Quasar Tomography: Unification of Echo Mapping and Photoionisation Models

Reverberation mapping uses time-delayed variations in photoionised emission lines to map the geometry and kinematics of emission-line gas in active galactic nuclei. In previous work, the light travel time delay tau=R(1+cos(theta))/c and Doppler shift v give a 2-d map Psi(tau,v) for each emission line. Here we combine the velocity-delay information with photoionisation physics in a maximum entropy fit to the full reverberating spectrum F_lam(lam,t) to recover a 5-d map of the differential covering fraction f(R,theta,n,N,v), with n and N the density and column density of the gas clouds. We test the method for a variety of geometries (shells, rings, disks, clouds, jets) by recovering a 3-d map f(R,theta,n) from reverberations in 7 uv emission lines. The best test recovers a hollow shell geometry, defining R to 0.15 dex, n to 0.3 dex, and ionisation parameter U ~ 1/(n R^2) to 0.1 dex. The results are sensitive to the adopted distance and luminosity, suggesting that these parameters may be measurable as well.Comment: Accepted 4 Sep 2002 for publication in MNRA

Quasar broad absorption line variability measurements using reconstructions of un-absorbed spectra

We present a two-epoch Sloan Digital Sky Survey and Gemini/GMOS+William Herschel Telescope/ISIS variability study of 50 broad absorption line quasars of redshift range 1.9 < z < 4.2, containing 38 Si IV and 59 C IV BALs and spanning rest-frame time intervals of approximately 10 months to 3.7 years. We find that 35/50 quasars exhibit one or more variable BALs, with 58% of Si IV and 46% of C IV BALs showing variability across the entire sample. On average, Si IV BALs show larger fractional change in BAL pseudo equivalent width than C IV BALs, as referenced to an unabsorbed continuum+emission-line spectrum constructed using non-negative matrix factorisation. No correlation is found between BAL variability and quasar luminosity, suggesting that ionizing continuum changes do not play a significant role in BAL variability (assuming the gas is in photoionization equilibrium with the ionizing continuum). A subset of 14 quasars have one variable BAL from each of Si IV and C IV with significant overlap in velocity space and for which variations are in the same sense (strengthening or weakening) and which appear to be correlated (98% confidence). We find examples of both appearing and disappearing BALs in weaker/shallower lines with disappearance rates of 2.3% for C IV and 5.3% for Si IV, suggesting average lifetimes of 142 and 43 years respectively. We identify 5 objects in which the BAL is coincident with the broad emission-line, but appears to cover only the continuum source. Assuming a clumpy inhomogeneous absorber model and a typical size for the continuum source, we infer a maximum cloud radius of 10^13 to 10^14 cm, assuming Eddington limited accretion.Comment: Accepted for publication in MNRAS. 22 pages, 12 figures, 7 table

Broad Absorption Line Quasar catalogues with Supervised Neural Networks

We have applied a Learning Vector Quantization (LVQ) algorithm to SDSS DR5 quasar spectra in order to create a large catalogue of broad absorption line quasars (BALQSOs). We first discuss the problems with BALQSO catalogues constructed using the conventional balnicity and/or absorption indices (BI and AI), and then describe the supervised LVQ network we have trained to recognise BALQSOs. The resulting BALQSO catalogue should be substantially more robust and complete than BI- or AI-based ones.Comment: 5 pages, 3 figures, to appear in the proceedings of "Classification and Discovery in Large Astronomical Surveys", Ringberg Castle, 14-17 October 200

Gamma-ray bursts: Restarting the Engine

Recent gamma-ray burst observations have revealed late-time, highly energetic events which deviate from the simplest expectations of the standard fireball picture. Instead they may indicate that the central engine is active or restarted at late times. We suggest that fragmentation and subsequent accretion during the collapse of a rapidly rotating stellar core offers a natural mechanism for this.Comment: 8 pages, 1 figure; accepted by ApJ Letter

Accretion disk reverberation with Hubble Space Telescope observations of NGC 4593 : evidence for diffuse continuum lags

K.H. acknowledges support from STFC grant ST/M001296/1.The Seyfert 1 galaxy NGC 4593 was monitored spectroscopically with the Hubble Space Telescope as part of a reverberation mapping campaign that also included Swift, Kepler and ground-based photometric monitoring. During 2016 July 12 - August 6, we obtained 26 spectra across a nearly continuous wavelength range of ~1150 - 10,000 Å. These were combined with Swift data to produce a UV/optical "lag spectrum", which shows the interband lag relative to the Swift UVW2 band as a function of wavelength. The broad shape of the lag spectrum appears to follow the τ ∝ λ 4/3 relation seen previously in photometric interband lag measurements of other active galactic nuclei (AGN). This shape is consistent with the standard thin disk model but the magnitude of the lags implies a disk that is a factor of ~3 larger than predicted, again consistent with what has been previously seen in other AGN. In all cases these large disk sizes, which are also implied by independent gravitational microlensing of higher-mass AGN, cannot be simply reconciled with the standard model. However the most striking feature in this higher resolution lag spectrum is a clear excess around the 3646 Å Balmer jump. This strongly suggests that diffuse emission from gas in the much larger broad-line region (BLR) must also contribute significantly to the interband lags. While the relative contributions of the disk and BLR cannot be uniquely determined in these initial measurements, it is clear that both will need to be considered in comprehensively modeling and understanding AGN lag spectra.Publisher PDFPeer reviewe

Evidence for variability time-scale-dependent UV/X-ray delay in Seyfert 1 AGN NGC 7469

MP acknowledges Royal Society-SERB Newton International Fellowship support funded jointly by the Royal Society, UK and the Science and Engineering Board of India (SERB) through Newton–Bhabha Fund. IMcH acknowledges support from a Royal Society Leverhulme Trust Research Fellowship LT160006 and from STFC grant ST/M001326/1. EMC gratefully acknowledges support from the National Science Foundation through award number AST-1909199. KH acknowledges support from STFC grant ST/R000824/1.Using a month-long X-ray light curve from RXTE/PCA and 1.5 month-long UV continuum light curves from IUE spectra in 1220-1970 Å, we performed a detailed time-lag study of the Seyfert 1 galaxy NGC 7469. Our cross-correlation analysis confirms previous results showing that the X-rays are delayed relative to the UV continuum at 1315 Å by 3.49 ± 0.22 d, which is possibly caused by either propagating fluctuation or variable Comptonization. However, if variations slower than 5 d are removed from the X-ray light curve, the UV variations then lag behind the X-ray variations by 0.37 ± 0.14 d, consistent with reprocessing of the X-rays by a surrounding accretion disc. A very similar reverberation delay is observed between Swift/XRT X-ray and Swift/UVOT UVW2, U light curves. Continuum light curves extracted from the Swift/GRISM spectra show delays with respect to X-rays consistent with reverberation. Separating the UV continuum variations faster and slower than 5 d, the slow variations at 1825 Å lag those at 1315 Å by 0.29 ± 0.06 d, while the fast variations are coincident (0.04 ± 0.12 d). The UV/optical continuum reverberation lag from IUE, Swift, and other optical telescopes at different wavelengths are consistent with the relationship: τ ∝  λ4/3, predicted for the standard accretion disc theory while the best-fitting X-ray delay from RXTE and Swift/XRT shows a negative X-ray offset of ∼0.38 d from the standard disc delay prediction.Publisher PDFPeer reviewe

Continuum reverberation mapping of Mrk 876 over three years with remote robotic observatories

Funding: Research at UC Irvine is supported by NSF grant AST-1907290. HL acknowledges a Daphne Jackson Fellowship sponsored by the Science and Technology Facilities Council (STFC), UK. ERC acknowledges support by the NRF of South Africa. TT acknowledges support from NSF through grant NSF-AST-1907208.Continuum reverberation mapping probes the sizescale of the optical continuum-emitting region in active galactic nuclei (AGN). Through 3 years of multiwavelength photometric monitoring in the optical with robotic observatories, we perform continuum reverberation mapping on Mrk~876. All wavebands show large amplitude variability and are well correlated. Slow variations in the light curves broaden the cross-correlation function (CCF) significantly, requiring detrending in order to robustly recover interband lags. We measure consistent interband lags using three techniques (CCF, JAVELIN, PyROA), with a lag of around 13~days from u to z. These lags are longer than the expected radius of 12~days for the self-gravitating radius of the disk. The lags increase with wavelength roughly following λ4/3, as would be expected from thin disk theory, but the lag normalization is approximately a factor of 3 longer than expected, as has also been observed in other AGN. The lag in the i band shows an excess which we attribute to variable Hα broad-line emission. A flux-flux analysis shows a variable spectrum that follows fν ∝ λ-1/3 as expected for a disk, and an excess in the i band that also points to strong variable Hα emission in that band.Publisher PDFPeer reviewe

The Swift X-ray flaring afterglow of GRB 050607

The unique capability of the Swift satellite to perform a prompt and autonomous slew to a newly detected Gamma-Ray Burst (GRB) has yielded the discovery of interesting new properties of GRB X-ray afterglows, such as the steep early lightcurve decay and the frequent presence of flares detected up to a few hours after the GRB trigger. We present observations of GRB 050607, the fourth case of a GRB discovered by Swift with flares superimposed on the overall fading X-ray afterglow. The flares of GRB 050607 were not symmetric as in previously reported cases, showing a very steep rise and a shallower decay, similar to the Fast Rise, Exponential Decay that are frequently observed in the gamma-ray prompt emission. The brighter flare had a flux increase by a factor of approximately 25,peaking for 30 seconds at a count rate of approximately 30 counts s-1, and it presented hints of addition short time scale activity during the decay phase. There is evidence of spectral evolution during the flares. In particular, at the onset of the flares the observed emission was harder, with a gradual softening as each flare decayed. The very short time scale and the spectral variability during the flaring activity are indicators of possible extended periods of energy emission by the GRB central engine. The flares were followed by a phase of shallow decay, during which the forward shock was being refreshed by a long-lived central engine or by shells of lower Lorentz factors, and by a steepening after approximately 12 ks to a decay slope considered typical of X-ray afterglows.Comment: 23 pages, 5 figures, Accepted by the Astrophysical Journa

A complex dust morphology in the high-luminosity AGN Mrk 876

Recent models for the inner structure of active galactic nuclei (AGN) advocate the presence of a radiatively accelerated, dusty outflow launched from the outer regions of the accretion disk. Here we present the first near-infrared (near-IR) variable (rms) spectrum for the high-luminosity, nearby AGN Mrk 876. We find that it tracks the accretion disk spectrum out to longer wavelengths than the mean spectrum due to a reduced dust emission. The implied outer accretion disk radius is consistent with the infrared results predicted by a contemporaneous optical accretion disk reverberation mapping campaign and much larger than the self-gravity radius. The reduced flux variability of the hot dust could be either due to the presence of a secondary, constant dust component in the mean spectrum or introduced by the destructive superposition of the dust and accretion disk variability signals or some combination of both. Assuming thermal equilibrium for optically thin dust, we derive the luminosity-based dust radius for different grain properties using our measurement of the temperature. We find that in all cases considered the values are significantly larger than the dust response time measured by IR photometric monitoring campaigns, with the least discrepancy present relative to the result for a wavelength-independent dust emissivity law, i.e. a blackbody, which is appropriate for large grain sizes. This result can be well explained by assuming a flared, disk-like structure for the hot dust.Comment: 18 pages, 7 figures; accepted to Ap

Scintillation-limited photometry with the 20-cm NGTS telescopes at Paranal Observatory

Ground-based photometry of bright stars is expected to be limited by atmospheric scintillation, although in practice observations are often limited by other sources of systematic noise. We analyse 122 nights of bright star (Gmag ≲ 11.5) photometry using the 20-cm telescopes of the Next-Generation Transit Survey (NGTS) at the Paranal Observatory in Chile. We compare the noise properties to theoretical noise models and we demonstrate that NGTS photometry of bright stars is indeed limited by atmospheric scintillation. We determine a median scintillation coefficient at the Paranal Observatory of CY=1.54⁠, which is in good agreement with previous results derived from turbulence profiling measurements at the observatory. We find that separate NGTS telescopes make consistent measurements of scintillation when simultaneously monitoring the same field. Using contemporaneous meteorological data, we find that higher wind speeds at the tropopause correlate with a decrease in long-exposure (t = 10 s) scintillation. Hence, the winter months between June and August provide the best conditions for high-precision photometry of bright stars at the Paranal Observatory. This work demonstrates that NGTS photometric data, collected for searching for exoplanets, contains within it a record of the scintillation conditions at Paranal