flow at the cores of GRBs is the most interesting current puzzle concerning GRBs. Numerical simulations have shown that the internal shocks' light curve reflects the activity of this inner engine. Using a simple analytic toy model we clarify the relations between the observed $ γ$-rays light curve and the inner engine's activity and the dependence of the light curves on the inner engine's parameters. This simple model also explains the observed similarity between the observed distributions of pulses widths and the intervals between pulses and the correlation between the width of a pulse and the length of the preceding interval. Our analysis suggests that the variability in the wind's Lorentz factors arises due to a modulation of the mass injected into a constant energy flow.

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arXiv2001

New results on the temporal structure of GRBs

We analyze the temporal structure of long ((T_{90}>2sec)) and short ((T_{90}<2sec)) BATSE bursts. We find that: (i) In many short bursts (δt_{min}/T\ll 1) (where (δt_{min}) is the shortest pulse). This indicates that short bursts arise, like long ones, in internal shocks. (ii) In long bursts there is an excess of long intervals between pulses (relative to a lognormal distribution). This excess can be explained by the existence of \emph{quiescent times}, long periods with no signal above the background that arise, most likely, from periods with no source activity. The lognormal distribution of the intervals (excluding the \emph{quiescent times}) is similar and correlated with the distribution of the pulses width, in agreement with the predictions of the internal shock model.

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