Radio Intra-Day Variability: Answers and Questions

Intra-day variability (IDV) of active galactic nuclei (AGN) has been detected from gamma-ray energies to radio wavelengths. At high energies, such variability appears to be intrinsic to the sources themselves. However, at radio wavelengths, brightness temperatures as high as10¹⁸ to 10²¹ K are encountered if the IDV is intrinsic to the source. We discuss here the accumulating evidence showing that, at radio wavelengths where the highest brightness temperatures are encountered, interstellar scintillation (ISS) is the principal mechanism causing IDV. While ISS reduces the implied brightness temperatures, they still remain uncomfortably high. This revised version was published online in July 2006 with corrections to the Cover Date.     

J1128+592: a highly variable IDV source

Short time‐scale radio variations of compact extragalactic radio quasars and blazars known as IntraDay Variability (IDV) can be explained in at least some sources as a propagation effect; the variations are interpreted as scintillation of radio waves in the turbulent interstellar medium of the Milky Way. One of the most convincing observational arguments in favor of a propagation‐induced variability scenario is the observed annual modulation in the characteristic time scale of the variation due to the Earth's orbital motion. So far there are only two sources known with a well‐constrained seasonal cycle. Annual modulation has been proposed for a few other less well‐documented objects. However, for some other IDV sources source‐intrinsic structural variations which cause drastic changes in the variability time scale were also suggested. J1128+592 is a recently discovered, highly variable IDV source. Previous, densely time‐sampled flux‐density measurements with the Effelsberg 100‐m radio telescope (Germany) and the Urumqi 25‐m radio telescope (China), strongly indicate an annual modulation of the time scale. The most recent 4 observations in 2006/7, however, do not fit well to the annual modulation model proposed before. In this paper, we investigate a possible explanation of this discrepancy. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

List of Participants

The possibility of obtaining information about small-scale inhomogenities of the electron component of the local interstellar medium (ISM) is investigated using interstellar scintillation of extragalactic radio sources. We analyze Culgoora array variability data at 80 and 160 MHz for 190 extragalactic radio sources distributed over most of the sky. The variability on time-scales of 1 month-15 years is interpreted as interstellar scintillations in rapidly-moving nearby (less than 150 pc) hot gas near shock waves in the local ISM. All-sky maps of scintillation index m averaged over 3-5 neighbouring sources and over m for time-scales of 1 month (m1) and one or several years (m12) show several maximum values for m. Locations of the maxima are insensitive to the method and number of points used for averaging. The positions of the maxima obtained in different ways agree to within 15-30 degrees on the sky; this is the angular resolution of this method. Two of the three most certain maxima are probably associated with Loop I, and the third coincides with a soft X-ray (0.1 - 0.3 keV) background maximum near the South Galactic Pole. Other less certain scintillation index maxima probably correspond to the Orion starformation region and to a soft X-ray maximum near the North Galactic Pole. A tunnel that is free of gas in the direction l = 240° is indicated by low values of m. The estimated time-scale for interstellar scintillations in these structures in the local ISM is in agreement with the time-scale for the observed radio source variations.     

Annual Modulation in Intraday Variability Timescales of Extragalactic Radio Sources

Annual modulations in timescales of intraday variability (IDY here-after) are discussed for six extragalactic sources:0716 714,0917 624,0954 65,1749 70,1803 78 and 2007 77.The timescales calculated from scintillation the-ory are compared with the observational data.It is emphasized that systematic observations are required to identify the phenomenon and to determine the motion of the interstellar medium with respect to the Local Standard of Rest.In addi-tion,significant deviations from the annual modulation could be due to intrinsic variations.     

Interstellar scintillation of compact extragalactic radio sources

The recent discovery of radio variability of a quasar on short time-scales (hours) prompts us to examine what is expected in respect of the interstellar scintillation of very compact, extragalactic radio sources. We find that large-amplitude, rapid, variability is predicted at commonly observed radio frequencies (1–20 GHz) over the vast majority of the extragalactic sky. As a guide to assist observers in understanding their data, we demonstrate simple techniques for predicting the effects of interstellar scintillation on any extragalactic source.     

Upper Limits on the Continuum Emission from Geminga at 74 and 326 MHz

We report a search for radio continuum emission from the gamma-ray pulsar Geminga. We have used the VLA to image the location of the optical counterpart of Geminga at 74 and 326 MHz. We detect no radio counterpart. We derive upper limits to the pulse-averaged flux density of Geminga, taking diffractive scintillation into account. We find that diffractive scintillation is probably quenched at 74 MHz and does not influence our upper limit, S<56 mJy (2 sigma), but that a 95% confidence level at 326 MHz is S<5 mJy. Owing to uncertainties on the other low-frequency detections and the possibility of intrinsic variability or extrinsic variability (refractive interstellar scintillation) or both, our nondetections are nominally consistent with these previous detections.     

Intraday polarization variability outside the VLBI core of the active galactic nucleus 0716+714

Rapid ('intraday') cm-wavelength variations in both total and polarized flux density have been observed in a number of strong extragalactic radio sources. It is difficult to explain these variations purely as propagation effects, but if they are intrinsic to the sources, implausibly high brightness temperatures are required. We discuss here rapid polarization variability during our λ =6 cm global very long-baseline interferometry (VLBI) observations of the active galactic nucleus 0716+714. Measurements made with the Very Large Array (VLA) during the VLBI observations indicate a ≃50° swing in the position angle χ of the VLA core polarization in 12 hours. Corresponding variations were observed only for short VLBI baselines, so that they could not have occurred in the VLBI core (the only feature detected in our VLBI polarization map). The fact that the variations appear both in the VLA data and in the VLBI data for short baselines makes it difficult to explain them as instrumental effects. This leads us to conclude that the rapid variations occurred outside the area covered by our VLBI map; we estimate that the variability occurred in some compact feature roughly 25 milliarcseconds from the nucleus. It is clear that compact structures on a wide range of scales must be taken into account in studies of intraday variability in AGN.     

Monitoring of interplanetary and ionospheric scintillation of an ensemble of radio sources

The results of a series of 24-hour observations of radio-source interplanetary and ionospheric scintillation performed on April 4–10, 2006, at the Pushchino Radio Astronomy Observatory are presented. The observations were carried out with the Large Phased Array radio telescope of the Lebedev Institute of Physics, Russian Academy of Sciences, at a frequency of 110 MHz. The scintillating fluxes of all radio sources that fall within a field of sky between declinations +28° and +31° were automatically recorded applying eight beams of the reception pattern operating simultaneously. All of the sources with flux densities of 0.2 Jy or higher were detected. The structure functions of the flux fluctuations were measured for time shifts 1 and 10 s, which characterize the interplanetary (1 s) and ionospheric (10 s) scintillation, respectively. The mean scintillation index m _IPP (on a characteristic time scale of 1 s) of an ensemble of radio sources located within a sky band 4° wide in declination and 1 h wide in right ascension was measured as the parameter that characterizes the interplanetary plasma. Diurnal variations of the interplanetary scintillation index were determined. The maximum m _IPP value at daytime equals 0.3, and the minimum value at nighttime equals 0.10. Weak interday variations of the mean daytime and nighttime scintillation indices were detected. The ionospheric scintillation indices m _Ion are small compared to m _IPP at daytime, but m _Ion ? m _IPP at nighttime. On the whole, both the interplanetary plasma and ionosphere were quiet during the observations.     

Radio Variability of First 3-Month Fermi Blazars at 5 GHz: Affected by Interstellar Scintillation?

Blazars from the first-three-months Fermi-AGN list were observed with the Urumqi 25-m radio telescope at 5 GHz in IDV (Intra-Day Variability) mode and inter-month observation mode. A significant correlation between the flux density at 5 GHz and the γ-ray intensity for the Fermi-LAT detected blazars is seen. There is a higher IDV detection rate in Fermi detected blazars than those reported for other samples. Stronger variability appears at lower galactic latitudes; IDV appears to be stronger in weaker sources, indicating that the variability is affected by interstellar scintillation.     

Annual cycles in the interstellar scintillation time-scales of PKS B1519–273 and PKS B1622–253

We have used the University of Tasmania's 30-m radio telescope at Ceduna in South Australia to regularly monitor the flux density of a number of southern blazars. We report the detection of an annual cycle in the variability time-scale of the centimetre radio emission of PKS B1622−253. Observations of PKS B1519−273 over a period of nearly 2 yr confirm the presence of an annual cycle in the variability time-scale in that source. These observations prove that interstellar scintillation is the principal cause of inter-day variability at radio wavelengths in these sources. The best-fitting annual cycle model for both sources implies a high degree of anisotropy in the scattering screen and that it has a large velocity offset with respect to the local standard of rest. This is consistent with a greater screen distance for these 'slow' intra-day variability (IDV) sources than for rapid scintillators such as PKS B0405−385 or J1819+3845.     

致密射电源的快速偏振变化和星际闪烁模型

以类星体０９１７＋６２４中１９８９年５月观测到的ＩＤＶ事件为实例,尝试提出一个４成分模型（１个稳定成分和 ３个闪烁成分）以充分解释在 ６ ｃｍ波长上观测到的偏振变化,包括流量和偏振流量的相关性和反相关性以及它们之间的快速转化．对于 ２０ ｃｍ波长上观测到的偏振变化,３成分模型（１个稳定成分和３个闪烁成分）已足以解释全部现象．文中提出的闪烁模型在解释ＩＤＶ事件的偏振变化方面改进了以前的模型拟合．     

Radio sources flickering and local interstellar medium (LISM) structure

A possibility of obtaining information on small-scale inhomogeneities of the electron component of the local interstellar medium (LISM) was investigated using interstellar scintillations of extragalactic radio sources. We analysed Culgoora array observational variability data of 190 extragalactic radio sources, covering most of the sky, at 80 and 160 MHz. The variability at time scales from 1 month to 15 years is interpreted as refractive interstellar scintillations in fast-moving nearby (less than 150 pc) hot gas, near shock waves in the LISM. An all-sky map of scintillation indices, m, averaged over three–five sources closest to one another, shows several m maxima. Two of the three most pronounced maxima are probably connected with Loop I; the third one coincides with the soft X-ray (0.1–0.3 keV) background maximum near the South Galactic Pole. Other, less certain, m maxima probably correspond to the Orion star-formation region and to a soft X-ray maximum near the North Galactic Pole. The ‘free-of-gas’ tunnel in the direction l=240° corresponds to low values of m. The estimated time scale of interstellar scintillations on the above-mentioned LISM structures is in agreement with that of the observed radio-source variations.     

The ATCA intraday variability survey of extragalactic radio sources

We present the results of an Australia Telescope Compact Array (ATCA) survey for intraday variability (IDV) of the total and polarized flux densities of 118 compact, flat-spectrum, extragalactic radio sources from the Parkes 2.7-GHz Survey. A total of 22 total flux density IDV sources were discovered and 15 sources were found to show IDV of their polarized flux density. We discuss the statistical properties of the IDV sources, including the distribution of source modulation indices, and the dependence of the variability amplitude on source spectral index and on Galactic position. We suggest interstellar scintillation (ISS) in the Galactic interstellar medium as the most likely mechanism for IDV. Even so, the inferred high brightness temperatures cannot be easily explained.     

Observational manifestations and intrinsic properties of the RCR sources in terms of a unified model

We present a summary results of the study of radio sources showing significant variations of integral flux density using the data from the RATAN-600 surveys of 1980–1994 at a frequency of 7.6 cm. The majority of the detected variable sources have flat radio spectra, although there are also all other spectrum types found. Point and compact sources predominate, although all known morphological structures are found in the sample. Variability is detected both in quasars and galaxies. Using the catalog data, we found brightness variations in the optical and/or infrared ranges for a half of host objects of radio sources. We analyzed the properties of nonvariable and variable RCR sources. We compared the ratio of absolute magnitude to radio luminosity for sources with the active nucleus types determined from the optical data. It is found that this parameter is approximately the same for quasars with different radio luminosity. It isminimum for the strongest radio galaxies and grows up to the level characteristic of quasars with the decrease of radio luminosity. Considering that the ratio depends on obscuring properties of a dust torus, such behavior can be explained if we assume that the torus geometry and its optical depth depend on the source long. This parameter is slightly higher among variable sources than among nonvariable ones which counts in favor of the nucleus more open to an observer.     

Variability Observations of a Complete Sample of Flat-Spectrum Radio Sources: Preliminary Results

This report presents preliminary results of daily observations, over 60and 100 days, of a complete, flux-limited sample of radio sources with flat spectra. The existence of flicker up to 21.7 GHz was confirmed, for sources with flat spectra, on a time-scale of 4 days. A model explaining the flux density variations of the unique radio source 0524+034, on long and short time-scales, by an intrinsic mechanism is proposed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Intrinsic intraday variability in the gravitational lens system B0218+357

Radio monitoring of the gravitational lens system B0218+357 reveals it to be a highly variable source with variations on time-scales of a few days correlated in both images. This shows that the variability is intrinsic to the background lensed source and suggests that similar variations in other intraday variable sources can also be intrinsic in origin.     

EGRET检测到的γ射线活动星系核的射电图像

研究表明活动星系核的高能γ射线辐射和低频射电辐射有着某些内在的联系。ＥＧＲＥＴ已检测到６６ 颗可信度很高的活动星系核。而这些高能的活动星系核有部分缺少足够的射电图象观测。为研究这些河外剧变源的射电辐射性质及致密结构,并研究活动量系核射电喷流弯曲与高能辐射的内在联系。我们利用国际欧洲ＶＬＢＩ网,英国的ＭＥＲＬＩＮ 和美国的ＶＬＡ对近２０ 颗γ射线活动星系核进行了观测。本文给出部分高能γ射线活动星系核在８ ．５ＧＨｚ的ＶＬＡ 观测图象。     

Correlated Radio-Optical Variations on Intraday Timescales

Correlated radio-optical variations on intraday timescales have been observed (e.g. In BLO 0716 714) and such radio intraday variability is suggested to have an intrinsic ori- gin. Recently, multi-wavelength observations, simultaneous at radio, mm-submm, optical and hard X-rays, of 0716 714, show that during a period of intraday/interday variations at ra- dio and mm wavelengths, the apparent brightness temperature of the source exceeded the Compton-limit (～1012 K) by 2--4 orders of magnitude, but no Compton catastrophe (or no high luminosity of inverse-Compton radiation) was detected. It is also found that the intra- day/interday variations at mm-submm wavelengths are consistent with the evolutionary be- havior of a standard synchrotron source and for the intraday/interday variations at centimeter wavelengths opacity effects can play a significant role, which is consistent with the interpreta- tion suggested previously by Qian et al. Thus the apparent high brightness temperatures may probably be explained in terms of Doppler boosting effects due to bulk relativistic motion of the source. We will argue a scenario to simulate the correlations between the radio and optical variations on intraday timescales observed in BLO 0716 714 in terms of a relativistic shock propagating through a jet with a dual structure.     

Surface trapping and leakage of low-frequency g modes in rotating early-type stars – I. Qualitative analysis

We summarize all the reported detections of, and upper limits to, the radio emission from persistent (i.e. non-transient) X-ray binaries. A striking result is a common mean observed radio luminosity from the black hole candidates (BHCs) in the low/hard X-ray state and the neutron star Z sources on the horizontal X-ray branch. This implies a common mean intrinsic radio luminosity to within a factor of 25 (or less, if there is significant Doppler boosting of the radio emission). Unless coincidental, these results imply a physical mechanism for jet formation that requires neither a black hole event horizon nor a neutron star surface. As a whole the populations of Atoll and X-ray pulsar systems are less luminous by factors of ≳5 and ≳10 at radio wavelengths than the BHCs and Z sources (while some Atoll sources have been detected, no high-field X-ray pulsar has ever been reliably detected as a radio source). We suggest that all of the persistent BHCs and the Z sources generate, at least sporadically, an outflow with physical dimensions 10¹² cm; that is, significantly larger than the binary separations of most of the systems. We compare the physical conditions of accretion in each of the types of persistent X-ray binary and conclude that a relatively low (10¹⁰ G) magnetic field associated with the accreting object, and a high (0.1 Eddington) accretion rate and/or dramatic physical change in the accretion flow, are required for formation of a radio-emitting outflow or jet.     

Low frequency observations of the scintillation of weak radio sources

Interplanetary scintillation observations of weak radio sources have been made at a frequency of 102.5 MHz. Sources have been chosen from the Bologna catalogue, with flux densities 0.5 Jy–4.0 Jy at 408 MHz.It has been found that the average scintillation visibility is in agreement with the results of the interferometer observations of Speed and Warwick (1978).A very wide spectral index distribution has been found for weak radio sources at low frequencies, perhaps indicating the presence of a new population of low luminosity, flat spectrum radio sources.