Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “synchrotrons”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

Rossi X-Ray Timing Explorer Observations of the Gamma-Ray Blazar PKS 2155-304

PKS 2155-304 is the archetypical X-ray-selected BL Lac object (XBL). It is one of the brightest BL Lacs at x-ray through optical wavelengths where it has a relatively featureless continuum and displays rapid, large amplitude variability. This continuum is thought to be direct synchrotron emission from a distribution of ultra-relativistic electrons which extends to unusually high energies (Edelson et al. 1995). The gamma-ray emission from PKS 2155-304 constitutes a second, separate, spectral component. Observations with the EGRET telescope aboard the Compton Gamma Ray Observatory (CGRO) show that the spectral energy distribution of this gamma-ray component must peak at energies above 10 GeV (Vestrand et al. 1995). This, plus the realization that the extension of the synchrotron component into the x-ray band meant that ambient photons would be scattered to TeV energies, led to predictions that PKS 2155-304 would be a detectable TeV gamma ray source (Vestrand et al. 1995; Stecker et al. 1996). The University of Durham group has recently reported the discovery of TeV gamma ray emission from PKS 2155-304 (Chadwick et al. 1998, 1999) . The TeV emission was detected in 1996 September and 1997 October/November, with the largest fluxes being measured in 1997 November. During 1997 November, we detected a record high GeV gamma-ray flux from PKS 2155-304 with CGRO/EGRET (Sreekumar and Vestrand 1997) and subsequently very high x-ray fluxes were measured with BeppoSAX (Chiapeeti et al. 1997). Here we report on the record x-ray fluxes measured with the Rossi X-Ray Timing Explorer (RXTE) during the GeV/TeV outburst.

Vestrand, W. Thomas↗
NASA NTRS1999

Origin of Gamma-Ray Emissions from the MeV Blazars

More than sixty gamma-ray emitting blazars have been detected by the Compton Gamma Ray Observatory (CGRO). A sub-class of these blazars is known as MeV blazars that are most luminous at the MeV energies. Spectra of these sources show a break somewhere between 1 MeV and 30 MeV with break amplitude as large as DELTA-GAMMA = 1.5. Different models like Compton cooling jet models, synchrotron self-Compton models, etc., have been suggested to explain the observed properties of the MeV luminous blazars. However these models are unable to account for spectral break amplitudes in excess of 0.5. Here we suggest an inverse-Compton model based on recent published experimental data where MeV gamma rays were produced via Compton backscattering of UV photons with 500 MeV electrons. We show that the high energy electrons of the relativistic jet can collide with the copious ultraviolet photons of the big blue bump of the blazar, and produce photons peaked in the MeV region. Also we show that the X rays and the soft gamma rays in these sources are produced through the synchrotron self-Compton process. Flux computed from our model have been used to compare with the observed X-ray/gamma-ray spectra of the MeV luminous blazars. Details of the model are discussed in the present paper.

Ghosh, K. K.↗
NASA NTRS1998

Correlation Between Radio-Millimeter and Gamma Ray Fluxes from Blazars

Using multifrequency databases, we have plotted radio (5 GHz, 22 GHz, and 37 GHz) and millimeter (90 GHz and 230 GHz) fluxes versus MeV and gamma-ray fluxes of the EGRET blazar sample. Strong correlations have been found between the emissions at the radio-millimeter wavelengths and they-rays. These correlations have been explained in the frame work of Synchrotron Self-Compton (SSC) processes between the radio-millimeter synchrotron emissions and the shock-accelerated electrons in the jets of blazars. Using this SSC model we are able to explain the GeV spectra of the EGRET blazars. The present model will also explain the physics of TeV flares in blazars and the time lag between the TeV and GeV photons from a blazar.

Ghosh, K. K.↗
NASA NTRS1999

Hard X-Ray Detection of the High Redshift Quasar 4C 71.07

BATSE/OSSE observations of the high redshift quasar 4C 71.07 indicate that this is the brightest and furthest AGN so far detected 20 keV. BATSE Earth occultation data have been used to search for emission from 4C 71.07 from nearly 3 years of observation. The mean source flux over the- whole period in the BATSE energy range 20-100 keV is (13.2 +/- 1.06) x 10(exp -11) erg/square cm/s corresponding to a luminosity of 2 x 10(exp 48 erg/s. The BATSE light curve over the 3 years of observations shows several flare-like events, one of which (in January 1996) is associated with an optical flare (R=16.1) but with a delay of 55 days. The OSSE/BATSE spectral analysis indicates that the source is characterized by a flat power spectrum (Gamma is approximately 1.1- 1.3) when in a low state: this spectral form is consistent within errors with the ASCA and ROSAT spectra. This means that the power law observed from 0.1 to 10 keV extends up to at least 1 MeV but steepens soon after to meet EGRET high energy data. BATSE data taken around the January 1996 flare suggests that the spectrum could be steeper when the source is in a bright state. The upsilon-F-upsilon representation of the source is typical of a low frequency peaked/ gamma- ray dominated blazar, with the synchrotron peak in the mm-FIR band and the Compton peak in the MeV band. The BATSE and OSSE spectral data seem to favour a model in which the high energy - flux is due to the sum of the synchrotron self-Compton and the external Compton contributions: this is also supported by the- variability behaviour of the source.

Malizia, A.↗
NASA NTRS1999

Probing Blazar Structure Through Multiwavelength Variability

This was a project to understand the multiwavelength variability of blazars, via monitoring with the XTE satellite and many other space- and ground-based observatories. Blazars are dominated by strong nonthermal emission, we now know, due to relativistic beaming in an aligned jet. Thus they are ideal laboratories for studying jet physics. The physical scales of the inner, energetically dominant jets are generally too small to resolve with current or next-generation instruments, so we used variability to probe the balance between acceleration/injection of energetic particles and radiative/other losses, and to infer jet structure. Our work has shown that the radio through UV-X-ray emission is produced via synchrotron radiation, and that at the peak frequency and above, the emission region is approximately homogeneous. A separate gamma-ray component, which is often energetically dominant, varies in a correlated way but with larger amplitude, suggesting it is produced when the synchrotron-emitting electrons Compton scatter soft photons (though other possibilities remain viable). With RXTE we carried out two intensive monitoring campaigns on the blazar PKS2155-304, in May 1996 and November 1996. Analysis and interpretation of these data constituted the main activities under this grant. The results have been presented at several meetings, and a number of publications have resulted from this work. In addition, a final paper for a refereed publication is in final draft form, as are companion papers based on simultaneous Rosat and EUVE results.

Urry, C. Megan↗
NASA NTRS1998

Origin of Gamma-Ray Emissions from the MeV Blazars

More than sixty gamma-ray emitting blazars have been detected by the Compton Gamma Ray Observatory (CGRO). A sub-class of these blazars is known as MeV blazars that are most luminous at the MeV energies. Spectra of these sources show a break somewhere between 1 MeV and 30 MeV with break amplitude as large as Delta(Gamma) = 1.5. Different models like Compton cooling jet models, synchrotron self-Compton models, etc., have been suggested to explain the observed properties of the MeV luminous blazars. However these models are unable to account for spectral break amplitudes in excess of 0.5. Here we suggest an inverse-Compton model based on recent published experimental data where MeV gamma rays were produced via Compton backscattering of UV photons with 500 MeV electrons. We show that the high energy electrons of the relativistic jet can collide with the copious ultraviolet photons of the big blue bump of the blazar, and produce photons peaked in the MeV region. Also we show that the X rays and the soft gamma rays in these sources are produced through the synchrotron self-Compton process. Flux computed from our model have been used to compare with the observed X-ray/ gamma-ray spectra of the MeV luminous blazars. Details of the model are discussed in the present paper.

Ghosh, K. K.↗
NASA NTRS1999

Optical Variability of BL Lacertae During the Major Outburst of 1997

We have undertaken an investigation of recent flux variability in BL Lac. We present optical observations taken over 12 nights documenting major as well as minor outbursts. This has been combined, for purposes of multifrequency analysis, with published X-ray and gamma ray data taken for an additional single night. On two nights in particular, which includes the night of the X-ray observations, a major outburst of about a full magnitude of variation was recorded. All the data have been analyzed with theoretical models. Attempts were made to use synchrotron self-Compton and external Comptonization models to explain the data; however both classes of models were found lacking. More satisfactory results were obtained using an analytical model proposed by Wang et al. that involves the evolution of synchrotron spectra in a homogeneous jet due to the injection of relativistic electrons, taking into account radiation losses during the outbursts. It is hoped that the results of this study of BL Lac, an archetype for the class of blazars in general, represent a more generic phenomenon applicable to the entire class.

Ghosh, K. K.↗
NASA NTRS1994

Nonthermal Radio Emission from Hot Star Winds: Its Origin and Physical Implications

Nonthermal radio emission has been observed from some of the most luminous hot star winds. It is understood to be synchrotron radiation of the relativistic electrons in the winds. To understand how the electrons are accelerated to such high energies and to correctly explain the observed radio flux and spectra require an exhaustive investigation of all the relevant physical processes involved and possibly point to a complex wind structure. In this paper we discuss the logical path toward a comprehensive model of the nonthermal radio emission from hot star winds. Based on the available observational data and fundamental theoretical considerations, we found that the only physically viable and self-consistent scenario is: the nonthermal radio emission is synchrotron radiation of relativistic electrons (left right harpoon) the electrons are accelerated by shocks via the first-order Fermi mechanism (left right harpoon) the acceleration has to be in situ in the radio emitting region (left right harpoon) the shocks formed at the base of the winds have to propagate to beyond the radio photosphere.

Chen, Wan↗
NASA NTRS2000

Structure of the X-Ray Emission from the Jet of 3C 273

We present images from four Chandra observations of the quasar 3C 273. The zeroth order images from two grating observations using the AXAF CCD Imaging Spectrometer (ACIS-S) detector are used to examine the structure and spectrum of the jet. The jet has at least four distinct features which are not resolved in previous observations. Using jet feature nomenclature based on HST observations, we find that knot Al is very bright in X-rays. We have measured the X-ray spectrum of this X-ray knot for the first time, obtaining a photon index of 1.36 +/- 0.11 and a flux density of 37 +/- 4 nJy at 1 keV. Combining this measurement with lower frequency data shows that a pure synchrotron model can fit the spectrum of knot Al from 4 GHz to 5 keV (over nine decades in energy) without a change of spectral slope. Knot A2 is also detected and is somewhat blended with knot B1 but synchrotron emission is not likely to explain the X-ray emission due to the spectral turnover observed in the optical-UV band. No other knots are clearly detected but there is an indication of weak emission from the eastern portion of knot H3. near the "head," which is radio-bright. There is diffuse flux which extends from 14 arcsec to 20 arcsec which shows curvature that is comparable to the optical flux found by Bahcall, et al.

Marshall, H. L.↗
NASA NTRS2000

Constraints on Particles and Fields from X-Ray Data

We review emission processes for the handful of extragalactic radio sources which produce X-rays from hotspots and knots in radio jets. In almost all cases, the emitting feature shows a large gradient in radio brightness, indicative of the presence of a strong shock. Since both thermal and synchrotron self Compton models face severe difficulties (except for the hotspots of Cygnus A and 3C 295), the most likely explanation is the presence of gamma = 10(exp 7) electrons producing X-ray synchrotron emission. The major problems remaining are why so few features have been detected so far and why has no optical emission been detected in some cases. While a somewhat unique situation exists for 3C 390.3, serious problems remain for Pictor A, 3C 120, and the quasar jets in 3C 273 and PKS 0637.

Harris, D. E.↗
NASA NTRS2000

Results of a Deep Chandra Observation of the Crab Nebula and Pulsar

The Crab Nebula and pulsar were observed for a total of 150 ksec with the LETG/HRC-S combination aboard the Chandra X-Ray Observatory in 2000, January. One of the principal aims of the experiment was to study the emission of from the pulsar as a function of pulse phase. Neutron stars are believed to be formed with core temperatures of 10(exp 11). As the pulsar is the youngest known neutron star with an age of only 940 yrs, it should be possible to observe thermal emission from the hot stellar surface which in turn constrains equations of state. The pulsar, on the other hand, is a powerful non-thermal emitter, powering an X-ray bright synchrotron nebula which, in Einstein and ROSAT observations, overshadowed the fainter thermal surface emission. Making use of the high angular resolution provided by Chandra we were able to detect X-rays from the Crab-pulsar at all pulse phases. We discuss whether this detection is indeed of thermal emission or of a faint synchrotron component of the pulsed emission from the magnetosphere. We further report on dynamical effects observed in the pulsar-wind outflow and the analysis of the LETG spectral data, especially near the oxygen edge. The results of the spectral analysis has interesting implications for the composition of the interstellar medium.

Weisskopf, M. C.↗
NASA NTRS2001

Results of a Deep Chandra Observation of the Crab Nebula and Pulsar

The Crab Nebula and pulsar were observed for a total of 150 ksec with the LETG/HRC-S combination aboard the Chandra X-Ray Observatory in 2000, January and February. One of the principal aims of the experiment was to study the emission from the pulsar as a function of pulse phase. Neutron stars are believed to be formed with core temperatures of 10(exp 11) K. As the pulsar is the best studied of the young known neutron stars with an age of only 940 yrs, it should be possible to observe thermal emission from the hot stellar surface which in turn constrains equations of state. The pulsar, on the other hand, is a powerful non-thermal emitter, powering an X-ray bright synchrotron nebula which, in Einstein and ROSAT observations, overshadowed the fainter thermal surface emission. Making use of the high angular resolution provided by Chandra we were able to detect X-rays from the Crab-pulsar at all pulse phases. We discuss whether this detection is indeed of thermal emission or of a faint synchrotron component of the pulsed emission from the magnetosphere. We further comment on dynamical effects observed in the pulsar-wind outflow and the analysis of the LETG spectral data, especially near the oxygen edge.

Weisskopf, M. C.↗
NASA NTRS2000

Optical Variability of BL Lacertae During the Major Outburst of 1997

We have undertaken an investigation of recent flux variability in BL Lac. We present optical observations taken over 22 nights documenting major as well as minor outbursts. This has been combined, for purposes of multifrequency analysis, with published X-ray and T-ray data taken for an additional single night, On two nights in particular, including the night of the X-ray observations, a major outburst of about a full magnitude of variation was recorded. All the data have been analyzed with theoretical models. Attempts were made to use synchrotron self-Compton and external Comptonization models to explain the data; however, both classes of models were found lacking. More satisfactory results were obtained using an analytical model proposed by Wang et al. that involves the evolution of synchrotron spectra in a homogeneous jet due to the injection of relativistic electrons, taking into account radiation losses during the outbursts. It is hoped that the results of this study of BL Lac, an archetype for the class of blazars in general, represent a more generic phenomenon applicable to the entire class.

Ghosh, K. K.↗
NASA NTRS1998

A Viewing Angle-Kinetic Luminosity Scheme for BL Lacertae Objects

We propose a unified classification for BL Lacertae objects (BLs), focusing on the synchrotron peak frequency of the spectral energy distribution. The unification scheme is based on the angle theta that describes the orientation of the relativistic jet and on the electron kinetic luminosity Lamba(sub kin) of the jet. We assume that Lamba(sub kin) scales with the size of the jet (r) in a self-similar fashion (Lamba(sub kin) is proportional to r(exp 2)), as supported by observational data. The jets are self-similar in geometry and have the same pressure and median magnetic field at the inlet, independent of size. The self-similarity is broken for the highest energy electrons, which radiate mainly at high frequencies, since for large sources they suffer more severe radiative energy losses over a given fraction of the jet length. We calculate the optically thin synchrotron spectrum using an accelerating inner jet model based on simple relativistic gas dynamics and show that it can fit the observed infrared-to-X-ray spectrum of PKS 2155-304. We couple the accelerating jet model to the unification scheme and compare the results to complete samples of BLs. The negative apparent evolution of X-ray-selected BLs is explained as a result of positive evolution of the jet electron kinetic luminosity Lamba(sub kin). We review observational arguments in favor of the existence of scaled-down accretion disks and broad emission-line regions in BLs. The proposed unification scheme can explain the lack of observed broad emission lines in X-ray-selected BLs as well as the existence of those lines preferentially in luminous radio-selected BLs. Finally, we review observational arguments that suggest the extension of this unification scheme to all blazars.

Georganopoulos, Markos↗
NASA NTRS2001

RXTE, VLBA, Optical, and Radio Monitoring of the Quasars 3C 279, PKS 1510--089, and 3C 273

We are continuing our combined RXTE X-ray, VLBA imaging (at 43 GHz), optical (several observatories), and radio (University of Michigan Radio Astronomy Observatory) monitoring of the quasars 3C 279 and PKS 1510-089, and have started similar monitoring of 3C 273. X-ray flares in 3C 279 and PKS 1510-089 are associated with ejections of superluminal components. In addition, there is a close connection between the optical and X-ray variability of 3C 279. There is a strong correlation between the 14.5 GHz and X-ray variability of PKS 1510-089 in 1997 and 1998 (with the radio leading the X-ray) that becomes weaker in subsequent years. X-ray fluctuations occur on a variety of timescales in 3C 273, with a major prolonged outburst in mid-2001. The lead author will discuss the correlations in terms of inverse Compton models for the X-ray emission coupled with synchrotron models for the lower-frequency radiation. Synchrotron self-Compton models can explain the "reverse" time lag in PKS 1510-089 is well as the variable correlation between the X-ray variations and those at lower frequencies in this object and in 3C 279.

Marscher, A. P.↗
NASA NTRS2002

Disentangling X-Ray Emission Processes In Vela-Like Pulsars

This grant is to support analysis of data from the X-ray Multi-mirror Mission (XMM). Specifically, we have been awarded time to observe two young neutron stars, B1823-13 and B1046-58, whose X-ray emission is expected to be a complicated combination of emission from an associated supernova remnant, from a wind-powered synchrotron nebula, from magnetospheric pulsations, and from the surface of the neutron star itself. It is only with XMM's unique combination of spectral, temporal and angular resolution that all these different processes can be separated and studied. Observations of B1823-13 have been conducted and analyzed. We interpret the data as follows: The unpulsed extended non-thermal nature of the central core argues that the extended source of emission corresponds to synchrotron emission from a nebula powered by the pulsar. The temperature of the diffuse component is too high to be interpreted as thermal emission; we rather argue that this extended component is non-thermal emission from a surrounding supernova remnant shell.

Gaensler, Bryan↗
NASA NTRS2002

X-Ray Spectral Variability Signatures of Flares in BL Lac Objects

We are presenting a detailed parameter study of the time-dependent electron injection and kinematics and the self-consistent radiation transport in jets of intermediate and low-frequency peaked BL Lac objects. Using a time-dependent, combined synchrotron-self-Compton and external-Compton jet model, we study the influence of variations of several essential model parameters, such as the electron injection compactness, the relative contribution of synchrotron to external soft photons to the soft photon compactness, the electron- injection spectral index, and the details of the time profiles of the electron injection episodes giving rise to flaring activity. In the analysis of our results, we focus on the expected X-ray spectral variability signatures in a region of parameter space particularly well suited to reproduce the broadband spectral energy distributions of intermediate and low-frequency peaked BL Lac objects. We demonstrate that SSC- and external-Compton dominated models for the gamma-ray emission from blazars are producing significantly different signatures in the X-ray variability, in particular in the soft X-ray light curves and the spectral hysteresis at soft X-ray energies, which can be used as a powerful diagnostic to unveil the nature of the high-energy emission from BL Lac objects.

Boettcher, Markus↗
NASA NTRS1999

Maximum Energies of Shock-Accelerated Electrons in Young Shell Supernova Remnants

Young supernova remnants (SNRs) are often assumed to be the source of cosmic rays up to energies approaching the slight steepening in the cosmic ray spectrum at around 1000 TeV, known as the "knee." We show that the observed X-ray emission of 14 radio-bright shell remnants, including all five historical shells, can be used to put limits on E(sub max), the energy at which the electron energy distribution must steepen from its slope at radio-emitting energies. Most of the remnants show thermal spectra, so any synchrotron component must fall below the observed X-ray fluxes. We obtain upper limits on E(sub max) by considering the most rapid physically plausible cutoff in the relativistic electron distribution, an exponential, which is as sharp or sharper than found in any more elaborate models. This maximally curved model then gives us the highest possible E(sub max) consistent with not exceeding observed X-rays. Our results are thus independent of particular models for the electron spectrum in SNRs. Assuming homogeneous emitting volumes with a constant magnetic field strength of 10 uG, no object could reach 1000 TeV, and only one, Kes 73, has an upper limit on E(sub max), above 100 TeV. All the other remnants have limits at or below 80 TeV. E(sub max) is probably set by the finite remnant lifetime rather than by synchrotron losses for remnants younger than a few thousand years, so that an observed electron steepening should be accompanied by steepening at the same energy for protons. More complicated, inhomogeneous models could allow higher values of E(sub max) in parts of the remnant, but the emission-weighted average value, that characteristic of typical electrons, should obey these limits. The young remnants are not expected to improve much over their remaining lives at producing the highest energy Galactic cosmic rays; if they cannot, this picture of cosmic-ray origin may need major alteration.

Reynolds, Stephen P.↗