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Search Results: 1 - 10 of 32452 matches for " John Middleditch "
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Pulsar-Driven Jets in Supernovae, Gamma-Ray Bursts, and the Universe
John Middleditch
Advances in Astronomy , 2012, DOI: 10.1155/2012/898907
Pulsar-Driven Jets in Supernovae, Gamma-Ray Bursts, and the Universe
John Middleditch
Advances in Astronomy , 2012, DOI: 10.1155/2012/898907
Abstract: The bipolarity of Supernova 1987A can be understood through its very early light curve from the CTIO 0.4?m telescope and IUE FES and following speckle observations of the “Mystery Spot”. These indicate a beam/jet of light/particles, with initial collimation factors >104 and velocities >0.95?c, involving up to 10?5? interacting with circumstellar material. These can be produced by a model of pulsar emission from polarization currents induced/(modulated faster than c) beyond the pulsar light cylinder by the periodic electromagnetic field (supraluminally induced polarization currents (SLIP)). SLIP accounts for the disruption of supernova progenitors and their anomalous dimming at cosmological distances, jets from Sco X-1 and SS 433, the lack/presence of pulsations from the high-/low-luminosity low-mass X-ray binaries, and long/short gamma-ray bursts, and it predicts that their afterglows are the pulsed optical-/near-infrared emission associated with these pulsars. SLIP may also account for the TeV e+/e? results from PAMELA and ATIC, the WMAP “Haze”/Fermi “Bubbles,” and the r-process. SLIP jets from SNe of the first stars may allow galaxies to form without dark matter and explain the peculiar nongravitational motions between pairs of distant galaxies observed by GALEX. 1. Introduction Supernova 1987A has provided astronomers with a wealth of data, some of which have not even now, a quarter century after the event, been satisfactorily accounted for by any model. One of the most remarkable features in the early study of SN 1987A was the “Mystery Spot,” with a thermal energy of ?erg, observed 50 days after the core-collapse event [1–3] and separated from the SN photosphere “proper” by ?arc?s at day 38 (Figure 1), with about 3% of this energy eventually radiated in the optical band. The possibility that the enormous energy implied for the Mystery Spot might somehow link it to gamma-ray bursts (GRBs) attracted little serious consideration at the time, or even since, beyond a very astute few [4–6]. The Mystery Spot was also observed at separations of ?arc?s on day 30 and ?arc?s on day 50 but always at an angle of 194°, consistent with the southern (and approaching) extension of the bipolarity [7]. The Mystery Spot offsets from SN 1987A imply a minimum projected separation of ~10 light-days ( t-d). Figure 1: Measurements of displacement (lower) and observed magnitude (upper) of the “Mystery Spot” from SN 1987A, at H and 533?nm, versus time, from [ 1– 3]. There is also a wealth of photometric and spectroscopic data from even earlier stages of SN 1987A, in
Pulsed Gamma-Ray-Burst Afterglows
John Middleditch
Physics , 2009,
Abstract: The bipolarity of Supernova 1987A can be understood in terms of its very early light curve as observed from the CTIO 0.4-m telescope, as well as the IUE FES, and the slightly later speckle observations of the "Mystery Spot" by two groups. These observations imply a highly directional beam of light and jet of particles, with initial collimation factors in excess of 10,000, velocities in excess of 0.95 c, as an impulsive event involving up to 0.00001 solar masses, which interacts with circumstellar material. The jet and beam coincide with the 194 degree angle of the bipolarity on the sky, and are oriented at 75 degrees to the line of sight to the Earth. By day 30 the collimation of the jet decreases, and its velocity declines to ~0.5 c. These observations and the resulting kinematic solution can be understood in terms of pulsar emission from polarization currents, induced by the periodically modulated electromagnetic field beyond the pulsar light cylinder, which are thus modulated at up to many times the speed of light. With plasma available at many times the light cylinder radius, as would be the case for a spinning neutron star formed at the center of its progenitor, pulsed emission is directed close to the rotation axis, eviscerating this progenitor, and continuing for months to years, until very little circumpulsar material remains. This model provides a candidate for the central engine of the gamma-ray burst (GRB) mechanism, both long and short, and predicts that GRB afterglows are the_pulsed_ optical/near infrared emission associated with these newly-born neutron stars.
Fourier Techniques for Very Long Astrophysical Time Series Analysis
Scott M. Ransom,Stephen S. Eikenberry,John Middleditch
Physics , 2002, DOI: 10.1086/342285
Abstract: In this paper, we present an assortment of both standard and advanced Fourier techniques that are useful in the analysis of astrophysical time series of very long duration -- where the observation time is much greater than the time resolution of the individual data points. We begin by reviewing the operational characteristics of Fourier transforms (FTs) of time series data, including power spectral statistics, discussing some of the differences between analyses of binned data, sampled data, and event data, and briefly discuss algorithms for calculating discrete Fourier transforms (DFTs) of very long time series. We then discuss the response of DFTs to periodic signals, and present techniques to recover Fourier amplitude "lost" during simple traditional analyses if the periodicities change frequency during the observation. These techniques include Fourier interpolation which allows us to correct the response for signals that occur between Fourier frequency bins. We then present techniques for estimating additional signal properties such as the signal's centroid and duration in time, the first and second derivatives of the frequency, the pulsed fraction, and an overall estimate of the significance of a detection. Finally, we present a recipe for a basic but thorough Fourier analysis of a time series for well-behaved pulsations.
A Maximum-Likelihood Analysis of Observational Data on Fluxes and Distances of Radio Pulsars: Evidence for Violation of the Inverse-Square Law
John Singleton,Pinaki Sengupta,John Middleditch,Todd L. Graves,Mario R. Perez,Houshang Ardavan,Arzhang Ardavan
Physics , 2009,
Abstract: We analyze pulsar fluxes at 1400 MHz ($S_{1400}$) and distances ($d$) extracted from the Parkes Multibeam Survey. Under the assumption that distribution of pulsar luminosities is distance-independent, we find that either (a) pulsar fluxes diminish with distance according to a non-standard power law, due, we suggest, to the presence of a component with $S_{1400} \propto 1/d$, or (b) that there are very significant (i.e. order of magnitude) errors in the dispersion-measure method for estimating pulsar distances. The former conclusion (a) supports a model for pulsar emission that has also successfully explained the frequency spectrum of the Crab and 8 other pulsars over 16 orders of magnitude of frequency, whilst alternative (b) would necessitate a radical re-evaluation of both the dispersion-measure method and current ideas about the distribution of free electrons within our Galaxy.
A new mechanism for generating broadband pulsar-like polarization
Houshang Ardavan,Arzhang Ardavan,Joseph Fasel,John Middleditch,Mario Perez,Andrea Schmidt,John Singleton
Physics , 2009,
Abstract: Observational data imply the presence of superluminal electric currents in pulsar magnetospheres. Such sources are not inconsistent with special relativity; they have already been created in the laboratory. Here we describe the distinctive features of the radiation beam that is generated by a rotating superluminal source and show that (i) it consists of subbeams that are narrower the farther the observer is from the source: subbeams whose intensities decay as 1/R instead of 1/R^2 with distance (R), (ii) the fields of its subbeams are characterized by three concurrent polarization modes: two modes that are 'orthogonal' and a third mode whose position angle swings across the subbeam bridging those of the other two, (iii) its overall beam consists of an incoherent superposition of such coherent subbeams and has an intensity profile that reflects the azimuthal distribution of the contributing part of the source (the part of the source that approaches the observer with the speed of light and zero acceleration), (iv) its spectrum (the superluminal counterpart of synchrotron spectrum) is broader than that of any other known emission and entails oscillations whose spacings and amplitudes respectively increase and decrease algebraically with increasing frequency, and (v) the degree of its mean polarization and the fraction of its linear polarization both increase with frequency beyond the frequency for which the observer falls within the Fresnel zone. We also compare these features with those of the radiation received from the Crab pulsar.
Infrared to Ultraviolet Wavelength-Dependent Variations Within the Pulse Profile Peaks of the Crab Nebula Pulsar
S. S. Eikenberry,G. G. Fazio,S. M. Ransom,J. Middleditch,J. Kristian,C. R. Pennypacker
Physics , 1996, DOI: 10.1086/310206
Abstract: We present evidence of wavelength-dependent variations within the infrared, optical, and ultraviolet pulse profile peaks of the Crab Nebula pulsar. The leading and trailing edge half-width half-maxima of the peaks display clear differences in their wavelength dependences. In addition, phase-resolved infrared-to-ultraviolet color spectra show significant variations from the leading to trailing edges of the peaks. The color variations between the leading and trailing edges remain significant over phase differences smaller than 0.0054, corresponding to timescales of $<180 \mu$s. These results are not predicted by any current models of the pulsar emission mechanism and offer new challenges for the development of such models.
A 16 Millisecond X-Ray Pulsar in the Crab-Like SNR N157B: Fast Times at 30 Doradus
E. V. Gotthelf,W. Zhang,F. E. Marshall,J. Middleditch,Q. D. Wang
Physics , 1998,
Abstract: The supernova remnant N157B (30 Dor B, SNR 0539-69.1, NGC 2060), located in the Tarantula Nebula of the Large Magellanic Cloud, has long been considered a possible Crab-like remnant. This hypothesis has been confirmed, quite spectacularly, with the discovery of PSR J0537-6910, the remarkable 16 ms X-ray pulsar in N157B. PSR J0537-6910 is the most rapidly spinning pulsar found to be associated with a supernova remnant. Here we report our discovery and summarize the properties of this pulsar and its supernova remnant.
Discovery of an Ultra-fast X-ray Pulsar in the Supernova Remnant N157B
F. E. Marshall,E. V. Gotthelf,W. Zhang,J. Middleditch,Q. D. Wang
Physics , 1998, DOI: 10.1086/311381
Abstract: We present the serendipitous discovery of 16 ms pulsed X-ray emission from the Crab-like supernova remnant N157B in the Large Magellanic Cloud. This is the fastest spinning pulsar associated with a supernova remnant (SNR). Observations with the Rossi X-ray Timing Explorer (RXTE), centered on the field containing SN1987A, reveal an X-ray pulsar with a narrow pulse profile. Archival ASCA X-ray data confirm this detection and locate the pulsar within 1' of the supernova remnant N157B, 14' from SN1987A. The pulsar manifests evidence for glitch(es) between the RXTE and ASCA observations which span 3.5 years; the mean linear spin-down rate is Pdot = 5.126 X 10E-14 s/s. The background subtracted pulsed emission is similar to other Crab-like pulsars with a power law of photon index of ~ 1.6. The characteristic spin-down age (~ 5000 years) is consistent with the previous age estimate of the SNR. The inferred B-field for a rotationally powered pulsar is ~ 1 X 10E12 Gauss. Our result confirms the Crab-like nature of N157B; the pulsar is likely associated with a compact X-ray source revealed by ROSAT HRI observations.
Hiccups in the night: X-ray monitoring of the two Crab-like LMC pulsars
E. V. Gotthelf,W. Zhang,F. E. Marshall,J. Middleditch,Q. D. Wang
Physics , 2001,
Abstract: We are undertaking an extensive X-ray monitoring campaign of the two Crab-like pulsars in the Large Magellanic Clouds, PSR B0540-69 and PSR J0537-6910. We present our current phase-connected timing analysis derived from a set of 50 pointed X-ray observations spanning several years. From our initial 1.2 yr monitoring program of the young 50 ms pulsar PSR B0540-69, we find the first compelling evidence for a glitch in its rotation. This glitch is characterized by dv / v = 1.90 +/- 0.05 X 10E-9 and $dvdot/ vdot = (8.5 +/- 0.5 X 10E-5. Taking into account the glitch event, we derive a braking index of n = 1.81 +/- 0.07, significantly lower than previous reported. For the 16 ms pulsar, PSR J0537-6910, we recorded 6 large glitch events during a period of nearly 3 years, the highest rate of all known Crab-like systems. Despite the extreme timing activity, the long term spin-down of this pulsar continues to average -1.9743 X 10E-10 Hz/s.
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