Jefferson M. Kommers

Bio: Jefferson M. Kommers is an academic researcher from Massachusetts Institute of Technology. The author has contributed to research in topics: Pulsar & Gamma-ray burst. The author has an hindex of 17, co-authored 32 publications receiving 1631 citations.

An X-ray pulsar with a superstrong magnetic field in the soft γ-ray repeater SGR1806 − 20

Abstract: Soft gamma-ray repeaters (SGRs) emit multiple, brief (approximately O.1 s) intense outbursts of low-energy gamma-rays. They are extremely rare; three are known in our galaxy and one in the Large Magellanic Cloud. Two SGRs are associated with young supernova remnants (SNRs), and therefore most probably with neutron stars, but it remains a puzzle why SGRs are so different from 'normal' radio pulsars. Here we report the discovery of pulsations in the persistent X-ray flux of SGR1806-20, with a period of 7.47 s and a spindown rate of 2.6 x 10(exp -3) s/yr. We argue that the spindown is due to magnetic dipole emission and find that the pulsar age and (dipolar) magnetic field strength are approximately 1500 years and 8 x 10(exp 14) gauss, respectively. Our observations demonstrate the existence of 'magnetars', neutron stars with magnetic fields about 100 times stronger than those of radio pulsars, and support earlier suggestions that SGR bursts are caused by neutron-star 'crust-quakes' produced by magnetic stresses. The 'magnetar' birth rate is about one per millenium, a substantial fraction of that of radio pulsars. Thus our results may explain why some SNRs have no radio pulsars.

A new type of transient high-energy source in the direction of the Galactic Centre

Abstract: * Universities Space Research Association, 4950 Corporate Drive, Suite100, Huntsville, Alabama 35806, USA¢NASA Marshall Space Flight Center, ES-84, Huntsville, Alabama 35812,USA$Astronomical Institute "Anton Pannekoek" and Center for High-EnergyAstrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands§Physics Department, University of Alabama in Huntsville, Huntsville,Alabama 35899, USAIIDepartment of Physics, Center for Space Research, MIT 37-627,Cambridge, Massachusetts 02139, USASotJaCES of high-energy (>20keV) bursts fall into two distincttypes: the non-repeating "/-ray bursters I, several thousand ofwhich have been detected but whose origin remains unknown,and the soft y-ray repeaters (SGRs), of which there are onlythree 2. The SGRs are known to be associated with supernovaremnants, suggesting that the burst events most probably origi-nate from young neutron stars 3. Here we report the detection of athird type of transient high-energy source. On 2 December 1995,we observed the onset of a sequence of hard X-ray bursts from adirection close to that of the Galactic Centre 4. The intervalbetween bursts was initially several minutes, but after two days,the burst rate had dropped to about one per hour and has beenlargely unchanged since then. More than 1,000 bursts have nowbeen detected, with remarkably similar light curves and inten-sities; this behaviour is unprecendented among transient X-rayand ,/-ray sources. We suggest that the origin of these burstsmight be related to the spasmodic accretion of material onto aneutron star.Visual inspection of the X-ray intensity records of the eightLarge Area Detectors (LADs) of BATSE 5 showed that bursts ofhard X-rays were emitted in rapid succession from a source in thegeneral direction of the Galactic Centre beginning 1995 Decem-ber 2 12:17 UT (Fig. 1). The bursts came at intervals of about 5minutes; they lasted for typically 30 seconds and reached a peakintensity of _300countss _ in the 25-611keV energy band.Spectral analysis of the events showed significant emission to60 kcV in virtually all bursts; in _ 17% of them, emission wasdetected to about 75 keV. The outbursts were also registered bythe Spectroscopy Detectors _ which look into the same sky direc-tions as the LADs and are sensitive down to _ 10 keV. The burstsusually show a clear single peak; in many cases a tail of extendedemission can be seen. The burst which we could unambiguouslyidentify as the first in this sequence of rapid hard X-ray burstsoccurred on December 2 08:22 UT. Between December 2 17:1111and 20:00 UT, the burst intervals clustered around (172 + 15)s.After 20:00 UT the intervals became more erratic and longer.Figure 2 shows the behaviour of the burst interval evolution duringthe first 4 days of emission. Between 4 and 16 December the burstdetection rate remained remarkably constant at an average of 18events per day (about one-third of the bursts were missed becauseof data gaps).We determined the location of the individual events from therelative strengths of the signals detected in different BATSEdetectors, using an algorithm that includes a detailed descriptionof the energy/angular detector response and the contribution fromX-rays scattered off the Earth atmosphere% Initially, the burstswere seen in two detectors only; to improve the constraints on thelocation, the Compton Gamma Ray Observatory (CGRO) wasreoriented on December 8 (10:57 trr, after which the events wereregistered by four detectors. The BATSE locations combined with(1) Earth occultation constraints of the source, (2) a galacticlongitude constraint from observations with the Oriented Scintil-lation Spectroscopy Experiment (OSSE) on CGRO v, and (3)Interplanetary Network (IPN) triangulation results _', producedan _ 2# × 6' error box centred on right ascension 17 h 44 min 28 s,declination -28' 45.0' (equinox 20110.0). A 3-arcmin error boxrecently m obtained with the X-Ray Timing Explorer (XTE) liesentirely within the IPN location. This error box does not containknown bright X-ray sources_LAs part of a current project .2we made a comprehensive searchfor events in the 20-60 keV energy, range in archival BATSE data,covering the period 13 January 1993 to 24 December 1993 (345FIG. 1 The BATSE(25-60 keY) intensity recordsobtained during three consecutive orbits of CGROon 2 December 1995 between 13:50 and 17:30UT;the data are displayed with a time resolution of4s. (The time scale starts at 00:00 UTon 2December 1995).

A Comparison between the Rapid Burster and GRO J1744–28

Abstract: Twenty years ago, the Rapid Burster (MXB 1730-335) was discovered. Its most salient feature was the occurrence of rapidly repetitive type II X-ray bursts, the release of gravitational potential energy due to spasmodic accretion onto a compact object. This is almost certainly due to an accretion disk instability whose origin is still not understood. With the recent appearance of GRO J1744-28, the Rapid Burster is no longer the only system to produce such bursts. Both systems are transient low-mass X-ray binaries in which the accretor is a neutron star. The Rapid Burster, located in a globular cluster, also produces type I bursts which are due to thermonuclear flashes on the neutron star's surface; no X-ray pulsations are observed. Its neutron star magnetic field is therefore relatively weak. In contrast, strong X-ray pulsations have been observed in the persistent flux as well as in the type II bursts from GRO J1744-28, but no type I bursts have been observed. Thus, the magnetic field of the neutron star in this system is probably stronger than is the case of the Rapid Burster. The fact that type II bursts occur in both systems may bring us closer to an understanding of the mechanism(s?) that produces them.

On the Association of Gamma-Ray Bursts with Supernovae

Abstract: The recent discovery of a supernova (SN 1998bw) seemingly associated with GRB 980425 adds a new twist to the decades-old debate over the origin of gamma-ray bursts. To investigate the possibility that some (or all) bursts are associated with supernovae, we performed a systematic search for temporal/angular correlations using catalogs of BATSE and BATSE/Ulysses burst locations. We find no associations with any of the precise BATSE/Ulysses locations, which allows us to conclude that the fraction of high-fluence gamma-ray bursts associated with known supernovae is small (<0.2%). For the more numerous weaker bursts, the corresponding limiting fraction of 2.5% is far less constraining due to the imprecise locations of these events. This fraction (2.5%) of bursts corresponds to approximately 30% of the recent supernovae used as a comparison data set. Thus, although we find no significant evidence to support a burst/supernova association, the possibility cannot be excluded for weak bursts.

The intensity distribution of faint gamma-ray bursts detected with BATSE

Abstract: We have recently completed a search of six years of archival Burst and Transient Source Experiment (BATSE) data for gamma-ray bursts (GRBs) that were too faint to activate the real-time burst detection system running on board the spacecraft. These 'nontriggered' bursts can be combined with the 'triggered' bursts detected on board to produce a GRB intensity distribution that reaches peak fluxes a factor of approximately two lower than could be studied previously. The value of the statistic (in Euclidean space) for the bursts we detect is 0.177 +/- 0.006. This surprisingly low value is obtained because we detected very few bursts on the 4.096 s and 8.192 s timescales (where most bursts have their highest signal-to-noise ratio) that were not already detected on the 1.024 s timescale. If allowance is made for a power-law distribution of intrinsic peak luminosities, the extended peak flux distribution is consistent with models in which the redshift distribution of the gamma-ray burst rate approximately traces the star formation history of the universe. We argue that this class of models is preferred over those in which the burst rate is independent of redshift. We use the peak flux distribution to derive a limit of 10% (99% confidence) on the fraction of the total burst rate that could be contributed by a spatially homogeneous (in Euclidean space) subpopulation of burst sources, such as type Ib/c supernovae. These results lend support to the conclusions of previous studies predicting that relatively few faint 'classical' GRBs will be found below the BATSE onboard detection threshold.

The Australia Telescope National Facility Pulsar Catalogue

Abstract: We have compiled a new and complete catalog of the main properties of the 1509 pulsars for which published information currently exists. The catalog includes all spin-powered pulsars, as well as anomalous X-ray pulsars and soft gamma-ray repeaters showing coherent pulsed emission, but excludes accretion-powered systems. References are given for all data listed. We have also developed a new World Wide Web interface for accessing and displaying either tabular or plotted data with the option of selecting pulsars to be displayed via logical conditions on parameter expressions. The Web interface has an expert mode giving access to a wider range of parameters and allowing the use of custom databases. For users with locally installed software and database on Unix or Linux systems, the catalog may be accessed from a command-line interface. C-language functions to access specified parameters are also available. The catalog is updated from time to time to include new information.

Transient Events from Neutron Star Mergers

Abstract: Mergers of neutron stars (NS + NS) or neutron stars and stellar-mass black holes (NS + BH) eject a small fraction of matter with a subrelativistic velocity. Upon rapid decompression, nuclear-density medium condenses into neutron-rich nuclei, most of them radioactive. Radioactivity provides a long-term heat source for the expanding envelope. A brief transient has a peak luminosity in the supernova range, and the bulk of radiation in the UV-optical domain. We present a very crude model of the phenomenon, and simple analytical formulae that can be used to estimate the parameters of a transient as a function of poorly known input parameters. The mergers may be detected with high-redshift supernova searches as rapid transients, many of them far away from the parent galaxies. It is possible that the mysterious optical transients detected by Schmidt et al. are related to neutron star mergers, since they typically have no visible host galaxy.

Beaming in gamma-ray bursts: evidence for a standard energy reservoir

Abstract: We present a comprehensive sample of all gamma-ray burst (GRB) afterglows with known distances, and we derive their conical opening angles based on observed broadband breaks in their light curves. Within the framework of this conical jet model, we correct for the geometry and we find that the gamma-ray energy release is narrowly clustered around 5 × 10^(50) ergs. We draw three conclusions. First, the central engines of GRBs release energies that are comparable to ordinary supernovae. Second, the broad distribution in fluence and luminosity for GRBs is largely the result of a wide variation of opening angles. Third, only a small fraction of GRBs are visible to a given observer, and the true GRB rate is several hundred times larger than the observed rate.

Short-Duration Gamma-Ray Bursts

Abstract: Gamma-ray bursts (GRBs) display a bimodal duration distribution with a separation between the short- and long-duration bursts at about 2 s. The progenitors of long GRBs have been identified as massive stars based on their association with Type Ic core-collapse supernovae (SNe), their exclusive location in star-forming galaxies, and their strong correlation with bright UV regions within their host galaxies. Short GRBs have long been suspected on theoretical grounds to arise from compact object binary mergers (neutron star–neutron star or neutron star–black hole). The discovery of short GRB afterglows in 2005 provided the first insight into their energy scale and environments, as well as established a cosmological origin, a mix of host-galaxy types, and an absence of associated SNe. In this review, I summarize nearly a decade of short GRB afterglow and host-galaxy observations and use this information to shed light on the nature and properties of their progenitors, the energy scale and collimation of the relativistic outflow, and the properties of the circumburst environments. The preponderance of the evidence points to compact object binary progenitors, although some open questions remain. On the basis of this association, observations of short GRBs and their afterglows can shed light on the on- and off-axis electromagnetic counterparts of gravitational wave sources from the Advanced LIGO/Virgo experiments.