The energy distribution of electrons in radio jets

Black-hole and neutron-star X-ray binaries exhibit compact radio jets, when they are in the so called quiescent, hard, or hard intermediate states. The radio spectrum in these states is flat to slightly inverted. It is widely accepted that the energy distribution of the electrons, in the rest frame of the jet, is a power law with index p. A power-law energy distribution of the electrons in the jet is sufficient to explain the flat to slightly inverted spectrum emitted by the jet from radio to near infrared wavelengths, but is it necessary? Contrary to what our thinking was decades ago, now we know that the jets originate in the hot inner flow around black holes and neutron stars. Thus, we have investigated the spectrum that is emitted by a thermal jet with kT in the range 100-250 keV. We have computed the emitted spectrum from radio to near infrared using either a thermal distribution of electrons or a power-law one. We have found that parabolic jets with a thermal distribution of electrons give inverted spectra with alpha in the range 0-0.4, while jets with a power-law distribution of electrons give inverted spectra with alpha in the range 0-0.2. The rest of the parameters are kept the same in the two cases. The break frequency, which marks the transition from optically thick to optically thin synchrotron emission, is comparable for the two electron energy distributions. Our conclusion is that, contrary to common belief, it is not necessary to invoke a power-law energy distribution of the electrons in a jet to explain its flat to slightly inverted radio spectrum. A relativistic Maxwellian produces similar spectra. Thus, the widely invoked corona around black holes in X-ray binaries may actually be the jet.Comment: 4 pages, 2 figures, accepted by A&A Letter

Linear polarization of radio frequency lines in molecular clouds and circumstellar envelopes

We predict that interstellar lines possess a few percent linear polarization provided that the optical depth in the source region is both anisotropic and of order unity and the radiative rates are at least comparable to the collision rates. These conditions are expected to be met in many sources which emit radio and far-infrared line radiation. Under circumstances in which the Zeeman splitting exceeds both the radiative and collisional rates the linear polarization is aligned either parallel or perpendicular to the projection of the magnetic field on the plane of the sky. This "strong magnetic field" limit is expected to apply to all radio frequency lines and to many of those far infrared lines which form between levels whose magnetic moments are comparable to the Bohr magneton. The "weak magnetic field" limit is relevant to most far-infrared lines formed between levels with magnetic moments of order the nuclear magneton. In this limit the polarization direction is determined by the orientation of the propagation direction with respect to the anisotropic optical depth

On mapping the magnetic field direction in molecular clouds by polarization measurements

We predict that interstellar radio-frequency lines possess a few percent linear polarization, provided that (1) the radiative transition rate is at least comparable to the collision rate, (2) the optical depth is moderate. and anisotropic, and (3) the number of extrema of the velocity component along the line of sight through the source is small. If the Zeeman splitting exceeds both the collisional frequency and the radiative transition rate, then the polarization is aligned either perpendicular to or parallel to the projection of the magnetic field on the plane of the sky

A quantitative explanation of the radio--X-ray correlation in black-hole X-ray binaries

The observed correlation between the radio and X-ray fluxes in the hard state of black-hole X-ray binaries (BHXRBs) has been around for more than two decades. It is currently accepted that the hard X-rays in BHXRBs come from Comptonization in the corona and the radio emission from the relativistic jet (Lorentz $\gamma >> 1$), which is a narrow structure of a few $R_g=GM/c^2$ at its base. The relativistic jet and the corona, however, are separate entities with hardly any communication between them, apart from the fact that both are fed from the accreting matter. It is also widely accepted that the accretion flow around black holes in BHXRBs consists of an outer thin disk and an inner hot flow. From this hot inner flow, an outflow emanates in the hard and hard-intermediate states of the source. By considering Compton up-scattering of soft disk photons in the outflow (i.e., in the outflowing corona, which is a wider structure, tens to hundreds of $R_g$ at its base, with low Lorentz gamma) as the mechanism that produces the hard X-ray spectrum, we have been able to explain quantitatively a number of observed correlations. Here, we demonstrate that this outflowing corona can also explain quantitatively the observed radio - X-ray correlation. In addition, we make the following theoretical predictions for GX 339-4: 1) the radio flux in the hard and hard-intermediate states should be a bell-shaped curve as a function of the photon-number spectral index Gamma, 2) the radio - X-ray correlation should break down when the source moves from the hard to the hard-intermediate state and instead the radio flux should first increase sharply in the hard-intermediate state and then decrease also sharply, in a very narrow range of the X-ray flux, and 3) the X-ray polarization will be parallel to the outflow in the hard state and perpendicular to it in the hard-intermediate one.Comment: Accepted for publication in A&

A multilevel implementation of the Goldreich-Kylafis effect into the radiative transfer code PyRaTE

Among all the available observational techniques for studying magnetic fields in the dense cold phase of the interstellar medium, linear polarization of spectral lines, referred to in the literature as the Goldreich-Kylafis effect (Goldreich & Kylafis 1981; hereafter "GK effect"), remains one of the most underutilized methods. In this study, we implement the GK effect into the multilevel, non-local thermodynamic equilibrium radiative transfer code PyRaTE. Different modes of polarized radiation are treated individually with separate optical depths computed for each polarization direction. We benchmark our implementation against analytical results and provide tests for various limiting cases. In agreement with previous theoretical results, we find that in the multilevel case the amount of fractional polarization decreases when compared to the two-level approximation, but this result is subject to the relative importance between radiative and collisional processes. Finally, we post-process an axially symmetric, non-ideal magnetohydrodynamic chemo-dynamical simulation of a collapsing prestellar core and provide theoretical predictions regarding the shape (as a function of velocity) of the polarization fraction of CO during the early stages in the evolution of molecular clouds. The code is freely available to download.Comment: 13 pages and 11 figures. Submitted to A&A. Comments welcom

Star Formation and Infrared Emission in Galaxies

The relationship between star formation and infrared emission in galaxies will be investigated. If galaxies were simple objects and young stars were completely covered with dust, then all the absorbed light of the young stars would be re-emitted in the infrared and from the infrared emission of galaxies we would infer the star formation rate (SFR) in them accurately. To show the complexities involved in real galaxies, we will use as a case study the late-type spiral galaxies. We will show that the heating of the dust is done mainly by the UV radiation of the young stars and therefore the infrared emission reveals the SFR in them. With a realistic model and its application to a number of galaxies, tight correlations are derived between SFR and total far infrared luminosity on one hand, and dust mass and 850 micron flux on the other. Other diagnostics of the SFR are examined and it is shown that there is consistency among them. Thus, the SFR for galaxies of all Hubble types has been determined as well as for interacting starburst galaxies. Combining different methods, the star-formation history of the universe has been determined and will be shown. Finally, some early results from the Spitzer Space Telescope will be presented.Comment: To be published in ``The many scales in the Universe - JENAM 2004 Astrophysics Reviews,'' from the Joint European and National Astronomical Meeting in Granada, Spain, September 13-17, 2004. Kluwer Academic Publishers, edited by Jose Carlos del Toro Iniesta, et a

Energy and time-lag spectra of black-hole systems in the low state

Most, probably all, accreting binaries that are believed to contain a black-hole emit radio waves when they are in the low/hard state. Whenever this radio emission has been resolved, a jet-like structure has become apparent. We propose that Compton upscattering of low-energy photons in the jet can explain both the energy spectra and the time lags versus Fourier frequency observed in the low/hard state of black-hole systems. The soft photons originate in the inner part of the accretion disk. We have performed Monte Carlo simulations of Compton upscattering in a jet and have found that for a rather wide range of values of the parameters we can obtain power-law high-energy X-ray spectra with photon-number index in the range 1.5 - 2 and power-law time lags versus Fourier frequency with index ~ 0.7. The black-hole source Cyg X-1 in the low/hard state is well described by our model.Comment: 4 pages 3 figures, to be published in A&A Letter

Time lags of the type-B QPO in MAXI J1348-630

The fast variability observed in the X-ray emission from black-hole binaries has a very complex phenomenology, but offers the possibility to investigate directly the properties of the inner accretion flow. In particular, type-B oscillations in the 2-8 Hz range, observed in the Soft-Intermediate state, have been associated to the emission from a relativistic jet. We present the results of the timing and spectral analysis of a set of observations of the bright transient MAXI J1348-630 made with the NICER telescope. The observations are in the brightest part of the outburst and all feature a strong type-B QPO at ~4.5 Hz. We compute the energy dependence of the fractional rms and the phase lags at the QPO frequency, obtaining high signal-to-noise data and sampling for the first time at energies below 2 keV. The fractional rms decreases from more than 10% at 9 keV to 0.6% at 1.5 keV, and is constant below that energy. Taking the 2-3 keV band as reference, photons at all energies show a hard lag, increasing with the distance from the reference band. The behaviour below 2 keV has never been observed before, due to the higher energy bandpass of previous timing instruments. The energy spectrum can be fitted with a standard model for this state, consisting of a thin disc component and a harder power law, plus an emission line between 6 and 7 keV. We discuss the results, concentrating on the phase lags, and show that they can be interpreted within a Comptonization model.Comment: 6, pages, 5 figures, accepted for publication in MNRA

X-ray Spectral Formation in a Converging Fluid Flow: Spherical Accretion into Black Holes

We study Compton upscattering of low-frequency photons in a converging flow of thermal plasma. The photons escape diffusively and electron scattering is the dominant source of opacity. We solve numerically and approximately analytically the equation of radiative transfer in the case of spherical, steady state accretion into black holes. Unlike previous work on this subject, we consider the inner boundary at a finite radius and this has a significant effect on the emergent spectrum. It is shown that the bulk motion of the converging flow is more efficient in upscattering photons than thermal Comptonization, provided that the electron temperature in the flow is of order a few keV or less. In this case, the spectrum observed at infinity consists of a soft component coming from those input photons which escaped after a few scatterings without any significant energy change and of a power law which extends to high energies and is made of those photons which underwent significant upscattering. The luminosity of the power law is relatively small compared to that of the soft component. The more reflective the inner boundary is, the flatter the power-law spectrum becomes. The spectral energy power-law index for black-hole accretion is always higher than 1 and it is approximately 1.5 for high accretion rates. This result tempts us to say that bulk motion Comptonization might be the mechanism behind the power-law spectra seen in black-hole X-ray sources.Comment: 37 pages, LaTex, AAS Macros, 8 ps figures, to appear in Ap

5cm OH masers as diagnostics of physical conditions in star-forming regions

We demonstrate that the observed characteristics of the 5 cm OH masers in star-forming regions can be explained with the same model and the same parameters as the 18 cm and the 6 cm OH masers. In our already published study of the 18 cm and the 6 cm OH masers in star-forming regions we had examined the pumping of the 5 cm masers, but did not report the results we had found because of some missing collision rate coefficients, which in principle could be important. The recently published observations on the 5 cm masers of OH encourage us to report our old calculations along with some new ones that we have performed. These calculations, in agreement with the observations, reveal the main lines at 5 cm as strong masers, the 6049 MHz satellite line as a weak maser, and the 6017 MHz satellite line as never inverted for reasonable values of the parametersComment: TeX 15 pages, 30 postscript figures, accepted by Ap