Radio-quiet Sources Research Articles

The Compact Central Object in the RX J0852.0−4622 Supernova Remnant

The central region of the recently discovered supernova remnant RX J0852.0-4622 was observed with the ACIS detector aboard the Chandra X-ray Observatory. We found only one relatively bright source, about 4' north of the SNR center, with a flux of $\sim 2\times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$ in the 0.5--10 keV band. The position of this point-like source, CXOU J085201.4-461753, rules out its association with the two bright stars in the field, HD 76060 and Wray 16-30. Observations of the field with the CTIO 0.9-m telescope show a star ($R\approx 17$, $B\approx 19$) at about 2\farcs4 from the nominal X-ray position. We consider association of this star with the X-ray source unlikely and estimate a limiting magnitude of the optical counterpart as $B \ge 22.5$ and $R \ge 21.0$. Based on the X-ray-to-optical flux ratio, we argue that the X-ray source is likely the compact remnant of the supernova explosion that created the RX J0852.0-4622 SNR. The observed X-ray spectrum of the source is softer than spectra of magnetospheric radiation of rotation-powered pulsars, but it is harder than spectra of cooling neutron stars emitting thermal radiation from the entire surface, similar to the central compact source of the Cas A SNR. We suggest that CXOU J085201.4-461753 belongs to the growing family of radio-quiet compact central sources, presumably neutron stars, recently discovered in a number of SNRs.

Searching for the Physical Drivers of the Eigenvector 1 Correlation Space

[Latex markup removed] We recently discussed an Eigenvector 1 (E1) parameter space that provides optimal discrimination between the principal classes of broad-line Active Galactic Nuclei (AGN). In this paper we begin a search for the most important physical parameters that are likely to govern correlations and data point distribution in E1 space. We focus on the principal optical parameter plane involving the width of the H-beta broad component and the equivalent width ratio between the FeII blend at 4570 A and H-beta broad component. We show that the observed correlation for radio-quiet sources can be accounted for if it is primarily driven by the ratio of AGN luminosity to black hole mass (L/M proportional to Eddington ratio) convolved with source orientation. L/M apparently drives the radio-quiet correlation only for FWHM(H-beta) < 4000 km/s which includes Narrow Line Seyfert 1 (NLSy1) galaxies and can be said to define an AGN ``main sequence.'' Source orientation plays an increasingly important role as FWHM(H-beta) increases. We also argue that AGN lying outside the radio-quiet ``main sequence,'' and specifically those with optical FeII much stronger than expected for a given FWHM(H-beta), may all be Broad Absorption Line QSOs.

Off-Beam Gamma-Ray Pulsars and Unidentified EGRET Sources in the Gould Belt

We investigate whether gamma-ray pulsars viewed at a large angle to the neutron star magnetic pole could contribute to the new population of Galactic unidentified EGRET sources associated with the Gould Belt. The faint, soft nature of these sources is distinctly different from both the properties of unidentified EGRET sources along the Galactic plane and the properties of the known gamma-ray pulsars. We explore the possibility, within the polar cap model, that some of these sources are emission from pulsars seen at lines of sight that miss both the bright gamma-ray cone beams and the radio beam. The off-beam gamma rays come from high-altitude curvature emission of primary particles, are radiated over a large solid angle and have a much softer spectrum than that of the main beams. We estimate that the relative detectability of such off-beam emission is about a factor of 4-5 higher than that of the on-beam emission. At least some of the radio-quiet Gould Belt sources detected by EGRET could therefore be such off-beam gamma-ray pulsars. The Gamma-ray Large Area Space Telescope should be able to detect pulsations in most of these sources.

Photon Splitting and Pair Creation in Highly Magnetized Pulsars

The absence of radio pulsars with long periods has lead to the popular notion of a high P ``death line.'' In the standard picture, beyond this boundary, pulsars with low spin rates cannot accelerate particles above the stellar surface to high enough energies to initiate pair cascades, and the pair creation needed for radio emission is strongly suppressed. In this paper we explore the possibility of another pulsar ``death line'' in the context of polar cap models, corresponding to high magnetic fields B in the upper portion of the period-period derivative diagram, a domain where few radio pulsars are observed. The origin of this high B boundary, which may occur when B becomes comparable to or exceeds $B_{\rm cr} = 4.4 \times 10^{13}$ Gauss, is also due to the suppression of magnetic pair creation, but primarily because of ineffective competition with magnetic photon splitting. Threshold pair creation also plays a prominent role in the suppression of cascades. We present Monte Carlo calculations of the pair yields in photon splitting/pair cascades which show that, in the absence of scattering effects, pair production is effectively suppressed, but only if all three modes of photon splitting allowed by QED are operating in high fields. This paper describes the probable shape and position of the new ``death line,'' above which pulsars are expected to be radio quiet, but perhaps still X-ray and gamma-ray bright. The hypothesized existence of radio-quiet sources finds dramatic support in the recent discovery of ultra-strong fields in Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars. Guidelines for moderate to high B pulsar searches at radio wavelengths and also in the soft and hard gamma-ray bands are presented.

The Central X‐Ray Point Source in Cassiopeia A

The spectacular ii —rst light ˇˇ observation by the Chandra X-Ray Observatory revealed an X-ray point source near the center of the 300 yr old Cas A supernova remnant. We present an analysis of the public X-ray spectral and timing data. No coherent pulsations were detected in the Chandra/HRC data. The 3 p upper limit on the pulsed fraction is less than 35% for P ( 20 ms. The Chandra/ACIS spectrum of the point source may be —tted with an ideal blackbody (kT 0.5 keV) or with blackbody models modi—ed by the presence of a neutron star atmosphere (kT 0.25¨0.35 keV), but the temperature is higher and the inferred emitting area lower than expected for a 300 yr old neutron star according to standard cooling models. The spectrum may also be —tted with a power-law model (photon index ! 2.8¨3.6). Both the spectral properties and the timing limits of the point source are inconsistent with a young Crab-like pulsar but are quite similar to the properties of the anomalous X-ray pulsars. The spectral parameters are also very similar to those of the other radio-quiet X-ray point sources in the supernova remnants Pup A, RCW 103, and PKS 1209(52. Current limits on an optical counterpart for the Cas A point source rule out models that invoke fallback accretion onto a compact object if fallback disk properties are similar to those in quiescent low-mass X-ray binaries. However, the optical limits are mar- ginally consistent with plausible alternative assumptions for a fallback disk. In this case, accreting neutron star models can explain the X-ray data, but an accreting black hole model is not promising. Subject headings: accretion, accretion disksblack hole physicsstars: neutron ¨ supernovae: individual (Cassiopeia A) ¨ supernova remnantsX-rays: stars

Phenomenology of Broad Emission Lines in Active Galactic Nuclei

▪ Abstract Broad emission lines hold fundamental clues about the kinematics and structure of the central regions in AGN. In this article we review the most robust line profile properties and correlations emerging from the best data available. We identify fundamental differences between the profiles of radio-quiet and radio-loud sources as well as differences between the high- and low-ionization lines, especially in the radio-quiet majority of AGN. An Eigenvector 1 correlation space involving FWHM Hβ, W(FeIIopt)/W(Hβ), and the soft X-ray spectral index provides optimal discrimination between all principal AGN types (from narrow-line Seyfert 1 to radio galaxies). Both optical and radio continuum luminosities appear to be uncorrelated with the E1 parameters. We identify two populations of radio-quiet AGN: Population A sources (with FWHM(Hβ) [Formula: see text] 4000 km s−1, generally strong FeII emission and a soft X-ray excess) show almost no parameter space overlap with radio-loud sources. Population B shows optical properties largely indistinguishable from radio-loud sources, including usually weak FeII emission, FWHM(Hβ) [Formula: see text] 4000 km s−1and lack of a soft X-ray excess. There is growing evidence that a fundamental parameter underlying Eigenvector 1 may be the luminosity-to-mass ratio of the active nucleus (L/M), with source orientation playing a concomitant role.

Some Recent Results on MHD Shocks and Winds

Work on the rotation of the magnetic field around the shock propagation direction in an oblique shock in a dusty medium is reviewed. A significant observational consequence is that the rotation lowers the minimum speed of a shock in which sputtering of grains is important. The effects of mass-loading and magnetic fields on the strengths of termination shocks of winds are considered. Such effects may be of relevance to the production of radio-quiet sources and are certainly important in the establishment of the ratio of the intensities of softer and harder radiation emitted by thermal processes in the winds.

Infrared Planetary Nebulae in the NRAO VLA Sky Survey

In order to construct a sample of planetary nebulae (PNe) unbiased by dust extinction, we first selected the 1358 sources in the IRAS Point Source Catalog north of J2000 declination δ = -40° having measured S(25 μm) ≥ 1 Jy and colors characteristic of PNe: detections or upper limits consistent with both S(12 μm) ≤ 0.35S(25 μm) and S(25 μm) ≥ 0.35S(60 μm). The majority are radio-quiet contaminating sources such as asymptotic giant branch stars. Free-free emission from genuine PNe should make them radio sources. The 1.4 GHz NRAO VLA Sky Survey (NVSS) images and source catalog were used to reject radio-quiet mid-infrared sources. We identified 454 IRAS sources with radio sources brighter than S ≈ 2.5 mJy beam-1 (equivalent to T ≈ 0.8 K in the 45'' FHWM NVSS beam) by positional coincidence. They comprise 332 known PNe in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae and 122 candidate PNe, most of which lie at very low Galactic latitudes. Exploratory optical spectroscopic observations suggest that most of these candidates are indeed PNe optically dimmed by dust extinction, although some contamination remains from H II regions, Seyfert galaxies, etc. Furthermore, the NVSS failed to detect only 4% of the known PNe in our infrared sample. Thus it appears that radio selection can greatly improve the reliability of PN candidate samples without sacrificingcompleteness.

Broad Emission Lines in AGN: Phenomenology and Models

We review the phenomenology of broad emission lines in AGN. We show that velocity displacements relative to the local rest frame of Hβ and CIVλ1549 are real. The most significant line displacement result involves a systematic blueshift for CIV, seen only in radio-quiet sources. We find some evidence that the amplitude of this displacement may correlate with source orientation. We show that disagreement between different studies of the CIV line properties is due to whether or not a narrow line component was subtracted. Finally, we consider evidence that optical and X-ray broad lines arise in an accretion disk. We show that the evidence for a disk origin is far from overwhelming for both Hβ and FeKα.

K[CLC]es[/CLC] 73: A Young Supernova Remnant with an X-Ray–bright, Radio-quiet Central Source

We clarify the nature of the small-diameter supernova remnant (SNR) Kes 73 and its central compact source, 1E 1841-045, using X-ray data acquired with ASCA. We introduce a spatiospectral decomposition technique necessary to disentangle the ASCA spectrum of the compact source from the barely resolved shell-type remnant. The source spectrum (1-8 keV) is characterized by an absorbed power law with a photon index α 3.4 and NH 3.0 × 1022 cm-2, possibly nonthermal in nature. This bright X-ray source is likely a slowly spinning pulsar, whose detection is reported in our companion paper. The SNR spectrum is characteristic of a thermal plasma, with kT 0.6 keV and emission lines typical of a young remnant. The element Mg and possibly O and Ne are found to be overabundant, qualitatively suggesting an origin from a massive progenitor. We find that Kes 73 is a young (2000 yr) Type II/Ib SNR containing a neutron star pulsar spinning anomalously slowly for its age. Kes 73 is yet another member of a growing class of SNRs containing radio-quiet compact sources with a hard spectral signature.

Constraints on the Dynamics of Electron-Positron Jets in Active Galactic Nuclei

view Abstract Citations (9) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Constraints on the Dynamics of Electron-Positron Jets in Active Galactic Nuclei Levinson, Amir Abstract The bulk acceleration of electron-positron jets in the presence of background radiation is examined, taking account of the conversion of electromagnetic energy into pairs and X-rays through the interaction of the cold e^+/-^ beam with the ambient radiation. It is shown that in order to avoid catastrophic radiative drag, the fraction of total jet power carried by pairs must not exceed the ratio of radiative cooling time of the outflowing pairs by soft photons to dynamical time. This requirement is automatically satisfied, owing to rapid pair annihilation, if the total jet power is not much smaller than the fraction of UV/soft X-ray luminosity intercepted by the inner jet, as measured in the comoving frame. It is found that the flow undergoes a sharp transition whereby the Poynting flux is converted into pairs and beamed X-rays, when the bulk Lorentz factor exceeds the corresponding threshold energy above which the pair-production opacity becomes larger than unity. When the scattered spectrum is sufficiently flat, such that the cooling rate is dominated by hard photons from near its upper cutoff, the Poynting flux is converted mostly into beamed pairs. However, if the scattered spectrum is steep at low energies, like those observed in radio-quiet sources, there is catastrophic X-ray production. The implications for jets from blazars are considered, and several ways in which X-ray overproduction may he avoided are discussed. Publication: The Astrophysical Journal Pub Date: August 1996 DOI: 10.1086/177632 Bibcode: 1996ApJ...467..546L Keywords: ELEMENTARY PARTICLES; GALAXIES: ACTIVE; GALAXIES: JETS; GALAXIES: NUCLEI; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (2)

Pair Cascades in Extragalactic Jets. II. The Beamed X-Ray Spectrum

view Abstract Citations (24) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Pair Cascades in Extragalactic Jets. II. The Beamed X-Ray Spectrum Levinson, Amir ; Blandford, Roger Abstract The X-ray through γ-ray emission from a relativistic jet is considered within the framework of an inhomogeneous pair-cascade model. It is shown that the overall spectrum from the model exhibits a spectral break at an energy of order (mec2)2/Eb, where Eb is the cutoff energy of the isotropic spectrum of scattered X-ray photons, above which the γ-rays are subject to significant absorption through pair production on the ambient soft radiation. Such a break has been observed thus far in the γ-ray spectra of several blazars; if blazars possess isotropic X-ray spectra similar to those observed in radio-quiet sources, then the relation between the break energy and the cutoff energy, predicted by our model, is roughly satisfied. It is argued that emission just below the γ-ray break originates near the base of the jet and is determined by the distribution function of cascading pairs there. Above the break, the slope of the emergent γ-ray spectrum reflects the radial variations of pair acceleration and soft photon intensity and should be steeper than the hard X-ray spectrum. For plausible particle acceleration prescriptions and soft radiation spectra, the characteristic hard X-ray spectra observed in some blazars are reproduced by our model. The X-ray cutoff energy is also related to the dynamics of the jet. It is argued that both the inertia and the radiative drag acting upon a jet increase sharply when the bulk Lorentz factor exceeds Γc ∼ (mec2/Eb) ∼ 10 and that jets will be accelerated up to this critical Lorentz factor which is of order that inferred from VLBI observations of superluminal radio sources. The utility of multiwaveband observations for advancing our understanding of blazars is stressed. Publication: The Astrophysical Journal Pub Date: August 1995 DOI: 10.1086/176034 Bibcode: 1995ApJ...449...86L Keywords: GALAXIES: JETS; GAMMA RAYS: THEORY; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (3) Related Materials (2) Part 1: 1995ApJ...441...79B Part 3: 1996ApJ...459..520L

Broad NE 8 lambda 774 emission from quasars in the HST-FOS snapshot survey (ABSNAP)

We discuss the strength and frequency of broad Ne VIII lambda 774 emission from quasars measured in the Hubble Space Telescope Faint Object Spectrograph (HST-FOS) snapshot survey (Absnap). Five sources in the survey have suitable redshifts (0.86 less than or equal to Z(sub em) less than or equal to 1.31), signal-to-noise ratios and no Lyman limit absorptions. Three of the five sources have a strong broad emission line near 774 A (rest), and the remaining two sources have a less securely measured line near this wavelength. We identify these lines with Ne VIII lambda 774 based on the measured wavelengths and theoretical estimates of various line fluxes (Hamann et al. 1995a). Secure Ne VIII detections occur in both radio-loud and radio-quiet sources. We tentatively conclude that broad Ne VIII lambda 774 emission is common in quasars, with typical strengths between approximately 25% and approximately 200% of O VI lambda 1034. These Ne VIII lambda 774 measurements imply that the broad emission line regions have a much hotter and more highly ionized component than previously recognized. They also suggest that quasar continua have substantial ionizing flux out to energies greater than 207 eV (greater than 15.2 ryd, lambda less than 60 A). Photoionization calculations using standard incident spectra indicate that the Ne VIII emission requires ionization parameters U greater than or = 5, total column densities N(sub H) greater than or = 10(sub 22)/sq cm and covering factors greater than or = 25%. The temperatures could be as high as approximately 10(exp 5) K. If the gas is instead collisionally ionized, strong Ne VIII would imply equilibrium temperatures in the range approximately 400,000 less than or approximately = T(sub e) less than or approximately = 10(exp 6) K. In either case, the highly ionized Ne VIII emission regions would appear as X-ray 'warm absorbers' if they lie along our line of sight to the X-ray continuum source.

Soft X-ray Emission of Quasars

From a list of known quasars compiled from various catalogues we selected all sources detected by the PSPC (0.1 – 2.4 keV) aboard ROSAT with more than 80 counts during the all sky survey. A sample of 102 sources resulted. At higher redshifts most of the selected sources are radio-loud. At a redshift smaller than 0.50 we found 54 radio-quiet quasars and 30 radio-loud sources. For this reduced sample the mean spectral index of the radio-quiet sources (&lt; Γ &gt; = 2.53) and that of the radio-loud ones (&lt; Γ &gt; = 2.26) are clearly different with a significance of 3.3 σ.About 2/3 of the bright quasars observed with Einstein also belong to our sample. The spectra observed with ROSAT are sytematically steeper than the ones observed with Einstein yielding a &lt; ΓROSAT – ΓEinstein &gt; of 0.66 ± 0.18 for radio quiet and of 0.68 ± 0.19 for radio-loud sources, respectively.For radio loud quasars, the mean spectral slope decreases from 2.3 to 1.5 when the redshift increases beyond 0.5 (figure 1). The fact that high redshift sources show a photon index of about 1.5, which is similar to the mean index observed with Einstein for radio-loud sources, suggests that this decrease towards higher redshifts can be interpreted by the shift of the soft X-ray excess outside of the ROSAT spectral band when the redshift increases. The solid lines in figure 1 represent theoretical pathes of the photon index as a function of the redshift as derived from simulations assuming a power law plus black body model spectrum for the quasars X-ray emission. In curve No 1 the powerlaw index is fixed to 1.4. To be compatible with the observation the temperature of the blackbody component must range between 50 and 70 eV. Curve No 2 asssumes the same model with a powerlaw index fixed to 1.8 to account for radio quiet sources.

Luminous Radio-quiet Sources in W3(Main)

We have resolved the 450-800 μm emission from the W3(Main) star forming region into three major peaks, using an 8″ beam on the James Clerk Maxwell Telescope (JCMT). One of the submillimeter sources is identified with W3-IRS 5, a well-known candidate protostar. However, to our surprise, we find that none of the submillimeter peaks coincides with any of the prominent compact H II regions in the area. We estimate that the three submillimeter sources together contribute 35%-50% of the total bolometric luminosity of the region and speculate that the contribution of luminous radio-quiet sources to the total luminosity of H II region/molecular cloud complexes may be larger than is often assumed

Prediction of the submillimeter spectrum of the cosmic background radiation by a plasma model

A non-Big-Bang plasma model is used to predict the submillimeter spectrum of the cosmic background radiation (CBR). Early intermediate-mass stars heat dust which, in turn, warms electrons trapped in force-free filaments emitted by galactic nuclei. Current stars contribute further heating by the same synchrotron mechanism. The electrons emit a nonblackbody radiation which matches the observed CBR spectrum and isotropy. The model also predicts intergalactic absorption of radio radiation, as confirmed by observations of spiral galaxies, and accurately predicts the spectrum of radio-quiet sources. >

Far-infrared observations of star-forming regions

view Abstract Citations (43) References (60) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Far-infrared observations of star-forming regions Sargent, A. I. ; van Duinen, R. J. ; Aalders, J. W. G. ; Fridlund, C. V. M. ; Nordh, H. L. Abstract Luminosities are determined, for embedded sources found in the course of mapping areas on the order of 20 x 30 arcmin in the molecular clouds associated with NGC 2071, Mon R2, ON 1, and IC 5146 at 85 and 150 microns. The energetics of each cloud are also examined, and it is found that external sources may contribute to the extended far-IR emission from these clouds. The observations indicate that radio-quiet, protostellar sources have much less effect on the observed temperature structure of star-forming clouds than do objects of comparable luminosity which have already formed compact H II regions, and that the radio-quiet sources appear to be more deeply embedded than those now associated with ionized gas. It is proposed that massive stars may form deep within molecular clouds rather than only at their surfaces. Publication: The Astrophysical Journal Pub Date: October 1981 DOI: 10.1086/159321 Bibcode: 1981ApJ...249..607S Keywords: Far Infrared Radiation; Infrared Astronomy; Interstellar Gas; Molecular Clouds; Stellar Evolution; Astronomical Maps; Brightness Distribution; Gas Ionization; H Ii Regions; Main Sequence Stars; Radiant Flux Density; Stellar Luminosity; Stellar Mass Accretion; Stellar Temperature; Astronomy full text sources ADS |