We report the results of intensive X-ray, UV and optical monitoring of the Seyfert 1 galaxy NGC 4593 with Swift. There is no intrinsic flux-related spectral change in the the variable components in any band with small apparent variations due only to contamination by a second constant component, possibly a (hard) reflection component in the X-rays and the (red) host galaxy in the UV/optical bands. Relative to the shortest wavelength band, UVW2, the lags of the other UV and optical bands are mostly in agreement with the predictions of reprocessing of high energy emission from an accretion disc. The U-band lag is, however, far larger than expected, almost certainly because of reprocessed Balmer continuum emission from the more distant broad line region gas. The UVW2 band is well correlated with the X-rays but lags by ∼ 6× more than expected if the UVW2 results from reprocessing of X-rays on the accretion disc. However, if the lightcurves are filtered to remove variations on timescales > 5d, the lag approaches the expectation from disc reprocessing. MEMEcho analysis shows that direct X-rays can be the driver of most of the variations in the UV/optical bands as long as the response functions for those bands all have long tails (up to 10d) in addition to a strong peak (from disc reprocessing) at short lag (< 1 d). We interpret the tails as due to reprocessing from the surrounding gas. Comparison of X-ray to UVW2 and UVW2 to V-band lags for 4 AGN, including NGC 4593, shows that all have UVW2 to V-band lags which exceed the expectations from disc resprocessing by ∼ < 2. However the X-ray to UVW2 lags are, mostly, in greater excess from the expectations from disc reprocessing and differ between AGN. The largest excess is in NGC 4151. Absorption and scattering may be affecting X-ray to UV lags.
We investigate the dependence of residual rotation measure (RRM) on intervening absorption systems at cosmic distances by using a large sample of 539 SDSS quasars in conjunction with the available rotation measure catalog at around 21cm wavelength. We found an excess extragalactic contribution in standard deviation of observed RRM (σ rrm ) of about 8.11±4.83 rad m −2 in our sample with intervening Mg ii absorber as compare to the sample without Mg ii absorber. Our results suggest that intervening absorbers could contribute to the enhancement of RRM at around 21cm wavelength, as was found earlier for RM measurements at around 6cm wavelength.
In recent years, several Radio-Loud Narrow-Line Seyfert 1 galaxies (RL-NLS1) possessing relativistic jets have come into attention with their detections in Very Large Baseline Array (VLBA) and in γ-ray observations. In this paper we attempt to understand the nature of radio-jets in NLS1s by examining the kpc-scale radio properties of, hitherto, the largest sample of 11101 optically-selected NLS1s. Using 1.4 GHz FIRST, 1.4 GHz NVSS, 327 MHz WENNS, and 150 MHz TGSS catalogues we find the radiodetection of merely ∼ 4.5 per cent (498/11101) NLS1s, with majority (407/498 ∼ 81.7 per cent) of them being RL-NLS1s. Our study yields the highest number of RL-NLS1s and it can only be a lower limit. We find that the most of our radio-detected NLS1s are compact (< 30 kpc), exhibit both flat as well as steep radio spectra, and are distributed across a wide range of 1.4 GHz radio luminosities (10 22 − 10 27 W Hz −1 ). At the high end of radio luminosity our NLS1s often tend to show blazar-like properties such as compact radio-size, flat/inverted radio spectrum, radio variability and polarization. The diagnostic plots based on the mid-IR colours suggest that the radio emission in NLS1s is mostly powered by AGN, while nuclear star-formation may have a significant contribution in NLS1s of low radio luminosities. The radio luminosity versus radio-size plot infers that the radio-jets in NLS1s are either in the early evolutionary phase or possibly remain confined within the nuclear region due to low-power or intermittent AGN activity.
The origin of short timescale (weeks/months) variability of AGN, whether due to intrinsic disc variations or reprocessing of X-ray emission by a surrounding accretion disc, has been a puzzle for many years. However recently a number of observational programmes, particularly of NGC5548 with Swift, have shown that the UV/optical variations lag behind the X-ray variations in a manner strongly supportive of X-ray reprocessing. Somewhat surprisingly the implied size of the accretion disc is ∼ 3× greater than expected from a standard, smooth, Shakura-Sunyaev thin disc model. Although the difference may be explained by a clumpy accretion disc, it is not clear whether the difference will occur in all AGN or whether it may change as, eg, a function of black hole mass, accretion rate or disc temperature. Measurements of interband lags for most AGN require long timescale monitoring, which is hard to arrange. However for low mass (< 10 6 M⊙) AGN, the combination of XMM-Newton EPIC (X-rays) with the optical monitor in fast readout mode allows an X-ray/UVoptical lag to be measured within a single long observation. Here we summarise previous related observations and report on XMM-Newton observations of NGC4395 (mass 100× lower, accretion rate ∼ 20× lower than for NGC5548). We find that the UVW1 lags the X-rays by ∼ 470s. Simultaneous observations at 6 different ground based observatories also allowed the g-band lag (∼ 800s) to be measured. These observations are in agreement with X-ray reprocessing but initial analysis suggests that, for NGC4395, they do not differ markedly from the predictions of the standard thin disc model. Models for UV/Optical Variability and relationship to X-ray VariabilityThe origin of UV/optical variability in AGN and its relationship to X-ray variability has been a puzzle for some time and there are two main possibilities for the origin of the UV/optical variability. The UV/optical variability could result from reprocessing of X-ray emission by the accretion disc or it could simply be the result of intrinsic variability of the thermal emission from the disc. These two models can, in principle, be distinguished simply by measuring the lag between the X-ray and UV/optical wavebands. In the reprocessing model, the UV/optical variations will lag behind the X-ray variations by the light travel time between the two emission regions. For a typical AGN this time will be a few hours. If the UV/optical variations are produced by intrinsic ⋆ Corresponding author: e-mail: email@example.com disc variations there are two possible lag timescales. If the UV/optical photons are the seed photons for the X-ray emission, being Compton up-scattered in the central corona, then if the X-ray variations are driven by seed photon variations, the X-ray emission will lag behind the UV/optical variations by the light travel time between the two emission regions, ie a few hours. Alternatively, if the UV/optical variations are caused by inwardly propagating accretion rate variations (Arévalo & Uttley 2006), these variations will ev...
For 3 radio-loud γ-ray detected Narrow-Line Seyfert 1 (γ-ray NLSy1) galaxies, we report optical variability on intra-night and/or week-like time scales, based on five 3 hours long monitoring sessions for each galaxy. The radio-loudness factors (R 1.4GHz ) 1 for these galaxies, namely 1H 0323+342 (z = 0.0629), PKS J1222+0413 (z = 0.966) and PKS J1505+0326 (z = 0.408) are ∼318, ∼1534 and ∼3364 at 1.4 GHz, respectively. For the most distant γ-ray NLSy1, PKS J1222+0413, Intra-Night Optical Variability (INOV) characterisation is presented for the first time. The blazar-like behaviour of the nearest γ-ray NLSy1 1H 0323+342, which showed strong INOV on 4 of the 5 nights, was unexpected in view of its recent reclassification as radio intermediate (R 5GHz 25). Its particularly violent INOV is manifested by two optical outbursts lasting ∼ 1 hour, whose rapid brightening phase is shown to imply a doubling time of ∼ 1 hour for the optical synchrotron flux, after (conservatively) deducting the thermal optical emission contributed by the host galaxy and the Seyfert nucleus. A more realistic decontamination could well reduce substantially the flux doubling time, bringing it still closer in rapidity to the ultra-fast VHE (> 100 GeV) flares reported for the blazars PKS 1222+216 and PKS 2155−304. A large contamination by thermal optical emission may, in fact, be common for NLSy1s as they are high Eddington rate accretors. The present study further suggests that superluminal motion in the radio jet could be a robust diagnostic of INOV.
In recent years, breakthroughs in methods and data have enabled gravitational time delays to emerge as a very powerful tool to measure the Hubble constant H0. However, published state-of-the-art analyses require of order 1 year of expert investigator time and up to a million hours of computing time per system. Furthermore, as precision improves, it is crucial to identify and mitigate systematic uncertainties. With this time delay lens modelling challenge we aim to assess the level of precision and accuracy of the modelling techniques that are currently fast enough to handle of order 50 lenses, via the blind analysis of simulated datasets . The results in Rung 1 and Rung 2 show that methods that use only the point source positions tend to have lower precision ($10-20\%$) while remaining accurate. In Rung 2, the methods that exploit the full information of the imaging and kinematic datasets can recover H0 within the target accuracy (|A| < 2%) and precision (<6% per system), even in the presence of a poorly known point spread function and complex source morphology. A post-unblinding analysis of Rung 3 showed the numerical precision of the ray-traced cosmological simulations to be insufficient to test lens modelling methodology at the percent level, making the results difficult to interpret. A new challenge with improved simulations is needed to make further progress in the investigation of systematic uncertainties. For completeness, we present the Rung 3 results in an appendix and use them to discuss various approaches to mitigating against similar subtle data generation effects in future blind challenges.
In a systematic program to characterise the intra-night optical variability (INOV) of different classes of narrow-line Seyfert 1 galaxies (NLSy1s), we report here the first comparative INOV study of NLSy1 sets detected in the X-ray and γ-ray bands. Our sample consists of 18 sources detected in X-ray but not in γ-rays (hereafter x NLSy1s) and 7 sources detected in γ-rays (hereafter g NLSy1s), out of which 5 are detected also in X-rays. We have monitored these two sets of NLSy1s, respectively, in 24 and 21 sessions of a minimum of 3 hours duration each. The INOV duty cycles for these two sets are found to be 12% and 53%, respectively (at a 99% confidence level). In the set of 18 x NLSy1s, INOV duty cycle is found to be zero for the 13 radio-quiet members (monitored in 14 sessions) and 43% for the 5 radio-loud members (10 sessions). The latter is very similar to the aforementioned duty cycle of 53% found here for the set of g NLSy1s (all of which are radio-loud). Thus it appears that the radio loudness level is the prime factor behind the INOV detection and the pattern of the high-energy radiation plays only a minor role.
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