341 research outputs found
The relationship between the x-ray and optical variability in Seyfert Galaxies
No full textThe relationship between the X-ray and optical variability in Seyfert galaxies by Elmé Breedt Lategan Seyfert galaxies have been known to vary since shortly after their discovery, but the origin of the optical variability and its relationship to the X-rays, have not been well established. Presented here is a comprehensive comparison between the long term X-ray and optical emission in a sample of seven Seyfert galaxies, to determine the relationship between the X-ray and optical variations. It is shown that the observed optical variability is the sum of several independent mechanisms: the short time-scale variability is dominated by X-ray reprocessing, but variations intrinsic to the disc, such as propagating accretion rate fluctuations, add considerable variability power on long time-scales.Cross-correlation analysis reveal a statistically significant correlation between the X-ray and optical emission of all the galaxies under investigation, with the optical variations following the X-rays by 1–2 days. This time-scale is consistent with the expected light travel time from the central X-ray source to the optically emitting region of the accretion disc. Although this is a strong indicator of X-ray reprocessing by the disc, it is also clear that reprocessing cannot account for all the optical variations observed. At least one source displays clear long time-scale (years) variations in the optical with no apparent counterpart in the X-rays, and in others the long term optical amplitude exceed that of the X-rays. It is shown that these long time-scale optical variations must originate from an independent variability mechanism, possibly variations in the accretion rate or the geometry of the system. There is also evidence that there may be a significant contribution to the optical emission from as far out as the dust torus.Finally it is shown that there may be a dependence of the strength of the X-ray/optical correlation on the disc temperature and black hole mass
Correlation and time delays of the X-ray and optical emission of the Seyfert Galaxy NGC 3783
No full textWe present simultaneous X-ray and optical B- and V-band light curves of the Seyfert Galaxy NGC 3783 spanning 2 years. The flux in all bands is highly variable and the fluctuations are significantly correlated. As shown before by Stirpe et al. the optical bands vary simultaneously, with a delay of less than 1.5 d but both B and V bands lag the X-ray fluctuations by 3–9 d. This delay points at optical variability produced by X-ray reprocessing and the value of the lag places the reprocessor close to the broad-line region. A power spectrum analysis of the light curve, however, shows that the X-ray variability has a power-law shape bending to a steeper slope at a time-scale of ?2.9 d while the variability amplitude in the optical bands continues to grow towards the longest time-scale covered, ?300 d. We show that the power spectra together with the small value of the time delay are inconsistent with a picture where all the optical variability is produced by X-ray reprocessing, though the small amplitude, rapid optical fluctuations might be produced in this way. We detect larger variability amplitudes on long time-scales in the optical bands than in the X-rays. This behaviour adds to similar results recently obtained for at least three other active galactic nuclei and indicates a separate source of long-term optical variability, possibly accretion rate or thermal fluctuations, in the optically emitting accretion disc.<br/
1000 cataclysmic variables from the Catalina Real-time Transient Survey
No full textOver six years of operation, the Catalina Real-time Transient Survey (CRTS) has identified 1043 cataclysmic variable (CV) candidates - the largest sample of CVs from a single survey to date. Here, we provide spectroscopic identification of 85 systems fainter than g > 19, including three AM Canum Venaticorum binaries, one helium-enriched CV, one polar and one new eclipsing CV. We analyse the outburst properties of the full sample and show that it contains a large fraction of low-accretion-rate CVs with long outburst recurrence times. We argue that most of the high-accretion-rate dwarf novae in the survey footprint have already been found and that future CRTS discoveries will be mostly low-accretion-rate systems. We find that CVs with white-dwarf-dominated spectra have significantly fewer outbursts in their CRTS light curves compared to disc-dominated CVs, reflecting the difference in their accretion rates. Comparing the CRTS sample to other samples of CVs, we estimate the overall external completeness to be 23.6 per cent, but show that as much as 56 per cent of CVs have variability amplitudes that are too small to be selected using the transient selection criteria employed by current ground-based surveys. The full table of CRTS CVs, including their outburst and spectroscopic properties examined in this paper, is provided in the online materials
Correlated X-ray/ultraviolet/optical variability in the very low mass AGN NGC 4395
No full textWe report the results of a 1-yr Swift X-ray/ultraviolet (UV)/optical programme monitoring the dwarf Seyfert nucleus in NGC4395 in 2008-2009. The UV/optical flux from the nucleus was found to vary dramatically over the monitoring period, with a similar pattern of variation in each of the observed UV/optical bands (spanning 1900-5500 Å). In particular, the luminosity of NGC4395 in the 1900 Å band changed by more than a factor of 8 over the monitoring period. The fractional variability was smaller in the UV/optical bands than that seen in the X-rays, with the X-ray/optical ratio increasing with increasing flux. Pseudo-instantaneous flux measurements in the X-ray and each UV/optical band were well correlated, with cross-correlation coefficients of ≥0.7, significant at 99.9per cent confidence. Archival Swift observations from 2006 sample the intra-day X-ray/optical variability on NGC4395. These archival data show a very strong correlation between the X-ray and b bands, with a cross-correlation coefficient of 0.84 (significant at >99per cent confidence). The peak in the cross-correlation function is marginally resolved and asymmetric, suggesting that X-rays lead the b band, but by ≤1h. In response to recent (2011 August) very high X-ray flux levels from NGC4395 we triggered Swift target of opportunity observations, which sample the intra-hour X-ray/UV variability. These observations indicate, albeit with large uncertainties, a lag of the 1900 Å band behind the X-ray flux of ∼400s. The tight correlation between the X-ray and UV/optical lightcurves, together with the constraints we place on the lag time-scale, is consistent with the UV/optical variability of NGC4395 being primarily due to reprocessing of X-ray photons by the accretion disc
Radio and X-ray variability in the Seyfert galaxy NGC 4051
No full textWe present intensive quasi-simultaneous X-ray and radio monitoring of the narrow line Seyfert 1 galaxy NGC 4051, over a 16-month period in 2000-01. The X-ray observations were made with the Proportional Counter Array on the Rossi Timing X-ray Explorer (RXTE) and radio observations were made at 8.4 and 4.8 GHz with the Very Large Array (VLA). In the X-ray band NGC 4051 behaves very much like the analogue of a Galactic black hole binary (GBH) system in a 'soft state'. In such systems, there has so far been no firm evidence for an active, radio-emitting jet like those found in 'hard-state' GBHs. VLBI observations of NGC 4051 show three well-separated compact components almost in a line. This structure resembles the core and outer hot spots seen in powerful, jet-dominated, extragalactic radio sources and, although no jet is visible in NGC 4051, suggests that a weak jet may exist. However it has not previously been clear whether the nucleus is currently active in the radio band and whether there is any link between the radio and X-ray emission processes.
Radio monitoring of the core of NGC 4051 is complicated by the presence of surrounding extended emission and by the changing array configurations of the VLA. Only in the A configuration is the core reasonably resolved. We have carefully removed the differing contaminations of the core by extended emission in the various arrays. The resulting lightcurve shows no sign of large amplitude variability (i.e. factor 50 per cent) over the 16-month period and is consistent with being constant. Within the 6 A configuration observations where we have greatest sensitivity we see marginal evidence for radio core variability of similar to 25 per cent (similar to 0.12 mJy at 8.4 GHz) on a 2-week time-scale, correlated with X-ray variations. These percentage variations are similar to those of the Seyfert galaxy NGC 5548, which is 10 times brighter. Even if the radio variations in NGC 4051 are real, the percentage variability is much less than in the X-ray band. Within the B configuration observations, although sensitivity is somewhat reduced, there is no sign of correlated X-ray/radio variability.
NGC 4051 is one decade lower in radio luminosity than the radio/X-ray/mass fundamental plane for jet-dominated hard-state black holes, although the scatter around the plane is of the same order. The lack of radio variability commonly seen in hard-state GBHs may be explained by orientation effects. Another possibility, consistent with the lack of radio variability, is that the radio emission arises from the X-ray corona although, in that case, the linear structure of the compact radio components is hard to explain. A combination of corona and jet may explain the observations
Pläne und Ansichten / 51 Beschryving van de stad Jedo, hoofdstad van't japansche ryk, en zetel van den opper monarch van Japan : = Plan de Jedo, capitale du Japon, et résidence du monarque seculier
No full textgetekent na een japansche kaart, die bewaart word in't kabinet van den Ridder Hans Sloan, van vier en een halve engelsche voet lang en zo veele breedt door J.G. ScheuchzerWappen links und rechts, bildliche Darstellung ("Marques d'honneurs ...") unten recht
Pläne und Ansichten / 52 Platte grondt van de stad Miaco, zetel van den geestelyken erf-keyzer van Japan : = Plan de Miaco résidence de l'empereur ecclésiastique
No full textna een origineele japansche kaart van vyf en een halve voet lang, en vier voet breedt, onder den Ridder Hans Sloane berustende, in't klein gebracht door J.G. ScheuchserNiederländischer Titel links, französischer Titel oben recht
Correlated X-ray/optical variability in the quasar MR?2251?178
No full textEmission from active galactic nuclei is known to vary strongly over time over a wide energy band, but the origin of the variability and especially of the interband correlations is still not well established. Here we present the results of our X-ray and optical monitoring campaign of the quasar MR 2251?178, covering a period of 2.5 years. The X-ray 2–10 keV flux is remarkably well correlated with the optical B, V and R bands, their fluctuations are almost simultaneous with a delay consistent with 0 d and not larger than 4 d in either direction. The amplitude of variations shows an intriguing behaviour: rapid, large amplitude fluctuations over tens of days in the X-rays have only small counterparts in the optical bands, while the long-term trends over hundreds of days are stronger in the B band than in X-rays. We show that simple reprocessing models, where all the optical variability arises from the variable X-ray heating, cannot simultaneously explain the discrepant variability amplitudes on different time-scales and the short delays between X-ray and optical bands. We interpret the variability and correlations, in the optically thick accretion disc plus corona scenario, as the result of intrinsic accretion rate variations modulating both X-ray and optical emission, together with reprocessing of X-rays by the accretion disc.<br/
The physical properties of AM CVn stars: new insights from Gaia DR2
No full textAM CVn binaries are hydrogen deficient compact binaries with an orbital period in the 5-65 min range and are predicted to be strong sources of persistent gravitational wave radiation. Using Gaia Data Release 2, we present the parallaxes and proper motions of 41 out of the 56 known systems. Compared to the parallax determined using the HST Fine Guidance Sensor we find that the archetype star, AM CVn, is significantly closer than previously thought. This resolves the high luminosity and mass accretion rate which models had difficulty in explaining. Using Pan-STARRS1 data we determine the absolute magnitude of the AM CVn stars. There is some evidence that donor stars have a higher mass and radius than expected for white dwarfs or that the donors are not white dwarfs. Using the distances to the known AM CVn stars we find strong evidence that a large population of AM CVn stars have still to be discovered. As this value sets the background to the gravitational wave signal of LISA, this is of wide interest. We determine the mass transfer rate for 15 AM CVn stars and find that the majority have a rate significantly greater than expected from standard models. This is further evidence that the donor star has a greater size than expected
The frequency of gaseous debris discs around white dwarfs
No full textA total of 1-3 per cent of white dwarfs are orbited by planetary dusty debris detectable as infrared emission in excess above the white dwarf flux. In a rare subset of these systems, a gaseous disc component is also detected via emission lines of the Ca ii 8600 Å triplet, broadened by the Keplerian velocity of the disc. We present the first statistical study of the fraction of debris discs containing detectable amounts of gas in emission at white dwarfs within a magnitude and signal-to-noise ratio limited sample. We select 7705 single white dwarfs spectroscopically observed by the Sloan Digital Sky Survey (SDSS) and Gaia with magnitudes g ≤ 19. We identify five gaseous disc hosts, all of which have been previously discovered. We calculate the occurrence rate of a white dwarf hosting a debris disc detectable via Ca ii emission lines as per cent. This corresponds to an occurrence rate for a dusty debris disc to have an observable gaseous component in emission as 4 ± per cent. Given that variability is a common feature of the emission profiles of gaseous debris discs, and the recent detection of a planetesimal orbiting within the disc of SDSS J122859.93+104032.9, we propose that gaseous components are tracers for the presence of planetesimals embedded in the discs and outline a qualitative model. We also present spectroscopy of the Ca ii triplet 8600 Å region for 20 white dwarfs hosting dusty debris discs in an attempt to identify gaseous emission. We do not detect any gaseous components in these 20 systems, consistent with the occurrence rate that we calculated
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