Nonthermal Continuum Research Articles

Jet and accretion power in the most powerfulFermiblazars

Among the blazars detected by the Fermi satellite, we have selected the 23 blazars that in the three months of survey had an average gamma-ray luminosity above 1e48 erg/s. For 17 out of the 23 sources we found and analysed X-ray and optical-UV data taken by the Swift satellite. With these data, implemented by archival and not simultaneous data, we construct the spectral energy distributions, and interpreted them with a simple one-zone, leptonic, synchrotron and inverse Compton model. When possible, we also compare different high energy states of single sources, like 0528+134 and 3C 454.3, for which multiple good sets of multi-wavelength data are available. In our powerful blazars the high energy emission always dominates the electromagnetic output, and the relatively low level of the synchrotron radiation often does not hide the accretion disk emission. We can then constrain the black hole mass and the disk luminosity. Both are large (i.e. masses equal or greater than 1e9 solar masses and disk luminosities above 0.1 Eddington). By modelling the non-thermal continuum we derive the power that the jet carries in the form of bulk motion of particles and fields. On average, the jet power is found to be slightly larger than the disk luminosity, and proportional to the mass accretion rate.

THE FAR-INFRARED-RADIO CORRELATION AT HIGH REDSHIFTS: PHYSICAL CONSIDERATIONS AND PROSPECTS FOR THE SQUARE KILOMETER ARRAY

(Abridged) I present a predictive analysis for the behavior of the FIR--radio correlation as a function of redshift in light of the deep radio continuum surveys which may become possible using the SKA. To keep a fixed ratio between the FIR and predominantly non-thermal radio continuum emission of a normal star-forming galaxy requires a nearly constant ratio between galaxy magnetic field and radiation field energy densities. While the additional term of IC losses off of the cosmic microwave background (CMB) is negligible in the local Universe, the rapid increase in the strength of the CMB energy density (i.e. $\sim(1+z)^{4})$ suggests that evolution in the FIR-radio correlation should occur with infrared (IR; $8-1000 \micron$)/radio ratios increasing with redshift. At present, observations do not show such a trend with redshift; $z\sim6$ radio-quiet QSOs appear to lie on the local FIR-radio correlation while a sample of $z\sim4.4$ and $z\sim2.2$ SMGs exhibit ratios that are a factor of $\sim$2.5 {\it below} the canonical value. I also derive a 5$\sigma$ point-source sensitivity goal of $\approx$20 nJy (i.e. $\sigma_{\rm RMS} \sim 4$ nJy) requiring that the SKA specified be $A_{\rm eff}/T_{\rm sys}\approx 15000$ m$^{2}$ K$^{-1}$; achieving this sensitivity should enable the detection of galaxies forming stars at a rate of $\ga25 M_{\sun} {\rm yr}^{-1}$, at all redshifts if present. By taking advantage of the fact that the non-thermal component of a galaxy's radio continuum emission will be quickly suppressed by IC losses off of the CMB, leaving only the thermal (free-free) component, I argue that deep radio continuum surveys at frequencies $\ga$10 GHz may prove to be the best probe for characterizing the high-$z$ star formation history of the Universe unbiased by dust.

Modulation of NTC frequencies by Pc5 ULF pulsations: Experimental test of the generation mechanism and magnetoseismology of the emitting surface

Nonthermal continuum (NTC) radiation is believed to be emitted by the conversion of an electrostatic wave into an electromagnetic one, which takes place at the Earth's magnetic equator. It is generally accepted that the frequency of the electrostatic wave at the source meets a local characteristic frequency placed in between two multiples of the electron cyclotron frequency, fce, which results in emission of a narrow band frequency element. In an event on 14 August 2003, we compare oscillations of the central frequency of distinct NTC frequency elements observed from Cluster orbiting near perigee, with simultaneous Pc5 Ultra Low Frequency (ULF) pulsations in the magnetic field observed from the same platform. The latter magnetic perturbations are interpreted as magnetohydrodynamic poloidal waves, where fundamental and second harmonic modes coexist. The NTC oscillation and the fundamental wave have similar periods, but are phase shifted by a quarter of period. From the correlation between both signals, and the proximity of the NTC source (localized via triangulation) with Cluster, we infer that the poloidal perturbations are spatially uniform between the source and the satellites. From the phase shift between signals, we conclude that the electrostatic wave which converts into NTC is mainly governed by the plasma density, affected by movements of the magnetic field lines. Furthermore, we demonstrate that the observations can be used to perform a magnetoseismology of the emitting surface. The results show a steepening of the plasmapause density profile near the satellites, which can be responsible for the generation of NTC emission.

An X-Ray Face-On View of the Sagittarius B Molecular Clouds Observed with Suzaku

Abstract We present a new methodology to derive the positions of the Sagittarius (Sgr) B molecular clouds (MCs) along the line of sight, as an application study of the Galactic center diffuse X-rays (GCDX). The GCDX are composed of hot plasma emission of about 7 keV and 1 keV temperatures, and non-thermal continuum emission including the 6.4 keV line from neutral irons. The former, the Galactic center plasma emission (GCPE), is uniformly distributed over 1$^\circ$ in longitude, while the latter is clumpy emission produced by Thomson scattering and fluorescence from MCs irradiated by external X-rays (the X-ray reflection nebula emission: XRNE). We examined the Suzaku X-ray spectra of the GCPE and XRNE near to the Sgr B MC complex, and found that the spectra suffer from two different absorptions of $N_{\rm H}$ (Abs1) $\geq$10$^{23}$H cm$^{-2}$ and $N_{\rm H}$ (Abs2) $\simeq$6 $\times$ 10$^{22}$H cm$^{-2}$. Abs1 is proportional to the 6.4 keV-line flux, and hence is due to the MCs, while Abs2 is typical of interstellar absorption toward the Galactic center. Assuming that the GCPE plasma is spherically-extended around Sgr A$^\ast$ with a uniform density and the same angular distribution of the two temperature components, we quantitatively estimated the line-of-sight positions of the MCs from the flux ratio the GCPE spectrum suffered by Abs1 and that with no Abs1. The results suggest that the Sgr B MCs are located at the near side of Sgr A$^\ast$ in the GCPE.

Near-Infrared Spectral Energy Distributions of Seyfert Galaxies: Stellar Population, Active Nucleus, and Hot Dust

We investigate the NIR spectra of 24 Seyfert galaxies observed with the instrument SpeX at the NASA Infrared Telescope Facility (ITRF) in the short cross-dispersed mode. The results of the spectral synthesis fitting procedure are presented and discussed in details by Riffel et al. (2009). The approach followed here is based on the starlight code (see Cid Fernandes et al. 2005). The spectral synthesis shows that the NIR continuum of active galaxies can be explained in terms of at least three components: a non-thermal continuum, dust emission, and the stellar population of the circumnuclear region. The study of the stellar population is a critical step in the analysis of the continuum emission of Seyfert galaxies. Moreover, our results are consistent with the predictions of the unified model for AGNs, as the non-thermal continuum and the hot dust emission are present in all Sy 1 sources and only in a small fraction of the Sy 2s. Regarding the stellar population component, our results point to a mean metallicity solar to above solar, if we consider the light-weighted values, while for the mass-weighted mean metallicity our results indicate a sub-solar value. We associate this discrepancy with the well known age–metallicity degeneracy: i.e., for a fixed mass, a high-metallicity stellar population looks cooler — and older — than a low-metallicity population, thus resulting in a higher M/L ratio. Moreover, this is consistent with a galaxy chemical enrichment scenario in which the young population is enriched by the evolution of the early massive stars. In this context, the light-weighted metallicity is more sensitive to the young component, while the mass-weighted metallicity to the old stellar population.

Measurements of stray antenna capacitance in the STEREO/WAVES instrument: Comparison of the radio frequency voltage spectrum with models of the galactic nonthermal continuum spectrum

The STEREO/WAVES instrument is designed to measure interplanetary radio emission and in situ plasma waves in the solar wind. The instrument uses three orthogonal monopole electric antennas as its sensor system in both a pseudodipole and monopole mode. At high radio frequencies, the capacitance of the antennas determines the system gain. Here we estimate the stray capacitance in the antenna system by comparing the measured voltage spectrum with a model of the galactic continuum spectrum, which is the instrument background at high frequencies. Together with the antenna free‐space capacitance, these measurements provide an absolute calibration of the STEREO/WAVES experiment at radio frequencies, a prerequisite for quantitative studies of solar and astrophysical radio emission.

Revealing the transition from post-AGB stars to planetary nebulae: non-thermal and thermal radio continuum observations

Midcourse Space eXperiment and Infrared Astrorcomical Satellite colour diagnostics as well as OH maser profile characteristics were used to select a sample of post-asymptotic giant branch (pAGB) candidates for a radio continuum detection experiment with the Australia Telescope Compact Array. Seven out of 28 sources, six of which are new detections, show a continuum. A planetary nebula serendipitously detected in the field of an undetected pAGB candidate also reveals radio continuum. The radio continuum properties of these eight sources are described. Almost half have non-thermal emission. DUSTY modelling of the infrared spectral energy distributions (SEDs) of the three strongest detections reveals that they all have central stars with temperatures substantially lower than that required for significant photoionization, leading us to infer that the radio continuum has arisen from wind-shock interactions. This hypothesis is consistent with the detection of non-thermal radio emission in one of these three objects.

X-Ray Observations of the Sagittarius D H ii Region toward the Galactic Center with Suzaku

Abstract We present a Suzaku X-ray study of the Sagittarius D (Sgr$\;$D) H ii region in the Galactic center region. Two 18$^{\prime}$$\times$ 18$^{\prime}$ images by the X-ray Imaging Spectrometer (XIS) encompass the entire Sgr$\;$D complex. Thanks to the low background, XIS discovered two diffuse sources with low surface brightness and obtained their high signal-to-noise ratio spectra. One is associated with the core of the Sgr$\;$D H ii region, arising from a young stellar cluster. The other object is new and reported here for the first time. We also present 3.5 cm and 6.0 cm radio continuum maps of the new source using the 100-m Green Bank Telescope. We conclude that the source is a new supernova remnant (SNR; G1.2$-$0.0) based on: (1) the 0.9$\pm$0.2 keV thermal X-ray spectrum with emission lines from highly ionized atoms; (2) the diffuse nature with an apparent extent of $\sim$10 pc at the Galactic center distance inferred from the X-ray absorption ($\sim$8.5 $\times$ 10$^{22}$cm$^{-2}$); and (3) the nonthermal radio continuum spectral index ($\sim-$0.5). Our discovery of an SNR in the Sgr$\;$D H ii region leads to a revision of the view of this system, which had been considered to be a thermal H ii region and its environment.

Medium‐latitude sources of plasmaspheric nonthermal continuum radiations observed close to harmonics of the electron gyrofrequency

Nonthermal continuum (NTC) radiation is, with auroral kilometric radiation (AKR), one of the two electromagnetic emissions generated within the Earth's magnetosphere and radiated into space. It is generally believed that NTC is emitted in the plasmapause density gradient after conversion of intense electrostatic waves, present near the magnetic equator, into electromagnetic waves. In this paper, we present a specific type of NTC event, of infrequent occurrence, displaying a finger‐like pattern not yet reported: banded emissions peaking at exact multiples of a common frequency, df, which decrease inbound toward the plasmapause boundary layer (PPBL). Analysis is presented that indicates that the corresponding sources are nearby sites of the PPBL where the local electron gyrofrequency fce equals df. The sources are radiating beams of limited cone angle size. The NTC sources for this event are shown to be located at about 20° magnetic latitude. This illustrates that the PPBL is active in radiating NTC waves not only near the magnetic equator but also up to the medium‐latitude range.

Swift observations of IBL and LBL objects

BL Lacs are an enigmatic class of active galactic nuclei (AGNs), characterized by the non-thermal continuum typically attributed to synchrotron and inverse Compton emission. Depending on the frequency location of the maxima of these components, they are subdivided into three subclasses LBLs, IBLs, and HBLs. We present the results of a set of observations of eight BL Lac objects of LBL and IBL type performed by the XRT and UVOT detectors onboard the Swift satellite between January 2005 and November 2006. We are mainly interested in measuring the spectral parameters, and particularly the steepness between the UV and the X-ray band, useful for determining the classification of these sources. We compare the behavior of these sources with previous XMM-Newton, BeppoSAX obser- vations and with historical data in the X-ray and in the optical band. We are also interested in classifying the sources in our sample on the basis of the observations and comparing them with their classification presented in literature. We performed X-ray spectral analysis of observed BL Lac objects using a simple powerlaw and in a few cases the log-parabolic model. We also combined the UV emission with the low energy X-ray data to We used observational data to classify sources in our sample and derived parameters of their spectral energy distribution. We found that for the IBLs X-rays low states show features of the high energy component, usually interpreted as due to inverse Compton emission. Sources in our sample exhibit a range of temporal UV and X-ray behaviors, some objects having clear and neat correlated UV and X-ray variations (e.g. ON231) and other objects showing no clear (e.g. AO 0235+164) UV and X-ray correlation. Finally, we also note that our estimates of spectral curvature are in the range of that measured for the HBLs.

Observations of electromagnetic emissions inside the Earth’s plasmasphere from the Interball-1 satellite

The spectrum analyzer AKR-X onboard the Interball-1 satellite at the beginning (August–October 1995) and at the end (August–October 2000) of satellite operation in perigees of its orbital motion recorded and analyzed electromagnetic emissions of the inner regions of the Earth’s plasmasphere in the frequency band 100–1500 kHz at distances of 1.1–1.8 R E. The observations have shown that the electromagnetic modes (the Z and LO modes escaping the magnetosphere) which are formed at the altitudes 600–4000 km are associated with the subauroral nonthermal continuum and with the recently discovered kilometric continuum. There are noticeable differences in the spectral character of these emissions during the minimum (1996) and maximum (2000) solar activity, when, as a rule, the LO mode escaping the plasmaphere and the continua are not present.

Smooth electron density transition from plasmasphere to the subauroral region

Upper hybrid resonance (UHR) noise band and the nonthermal continuum (NTC) radiation were routinely observed by the radio plasma imager (RPI) onboard the IMAGE satellite when it transited the plasmasphere and subauroral regions. The lower cutoff frequencies of the UHR band and the NTC radiation provide an estimate of the electron plasma frequencies along the satellite orbit. A steep electron density gradient, which defines the plasmapause, was commonly observed when IMAGE transited from the plasmasphere to the subauroral region or vice versa. It appears however that, on many occasions, the electron density transition from the plasmasphere to the subauroral region is smooth without a clear signature of the plasmapause. Such smooth transitions can occur at various magnetic local times. The events presented in this study were observed after geomagnetic activities had been quiet for 2 or more days with Kp primarily less than 3. The smooth density transitions from the plasmasphere to the subauroral region may be an observational evidence of the suggested plasmaspheric wind.

A quantitative test of Jones NTC beaming theory using CLUSTER constellation

Abstract. Non-thermal continuum (NTC) radiation is, with auroral kilometric radiation (AKR), one of the two electromagnetic emissions generated within the Earth's magnetosphere and radiated into space. The location of the source of NTC has been sought for several decades, with only limited success. The constellation formed by the four CLUSTER spacecraft provides the possibility of triangulation in the vicinity of the source, thus allowing progress in source localisation, while simultaneously revealing the beaming properties of NTC radio sources. We present a case event showing two beams localised on opposite sides of the magnetic equator. At any selected frequency, triangulation points to a single region source of small size. Its position is compatible with the range of possible loci of sources predicted by the radio window theory of Jones (1982) in a frame of constraints relaxed from the simple sketch proposed in early works. The analysis of similar observations from the Dynamics Explorer 1 by Jones et al. (1987) enabled the authors to claim validation of the radio window theory. CLUSTER observations, however, reveal a large beaming cone angle projected onto the ecliptic plane, a feature unobservable by Dynamics Explorer which had a different spin axis orientation. According to the radio window theory, such a large observed cone angle can only be formed by a series of point sources, each beaming in a narrow cone angle. This study demonstrates the difficulty of validating NTC linear generation mechanisms using global beaming properties alone.

A search for water maser emission from post-AGB stars

AbstractWe have used the Tidbinbilla 70-m antenna to search for 22 GHz H2O maser emission from a sample of 85 evolved stars. 21 detections were made. Of these 15 were from massive AGB stars. High-velocity H2O maser emission was detected from five sources, of which four are post-AGB stars. Three of the high-velocity sources, b292 (IRAS 18043–2116), d46 (IRAS 15445–5449), and d62 (IRAS 15544–5332) were new discoveries. d46 is also a source of non-thermal radio continuum emission. The high-velocity H2O maser emission and the radio continuum from post-AGB stars are probably associated with shocks that form from wind-wind interactions.

The Origin of Diffuse X‐Ray and γ‐Ray Emission from the Galactic Center Region: Cosmic‐Ray Particles

The inner couple of hundred parsecs of our Galaxy are characterized by a significant amount of synchrotron-emitting gas. Many of best studied sources in this region exhibit a mixture of 6.4 keV Fe Kα emission, molecular line emission, and nonthermal radio continuum radiation. The spatial correlation between fluorescent Fe Kα line emission at 6.4 keV and molecular line emission from Galactic center molecular clouds has been explained as reflected X-rays from a past outburst of Sgr A*. Here we present a multiwavelength study of this region and find a correlation between nonthermal radio filaments and X-ray features. This correlation, when combined with distribution of molecular gas, suggests against irradiation model. Instead, we account for this distribution in terms of impact of relativistic particles from local (nonthermal filaments) and extended sources with diffuse neutral gas producing both nonthermal bremsstrahlung X-ray continuum emission and diffuse 6.4 keV line emission. The production rate of Fe Kα photons associated with injection of electrons into a cloud as a function of column density is calculated. The required energy density of low-energy cosmic rays associated with synchrotron-emitting radio filaments or extended features is estimated to be in range between 20 and ~103 eV cm-3 for Sgr C, Sgr B1, Sgr B2, and the 45 and -30 km s-1 clouds. We also generalize this idea to explain cosmic-ray heating of molecular gas, interstellar cosmic-ray ionization, pervasive production of diffuse Kα line, and TeV emission from Galactic center molecular clouds. In particular, we suggest that inverse Compton scattering of submillimeter radiation from dust by relativistic electrons may contribute substantially to large-scale diffuse TeV emission observed toward central regions of Galaxy.

Iron Line and Diffuse Hard X‐Ray Emission from the Starburst Galaxy M82

We examine the properties of the diffuse hard X-ray emission in the classic starburst galaxy M82. We use new Chandra ACIS-S observations in combination with reprocessed archival Chandra ACIS-I and XMM-Newton observations. We find E~6.7 keV Fe He-alpha emission is present in the central |r| < 200 pc, |z| < 100 pc of M82 in all datasets at high statistical significance, in addition to a possibly non-thermal X-ray continuum and marginally significant E=6.4 keV Fe K-alpha line emission. No statistically significant Fe emission is found in the summed X-ray spectra of the point-like X-ray sources or the ULX in the two epochs of Chandra observation. The total nuclear region iron line fluxes in the 2004 April 21 XMM-Newton observation are consistent with those of the Chandra-derived diffuse component, but in the 2001 May 6 XMM-Newton observation they are significantly higher and also both E=6.4 and E=6.9 keV iron lines are detected. We attribute the excess iron line emission to the Ultra-Luminous X-ray source in its high state. In general the iron K-shell luminosity of M82 is dominated by the diffuse component. The total X-ray luminosity of the diffuse hard X-ray emission (corrected for emission by unresolved low luminosity compact objects) is L_X ~ 4.4 x 10^39 erg/s in the E=2-8 keV energy band, and the 6.7 keV iron line luminosity is L_X ~ (1.1 -- 1.7) x 10^38 erg/s. The 6.7 keV iron line luminosity is consistent with that expected from the previously unobserved metal-enriched merged supernova ejecta that is thought to drive the larger-scale galactic superwind. The iron line luminosity implies a thermal pressure within the starburst region of P/k ~ 2 x 10^7 K/cm^3, which is consistent with independent observational estimates of the starburst region pressure [Abstract abridged].

The Signature of 44 Ti in Cassiopeia A Revealed by IBIS/ISGRI on INTEGRAL

We report the detection of both the 67.9 and 78.4 keV 44Sc gamma-ray lines in Cassiopeia A with the INTEGRAL IBIS/ISGRI instrument. Besides the robustness provided by spectro-imaging observations, the main improvements compared to previous measurements are a clear separation of the two 44Sc lines together with an improved significance of the detection of the hard X-ray continuum up to 100 keV. These allow us to refine the determination of the 44Ti yield and to constrain the nature of the nonthermal continuum emission. By combining COMPTEL, BeppoSAX/PDS and ISGRI measurements, we find a line flux of (2.5 +/- 0.3)*10(-5) cm(-2) s(-1) leading to a synthesized 44Ti mass of 1.6 (+0.6-0.3)*10(-4) solar mass. This high value suggests that Cas A is peculiar in comparison to other young supernova remnants, from which so far no line emission from 44Ti decay has been unambiguously detected.

Radio observations of the planetary nebula around the OH/IR star OH 354.88-0.54 (V1018 Sco)

We present radio observations of the unique, recently formed, planetary nebula (PN) associated with a very long-period OH/IR variable star V1018 Sco that is unequivocally still in its asymptotic giant branch phase. Two regions within the optical nebula are clearly detected in nonthermal radio continuum emission, with radio spectral indices comparable to those seen in colliding-wind Wolf-Rayet binaries. We suggest that these represent shocked interactions between the hot, fast stellar wind and the cold nebular shell that represents the PN’s slow wind moving away from the central star. This same interface produces both synchrotron radio continuum and the optical PN emission. The fast wind is neither spherical in geometry nor aligned with any obvious optical or radio axis. We also report the detection of transient H2O maser emission in this nebula.

VLA H53α Radio Recombination Line Observations of the Ultraluminous Infrared Galaxy Arp 220

We present high angular resolution (07) observations made with the Very Large Array (VLA) of the radio recombination line (RRL) H53α and radio continuum emission at 43 GHz from the ultraluminous infrared galaxy (ULIRG) Arp 220. The 43 GHz continuum emission shows a compact structure (~2'') with two peaks separated by ~1'', the east (E) and west (W) components, that correspond to each galactic nucleus of the merger. The spectral indices for both the E and W components, using radio continuum images at 8.3 and 43 GHz are typical of synchrotron emission (α ~ -1.0). Our 43 GHz continuum and H53α line observations confirm the flux densities predicted by the models proposed by Anantharamaiah et al. (2000). This agreement with the models implies the presence of high-density (~105 cm-3) compact H II regions (~0.1 pc) in Arp 220. The integrated H53α line emission is stronger toward the nonthermal radio continuum peaks, which are also coincident with the peaks of molecular emission of the H2CO. The coincidence between the integrated H53α and the H2CO maser line emission suggests that the recent star-forming regions, traced by the high-density gas, are located mainly in regions that are close to the two radio continuum peaks. A velocity gradient of ~0.30 km s-1 pc-1 in the H53α RRL is observed toward the E component, and a second velocity gradient of ~0.15 km s-1 pc-1 is detected toward the W component. The orientations of these velocity gradients are in agreement with previous CO, H I, and OH observations. The kinematics of the high-density ionized gas traced by the H53α line are consistent with two counterrotating disks as suggested by the CO and H I observations.

Near‐Infrared, Kilosecond Variability of the Wisps and Jet in the Crab Pulsar Wind Nebula

We present a time-lapse sequence of 20 near-infrared (J- and K'-band) snapshots of the central 20x20 arcsec of the Crab pulsar wind nebula, taken at subarcsecond resolution with the Hokupa'a/QUIRC adaptive optics camera on the Gemini North Telescope, and sampled at intervals of 10 min and 24 hr. It is observed that the equatorial wisps and polar knots in the termination shock of the pulsar wind appear to fluctuate in brightness on kilosecond time-scales. Maximum flux variations of +/-24 +/-4 and +/-14 +/-4 per cent relative to the mean (in 1.2 ks) are measured for the wisps and knots respectively, with greatest statistical significance in J band where the nebula background is less prominent. The J and K' flux densities imply different near-infrared spectra for the nonthermal continuum emission from the wisps and outermost polar knot (sprite), giving F_nu ~ nu^{-0.56+/-0.12} and F_nu ~ nu^{-0.21+/-0.13) respectively. The data are compared with existing optical and UV photometry and applied to constrain theories of the variability of the wisps (relativistic ion-cyclotron instability) and knots (relativistic fire hose instability).