Extragalactic Active Galactic Nuclei Research Articles

Revealing two radio-active galactic nuclei extremely near PSR J0437−4715

Newton's gravitational constant $G$ may vary with time at an extremely low level. The time variability of $G$ will affect the orbital motion of a millisecond pulsar in a binary system and cause a tiny difference between the orbital period-dependent measurement of the kinematic distance and the direct measurement of the annual parallax distance. PSR J0437$-$4715 is the nearest millisecond pulsar and the brightest at radio. To explore the feasibility of achieving a parallax distance accuracy of one light-year, comparable to the recent timing result, with the technique of differential astrometry, we searched for compact radio sources quite close to PSR J0437$-$4715. Using existing data from the Very Large Array and the Australia Telescope Compact Array, we detected two sources with flat spectra, relatively stable flux densities of 0.9 and 1.0 mJy at 8.4 GHz and separations of 13 and 45 arcsec. With a network consisting of the Long Baseline Array and the Kunming 40-m radio telescope, we found that both sources have a point-like structure and a brightness temperature of $\geq$10$^7$ K. According to these radio inputs and the absence of counterparts in the other bands, we argue that they are most likely the compact radio cores of extragalactic active galactic nuclei rather than Galactic radio stars. The finding of these two radio active galactic nuclei will enable us to achieve a sub-pc distance accuracy with the in-beam phase-referencing very-long-baseline interferometric observations and provide one of the most stringent constraints on the time variability of $G$ in the near future.

Non-thermal radio astronomy

This presentation starts with Karl Jansky’s discovery of cosmic radio emission in 1933 and notes the striking similarities to Hess’s discovery of cosmic-rays in 1912. At first it was assumed that this radio emission was thermal but in 1939 Grote Reber discovered that it was stronger at longer wavelengths, requiring a non-thermal emission process. These discoveries had a revolutionary impact on astronomy and radio astronomy was born.The interpretation of this non-thermal radiation as synchrotron emission from high energy particles in the interstellar medium did not occur until the late 1940s but then it provided the link between radio astronomy and cosmic-ray research. Ginzburg, in particular, saw that cosmic-ray astrophysics was now possible using radio waves to trace sources of cosmic-rays.We discuss the discovery of extragalactic active galactic nuclei leading to the discovery of quasars and the first evidence for black holes in the nuclei of galaxies. We summarise the present status and future of some of the main radio telescopes used to image the non-thermal emission from external galaxies.Finally, we include a short description of the use of radio signals for the direct detection of cosmic-rays and UHE neutrinos.

Subtraction of Bright Point Sources from Synthesis Images of the Epoch of Reionization

AbstractBright point sources associated with extragalactic active galactic nuclei and radio galaxies are an important foreground for low-frequency radio experiments aimed at detecting the redshifted 21-cm emission from neutral hydrogen during the epoch of reionization. The frequency dependence of the synthesized beam implies that the sidelobes of these sources will move across the field of view as a function of observing frequency, hence frustrating line-of-sight foreground subtraction techniques. We describe a method for subtracting these point sources from dirty maps produced by an instrument such as the MWA. This technique combines matched filters with an iterative centroiding scheme to locate and characterize point sources in the presence of a diffuse background. Simulations show that this technique can improve the dynamic range of epoch-of-reionization maps by 2—3 orders of magnitude.

A 31 GHz SURVEY OF LOW-FREQUENCY SELECTED RADIO SOURCES

The 100-m Robert C. Byrd Green Bank Telescope (GBT) and the Owens Valley Radio Observatory (OVRO) 40-m radio telescope have been used to conduct a survey of 3165 known extragalactic radio sources over 143 square degrees of the sky. Target sources were selected from the NRAO VLA Sky Survey in fields observed by the Cosmic Background Imager (CBI); most are extragalactic active galactic nuclei (AGN) with 1.4 GHz flux densities of 3 to 10 mJy. The resulting 31 GHz catalogs are presented in full online. Using a Maximum-Likelihood analysis to obtain an unbiased estimate of the distribution of the 1.4 to 31 GHz spectral indices of these sources, we find a mean 31 to 1.4 GHz flux ratio of 0.110 +/- 0.003 corresponding to a spectral index of alpha=-0.71 +/- 0.01 (S ~ nu^alpha); 9.0 +/- 0.8 % of sources have alpha > -0.5 and 1.2 +/- 0.2 % have alpha > 0. By combining this spectral index distribution with 1.4 GHz source counts we predict 31 GHz source counts in the range 1 mJy < S_31 < 4 mJy, N(>S_31) = (16.7 +/- 1.7) deg^2 (S_31/1 mJy)^(-0.80 +/- 0.07). We also assess the contribution of mJy-level (S_1.4 < 3.4 mJy) radio sources to the 31 GHz CMB power spectrum, finding a mean power of ell (ell+1) C^src_ell/(2 pi) = 44 +/- 14 micro-Kelvin^2 and a 95% upper limit of 80 micro-Kelvin^2 at ell = 2500. Including an estimated contribution of 12 micro-Kelvin^2 from the population of sources responsible for the turn-up in counts below S_1.4 = 1 mJy this amounts to 21 +/- 7 % of what is needed to explain the CBI high-ell excess signal, 275 +/- 63 micro-Kelvin^2. These results are consistent with other measurements of the 31 GHz point source foreground.

Connection between the Accretion Disk and Superluminal Radio Jets and the Role of the Radio Plateau State in GRS 1915+105

We investigate the association between the accretion disk during the radio plateau state and the following superluminal relativistic radio jets with peak intensity varying from 200 to 1000 mJy observed over a period of 5 yr and present evidence of a direct accretion disk-jet connection in the microquasar GRS 1915+105. We have analyzed RXTE PCA/HEXTE X-ray data and have found that the accretion rate, accr, as inferred from the X-ray flux, is very high during the radio plateaux. We suggest that the accretion disk during the radio plateaux is always associated with radiation-driven wind, which is manifested in the form of enhanced absorption column density for X-rays and depleted IR emission. We find that the wind density increases with the accretion disk luminosity during the radio plateaux. The wind density is similar to the density of the warm absorber proposed in extragalactic active galactic nuclei (AGNs) and quasars. We suggest a simple model for the origin of superluminal relativistic jets. Finally, we discuss the implications of this work for Galactic microquasars and extragalactic AGNs and quasars.

The Effect of External Winds on Relativistic Jets

Relativistic jets in Galactic superluminals and extragalactic active galactic nuclei may be surrounded by a wind near the central engine. Theoretical analysis and numerical simulation reveal considerable stabilization of relativistic jet flow by a wind to helical and higher order asymmetric modes of jet distortion. When velocities are measured in the source (inlet) frame, reduction in the absolute velocity difference between jet and wind, Δv = vjet - vwnd, provides stabilization in addition to stabilization provided by a high jet Lorentz factor, but a high Lorentz factor wind is not needed to stabilize a high Lorentz factor jet. However, the fundamental pinch mode is not similarly affected, and knots with spacing a few times the jet radius are anticipated to develop in such flows. Thus, we identify a mechanism that can suppress large-scale asymmetric structures while allowing axisymmetric structures to develop. Relativistic jets surrounded by outflowing winds will be more stable than if surrounded by a stationary or backflowing external medium. Knotty structures along a straight jet like that in 3C 175 could be triggered by pinching of an initially low Mach number jet surrounded by a suitable wind. As the jet enters the radio lobe, suppression of any surrounding outflow or backflow associated with the high-pressure lobe triggers exponential growth of helical twisting.

Cosmic gamma rays from extragalactic objects

The authors have analysed the database of cosmic gamma rays above 100 MeV from the SAS-2 satellite to search for gamma -ray fluxes from extragalactic active galactic nuclei (AGN). The correlation function method devised by the COS-B group to search for galactic gamma -ray sources has been used. They have thus identified 15 gamma -ray AGN, including 11 quasars, 3 BL Lac objects and 1 radiogalaxy. They have further investigated the relationship between their gamma -ray fluxes in different energy regions (>100 MeV and 35-100 MeV) and the relationship between their gamma -ray fluxes and their red shifts, and they have also derived their gamma -ray spectral indices. The results give further support to the claim that the sources detected are genuine extragalactic objects.

Cosmic gamma rays from active galactic nuclei

We have used the SAS-2 γ-ray data to search for γ-rays of energy >100 MeV from extragalactic active galactic nuclei (AGNs). The correlation function devised by the COS-B group to search for galactic γ-ray sources has been used. We have identified 18 such AGNs, including 13 quasars, 3 BL Lac objects, 1 Seyfert galaxy and 1 radiogalaxy. We have further investigated the relationship between the γ-ray fluxes in the energy regions > 100 MeV and 35–100 MeV and their redshifts, and we have also derived their γ-ray spectral indices. The results give further support to our claim that the sources detected are genuine extragalactic objects.