Evidence for accretion disc reprocessing in QSO 0957+561

We use archival g - and r -band photometry of the gravitational lens system QSO 0957+561A, B to estimate the intrinsic variability of the quasar during 1996 February–June. The light curves span 234 d with temporal resolutions of about 2.5 d. Both light curves display a single large-amplitude event, of ∼0.1 mag (max-to-min) in about 100 d, followed by small-amplitude variations of ∼0.02 mag on time-scales of tens of days. We find the r -band variations lag those at g by 3.4_−1.4^+1.5 d for the large-amplitude event. This lag is greater than zero at no less than 98 per cent confidence. The delayed coupling of the rest-frame UV intrinsic variations strongly suggests the existence of a stratified reprocessing region extending ∼light-days from the putative central black hole source. The observed lag is consistent with that expected from a reverberation within an irradiated accretion disc structure. However, any definitive statement requires further detailed theoretical modelling and high-quality, signal-to-noise ratio of about 100, optical/IR simultaneous monitoring with about 3-d resolution for approximately 6 months.     

Variable Radio Echoes from a Jet in QSO 0957 + 561. II. Statistical Significance of the Result

At the previous publication (Oknyanskij, 1997) we considered and used a new algorithm for time-delay investigations in case when the time delay was a linear function of time and the echo response intensity was a power-low function of the delay. We applied this method to investigate optical-to-radio delay in the double quasar 0957 + 561 (generally accepted to be a case of gravitational lensing). It was found in this way that the radio variations (5 MHz) followed the optical ones, but the time delay was a linear function of time with the mean value being about 2370 day and with the rate of increase V≈ 110 days/year. Here we use Monte-Carlo simulations to estimate the significance of the results. We estimate (with the 95% confidence level) that the probability of getting the same result by chance (if real V value was equal to 0 days/year) is less then 5%. We also show that the method can determinate the actual rate of increase Va of the time delay in the artificial light curves, which have the same data spacing, power spectrum and noise level as real ones. We briefly consider some other possible fields for using the method. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of the Difference Light Curve of the Gravitational Mirage QSO 0957+561

From optical data of Q0957+561 for 1995/1996 seasons (in the g band) and 1996/1997 seasons (in the R band) they are derived upper limits on the amplitude of the rapid microlensing fluctuations of 0.07 mag and 0.05 mag, respectively. The photometry in the R band (1996/1997 seasons) was taken at Teide Observatory (IAC-80 Telescope), Instituto de Astrofisica de Canarias, Spain. In this contribution, we also study the microlensing history (from 1981 to 1996) of the Twin QSO, which is a curious system with two optical images of similar brightness. This revised version was published online in July 2006 with corrections to the Cover Date.     

GLITP optical monitoring of QSO 0957+561: VR light curves and variability

The Gravitational Lenses International Time Project (GLITP) collaboration observed the first gravitational lens system (QSO 0957+561) from 2000 February 3 to March 31. The daily VR observations were made with the 2.56-m Nordic Optical Telescope at Roque de los Muchachos Observatory, La Palma, Spain. We have derived detailed and robust VR light curves of the two components Q0957+561A and Q0957+561B. In spite of the excellent sampling rate, we have not found evidence in favour of true daily variability. With respect to variability on time-scales of several weeks, we measure VR gradients of about −0.8 mmag d⁻¹ in Q0957+561A and +0.3 mmag d⁻¹ in Q0957+561B. The gradients are very probably originated in the far source. Thus, adopting this reasonable hypothesis (intrinsic variability), we compare them to the expected gradients during the evolution of a compact supernova remnant at the redshift of the source quasar. The starburst scenario is roughly consistent with some former events, but the new gradients do not seem to be caused by supernova remnant activity.     

Five nights of intensive R- and V-band photometry of QSO 0957+561A,B

We present R - and V -band photometry of the gravitational lens system QSO 0957+561 from five nights (one in 2000 January and four in 2001 March, corresponding to the approximate time delay for the system) of uninterrupted monitoring at the Nordic Optical Telescope. In the photometry scheme we have stressed careful magnitude calibration as well as corrections for the lens galaxy contamination and the crosstalk between the twin (A and B) quasar images. The resulting, very densely sampled, light curves are quite stable, in conflict with earlier claims derived from the same data material. We estimate high-precision timelag-corrected B/A flux ratios in both colour bands, as well as V – R colour indices for A and B, and discuss the short time-scale variability of the system.     

CCD Photometry of QSO 0957+561 A & B for the period 1987–1992 using La Palma archive

We present the results of CCD photometry of the first gravitational lenssystem QSO 0957+561, obtained on the base of La Palma archive. Usingcross-correlation analysis we have found the time delay between A and Bquasar images to be 430 ± 15 days. Our photometry of QSO 0957+561 A & Bgives additional independent confirmation of the recent results on the time delay about 425 days.     

Short time-scale fluctuations in the difference light curves of QSO 0957+561A,B: microlensing or noise?

From optical R -band data of the double quasar QSO 0957+561A,B, we made two new difference light curves (with an overlap of about 330 d between the time-shifted light curve for the A image and the magnitude-shifted light curve for the B image). We observed noisy behaviours around the zero line and no short time-scale events (with a duration of months), where the term 'event' refers to a prominent feature that may be a result of microlensing or another source of variability. Only one event lasting two weeks and rising −33 mmag was found . Measured constraints on the possible microlensing variability can be used to obtain information on the granularity of the dark matter in the main lensing galaxy and the size of the source. In addition, one can also test the ability of the observational noise to cause the rms averages and the local features of the difference signals. We focused on this last issue. The combined photometries were related to a process consisting of an intrinsic signal plus a Gaussian observational noise. The intrinsic signal has been assumed to be either a smooth function (polynomial), a smooth function plus a stationary noise process, or a correlated stationary process . Using these three pictures without microlensing, we derived some models totally consistent with the observations. We finally discussed the sensitivity of our telescope (at Teide Observatory) to several classes of microlensing variability.     

Multiple radio echoes in the solar corona

Echo-type solar radio bursts are associated with preceding short-lived bursts in double events. The peculiar and rather rare decameter echoes are observed with a UTR-2 antenna. The initial narrow-band burst is followed, some 5 to 10 s later, by an echo-like burst at the same frequency. The observational data obtained for decameter echo evens are, on the whole, consistent with the model of a pulsed source emitting radio signals at the plasma-frequency harmonic, which is placed in a non-uniform corona and rotates together with the Sun.Intensity-time profiles of 25 MHz echo bursts are of an unusual shape, featuring an extended leading edge and an abrupt decay at the trailing edge and also showing some fine structures in the form of an additional, weakly pronounced maximum or a step at the final stage of the burst. Time parameters characterizing the profiles are evaluated. The step is delayed with respect to the main pulse at about two times longer than the principal echo maximum. At the same time, the time delays depend essentially on the heliolongitude of the active region and achieve their maximum values at the meridian. The step height does not exceed 0.5 of the echo maximum. At this level, the echo-decay time almost coincides with the initial burst duration but is about 1.7 times less than the echo-rise time. The feature at the echo tail can be interpreted as a result of a repeated reflection of the burst from the source region. The causes and conditions for the formation of multiple echoes are discussed. The extended leading edge of the echo permits us to assume a quasi-radial fibrous structure of the corona, capable of back-scattering the incident radiation.     

Changes in the trajectory of the radio jet in 0735+178?

We present multi-epoch 8.4- and 43-GHz Very Long Baseline Array images of the BL Lac object 0735+178. The images confirm the presence of a twisted jet with two sharp apparent bends of 90° within two milliarcseconds of the core, resembling a helix in projection. The observed twisted geometry could be the result of precession of the jet inlet, but is more likely produced by pressure gradients in the external medium through which the jet propagates. Quasi-stationary components are observed at the locations of the 90° bends, possibly produced by differential Doppler boosting.
Identification of components across epochs, since the earliest VLBI observations of this source in 1979.2, proves difficult due to the sometimes large time gaps between observations. One possible identification suggests the existence of superluminal components following non-ballistic trajectories with velocities up to     . However, in images obtained after mid-1995, components show a remarkable tendency to cluster near several jet positions, suggesting a different scenario in which components have remained nearly stationary in time, at least since mid-1995. Comparison with the earlier published data, covering more than 19 years of observations, suggests a striking qualitative change in the jet trajectory sometime between mid-1992 and mid-1995, with the twisted jet structure with stationary components becoming apparent only at the later epochs. This would require a re-evaluation of the physical parameters estimated for 0735+178, such as the observing viewing angle, the plasma bulk Lorentz factor, and those deduced from these.     

A transient relativistic radio jet from Cygnus X-1

We report the first observation of a transient relativistic jet from the canonical black hole candidate, Cygnus X-1, obtained with the Multi-Element Radio-Linked Interferometer Network (MERLIN). The jet was observed in only one of six epochs of MERLIN imaging of the source during a phase of repeated X-ray spectral transitions in 2004 Jan–Feb, and this epoch corresponded to the softest 1.5–12 keV X-ray spectrum. With only a single epoch revealing the jet, we cannot formally constrain its velocity. Nevertheless, several lines of reasoning suggest that the jet was probably launched 0.5–4.0 d before this brightening, corresponding to projected velocities of  0.2 c ≲ v _app≲ 1.6 c   , and an intrinsic velocity of  ≳0.3 c   . We also report the occurrence of a major radio flare from Cyg X-1, reaching a flux density of ∼120 mJy at 15 GHz, and yet not associated with any resolvable radio emission, despite a concerted effort with MERLIN. We discuss the resolved jet in terms of the recently proposed 'unified model' for the disc–jet coupling in black hole X-ray binaries, and tentatively identify the 'jet line' for Cyg X-1. The source is consistent with the model in the sense that a steady jet appears to persist initially when the X-ray spectrum starts softening, and that once the spectral softening is complete the core radio emission is suppressed and transient ejecta/shock observed. However, there are some anomalies, and Cyg X-1 clearly does not behave like a normal black hole transient in progressing to the canonical soft/thermal state once the ejection event has happened.     

The dying accretion and jet in a powerful radio galaxy of Hercules A

Hercules A(Her A) is one of a rare class of dying and transition-type objects, which has a pair of giant, powerful radio lobes and a weak radio core. We reduce and analyze the radio data of Her A that were observed by the Expanded Very Large Array(EVLA) during 2010-2011 at C band. The intensity distribution is very smooth along the edge of the lobe front and the intensity also sharply decreases at the edge, which supports that magnetic fields may play an important role in radio lobes. The spectrum of the weak core is very steep and the core flux becomes weaker by about ten percent when compared to what was observed twenty years ago, which suggest that the central engine is still dying quickly. Her A deviates a lot from the relation between [O III] luminosity and low-frequency 178 MHz luminosity(LO III-L178 MHz) as defined by other FR I/II sources. However, when only radio core emission is considered, it roughly follows an LO III-L178 MHzcorrelation. This result supports that the black-hole accretion and large-scale jet in Her A did not evolve simultaneously, and indicates that although the large-scale jet is still powerful, the accretion and inner jet have changed into an inactive state. Based on the estimated Bondi accretion rate, we model the spectrum of Her A with a radiatively inefficient accretion flow and jet model.     

A possible EGRET source with a two-sided radio jet structure on a parsec scale – 3EG J0808+5114

Recent observations have revealed that damped Lyα clouds (DLAs) host star formation activity. In order to examine if such star formation activity can be triggered by ionization fronts, we perform high-resolution hydrodynamics and radiative transfer simulations of the effect of radiative feedback from propagating ionization fronts on high-density clumps. We examine two sources of ultraviolet (UV) radiation field to which high-redshift ( z ∼ 3) galaxies could be exposed: one corresponding to the UV radiation originating from stars within the DLA, itself, and the other corresponding to the UV background radiation. We find that, for larger clouds, the propagating I-fronts created by local stellar sources can trigger cooling instability and collapse of significant part, up to 85 per cent, of the cloud, creating conditions for star formation in a time-scale of a few Myr. The passage of the I-front also triggers collapse of smaller clumps (with radii below ∼4 pc), but in these cases the resulting cold and dense gas does not reach conditions conducive to star formation. Assuming that 85 per cent of the gas initially in the clump is converted into stars, we obtain a star formation rate of  ∼0.25 M_⊙ yr⁻¹ kpc⁻²  . This is somewhat higher than the value derived from recent observations. On the other hand, the background UV radiation which has harder spectrum fails to trigger cooling and collapse. Instead, the hard photons which have long mean free-path heat the dense clumps, which as a result expand and essentially dissolve in the ambient medium. Therefore, the star formation activity in DLAs is strongly regulated by the radiative feedback, both from the external UV background and internal stellar sources and we predict quiescent evolution of DLAs (not starburst-like evolution).     

Three episodes of jet activity in the Fanaroff–Riley type II radio galaxy B0925+420

We present Very Large Array images of a 'double–double radio galaxy', a class of objects in which two pairs of lobes are aligned either side of the nucleus. In this object, B0 925+420, we discover a third pair of lobes, close to the core and again in alignment with the other lobes. This first-known 'triple–double' object strongly increases the likelihood that these lobes represent multiple episodes of jet activity, as opposed to knots in an underlying jet. We model the lobes in terms of their dynamical evolution. We find that the inner pair of lobes are consistent with the outer pair having been displaced buoyantly by the ambient medium. The middle pair of lobes are more problematic – to the extent where an alternative model interpreting the middle and inner 'lobes' as additional bow shocks within the outer lobes may be more appropriate – and we discuss the implications of this on our understanding of the density of the ambient medium.     

On the resolved radio emission from AG Draconis: evidence for jet ejection?

The recent detection of resolved radio emission from AG Dra by MERLIN reported by Ogley et al. is discussed in the context of the wind environment and the physical parameters and geometry of this symbiotic binary system. In particular, it is shown that the two radio components are closely aligned with the binary axis, and their separation suggests their origin in jets ejected from AG Dra during the recent 1995–98 series of oubtbursts.     

ROSAT X射线源1057＋4316被证认为一个z＝0．32的类星体

根据北京天文台２．１６ｍ望远镜的有缝光谱观测，证认出位于１０ｈ５７ｍ４８．５ｓ＋４３°１６′１３″（１９５０．０）的一个ＲＯＳＡＴＸ射线源是一个ｚ＝０．３２的类星体。     

Pair annihilation and radio emission from galactic jet sources: the case of Nova Muscae

In the hard X-ray spectra of some X-ray binaries line features at around 500 keV are detected. We interpret these as arising from pair annihilation in relativistic outflows leading to a significant Doppler shift of the frequencies of the lines. We show how this can be used to accurately determine the bulk velocity and orientation to the line of sight of the outflows. Constraints on the energy requirements of such outflows are also derived. Furthermore, we show that a small fraction of pairs escaping the annihilation region may give rise to the radio synchrotron emission observed in some of these objects. We apply these ideas to the hard X-ray and radio observations of Nova Muscae 1991. In this object, the energy requirements seem to rule out a large proton fraction in the outflows.