AGN STORM 2. I. First results: A Change in the Weather of Mrk 817

We present the first results from the ongoing, intensive, multiwavelength monitoring program of the luminous Seyfert 1 galaxy Mrk 817. While this active galactic nucleus was, in part, selected for its historically unobscured nature, we discovered that the X-ray spectrum is highly absorbed, and there are new blueshifted, broad, and narrow UV absorption lines, which suggest that a dust-free, ionized obscurer located at the inner broad-line region partially covers the central source. Despite the obscuration, we measure UV and optical continuum reverberation lags consistent with a centrally illuminated Shakura–Sunyaev thin accretion disk, and measure reverberation lags associated with the optical broad-line region, as expected. However, in the first 55 days of the campaign, when the obscuration was becoming most extreme, we observe a de-coupling of the UV continuum and the UV broad emission-line variability. The correlation recovered in the next 42 days of the campaign, as Mrk 817 entered a less obscured state. The short C iv and Lyα lags suggest that the accretion disk extends beyond the UV broad-line region.

Properties of Very Broad Line MgII Radio-Loud and Radio-Quiet Quasars

We perform an analysis of the properties of radio-loud (RL) and radio-quiet (RQ) quasars with MgII broad emission line (i-band magnitude ≤19.1 and z ≤1.9), selected from the parent sample of SDSS DR7 catalogue. For sources with full-width half maxima (FWHM) greater than 15,000 km s−1 (very broad line sample; VBL) we find the radio loud fraction (RLF) to be about 40%. To further investigate this result we compare the bolometric luminosity, optical continuum luminosity, black hole (BH) mass and Eddington ratios of our VBL sample of RL and RQ quasars. Our analysis shows that in our VBL sample space, RL quasars have higher luminosities and BH mass than RQ quasars. The similarity in the distribution of their covering fraction (CF) shows that there is no difference in dust distribution between VBL RL and RQ quasars and hence dust is not affecting our results. We also find that there is no correlation of RL quasar properties with optical continuum luminosity and BH mass.

Anomalous Broad-Line Region responses to continuum variability in Active Galactic Nuclei – I. Hβ variability

In the standard AGN reverberation-mapping model, variations in broad-line region (BLR) fluxes are predicted from optical continuum variability (taken as a proxy for the ionizing continuum) convolved with a response function that depends on the geometry. However, it has long been known that BLR variability can deviate from these predictions. We analyse both extensive long-term Hβ and continuum monitoring of NGC 5548 and a large sample of high-quality Hβ light curves of other AGNs to investigate the frequency and characteristics of anomalous responses of the BLR. We find that anomalies are very common and probably occur in every object. Onsets can be on a timescale only slightly longer than the light-crossing time and durations are of the order of the characteristic timescale of variability of the optical continuum to several times longer. Anomalies are larger when NGC 5548 is in a low state, but otherwise there is no correlation with continuum variability. There is abundant evidence for the optical continuum of AGNs varying independently of the higher-energy continua and this is sufficient to explain the anomalous responses of the total BLR flux. There are good reasons for believing that the frequent lack of correlation between spectral regions is due to anisotropic and non-axisymmetric emission. Rapid changes in line profiles and velocity-dependent lags are consistent with this. Motion of compact absorbing clouds across the line of sight is another possible cause of anomalies. The prevalence of anomalies should be considered when planning reverberation-mapping campaigns.

A characteristic optical variability time scale in astrophysical accretion disks

Accretion disks around supermassive black holes in active galactic nuclei produce continuum radiation at ultraviolet and optical wavelengths. Physical processes in the accretion flow lead to stochastic variability of this emission on a wide range of time scales. We measured the optical continuum variability observed in 67 active galactic nuclei and the characteristic time scale at which the variability power spectrum flattens. We found a correlation between this time scale and the black hole mass extending over the entire mass range of supermassive black holes. This time scale is consistent with the expected thermal time scale at the ultraviolet-emitting radius in standard accretion disk theory. Accreting white dwarfs lie close to this correlation, suggesting a common process for all accretion disks.

Characterizing the abundance, properties, and kinematics of the cool circumgalactic medium of galaxies in absorption with SDSS DR16

ABSTRACT In order to study the circumgalactic medium (CGM) of galaxies we develop an automated pipeline to estimate the optical continuum of quasars and detect intervening metal absorption line systems with a matched kernel convolution technique and adaptive S/N criteria. We process ∼ one million quasars in the latest Data Release 16 (DR16) of the Sloan Digital Sky Survey (SDSS) and compile a large sample of ∼ 160 000 Mg ii absorbers, together with ∼ 70 000 Fe ii systems, in the redshift range 0.35 < zabs < 2.3. Combining these with the SDSS DR16 spectroscopy of ∼1.1 million luminous red galaxies (LRGs) and ∼200 000 emission line galaxies (ELGs), we investigate the nature of cold gas absorption at 0.5 < z < 1. These large samples allow us to characterize the scale dependence of Mg ii with greater accuracy than in previous work. We find that there is a strong enhancement of Mg ii absorption within ∼50 kpc of ELGs, and the covering fraction within 0.5rvir of ELGs is 2–5 times higher than for LRGs. Beyond 50 kpc, there is a sharp decline in Mg ii for both kinds of galaxies, indicating a transition to the regime where the CGM is tightly linked with the dark matter halo. The Mg ii-covering fraction correlates strongly with stellar mass for LRGs, but weakly for ELGs, where covering fractions increase with star formation rate. Our analysis implies that cool circumgalactic gas has a different physical origin for star-forming versus quiescent galaxies.

Modeling the UV/optical continuum time-lags in AGN

Thermal reverberation in accretion discs of active galactic nuclei is thought to be the reason of the continuum UV/optical time lags seen in these sources. Recently, we studied thermal reverberation of a standard Novikov-Thorne accretion disc illuminated by an X–ray point-like source, and we derived an analytic prescription for the time lags as function of wavelength. In this work, we use this analytic function to fit the time-lags spectra of seven Seyferts, that have been intensively monitored, in many wave-bands, in the last few years. We find that thermal reverberation can explain the observed UV/optical time lags in all these sources. Contrary to previous claims, the magnitude of the observed UV/optical time-lags is exactly as expected in the case of a standard accretion disc in the lamp-post geometry, given the black hole mass and the accretion rate estimates for the objects we study. We derive estimates of the disc accretion rates and corona height for a non-spinning and a maximally spinning black hole scenarios. We also find that the modelling of the continuum optical/UV time-lags can be used to estimate the black hole spin, when combined with additional information. We also find that the model under-predicts the observed X–ray to UV time-lags, but this difference is probably due to the broad X-ray auto-correlation function of these sources.

NGC 2770: High supernova rate due to interaction

Context. Galaxies that hosted many core-collapse supernova (SN) explosions can be used to study the conditions necessary for the formation of massive stars. NGC 2770 was dubbed an SN factory because it hosted four core-collapse SNe in 20 years (three type Ib and one type IIn). Its star formation rate (SFR) was reported to not be enhanced and, therefore, not compatible with such a high SN rate. Aims. We aim to explain the high SN rate of NGC 2770. Methods. We used archival H I line data for NGC 2770 and reinterpreted the Hα and optical continuum data. Results. Even though the continuum-based SFR indicators do not yield high values, the dust-corrected Hα luminosity implies a high SFR, consistent with the high SN rate. Such a disparity between the SFR estimators is an indication of recently enhanced star formation activity because the continuum indicators trace long timescales of the order of 100 Myr, unlike the line indicators, which trace timescales of the order of 10 Myr. Hence, the unique feature of NGC 2770 compared to other galaxies is the fact that it was observed very shortly after the enhancement of the SFR. It also has high dust extinction, E(B − V) above 1 mag. We provide support for the hypothesis that the increased SFR in NGC 2770 is due to the interaction with its companion galaxies. We report an H I bridge between NGC 2770 and its closest companion and the existence of a total of four companions within 100 kpc (one identified for the first time). There are no clear H I concentrations close to the positions of SNe in NGC 2770 such as those detected for hosts of gamma-ray bursts (GRBs) and broad-lined SNe type Ic (IcBL). This suggests that the progenitors of type Ib SNe are not born out of recently accreted atomic gas, as was suggested for GRB and IcBL SN progenitors.

Optical and X-ray observations of stellar flares on an active M dwarf AD Leonis with the Seimei Telescope, SCAT, NICER, and OISTER

We report on multi-wavelength monitoring observations of an M-dwarf flare star AD Leonis with the Seimei Telescope (6150–7930 Å), SCAT (Spectroscopic Chuo-university Astronomical Telescope; 3700–7500 Å), and NICER (Neutron Star Interior Composition Explorer; 0.2–12.0 keV), with the collaboration of the OISTER (Optical and Infrared Synergetic Telescopes for Education and Research) program. Twelve flares are detected in total, including ten Hα, four X-ray, and four optical-continuum flares; one of them is a superflare with a total energy of ∼2.0 × 1033 erg. We found that: (1) during the superflare, the Hα emission line full width at 1/8 maximum dramatically increases to 14 Å from 8 Å in the low-resolution spectra (R ∼ 2000) accompanied by large white-light flares, (2) some weak Hα/X-ray flares are not accompanied by white-light emissions, and (3) the non-flaring emissions show clear rotational modulations in X-ray and Hα intensity in the same phase. To understand these observational features, one-dimensional hydrodynamic flare simulations are performed using the RADYN code. We find the simulated Hα line profiles with hard and high-energy non-thermal electron beams to be consistent with the initial phase line profiles of the superflares, while those with a softer and/or weak-energy beam are consistent with those in decay phases, indicating the changes in the energy fluxes injected to the lower atmosphere. Also, we find that the relation between the optical continuum and Hα intensity is nonlinear, which can be one cause of the non-white-light flares. The flare energy budget exhibits diversity in the observations and models, and more observations of stellar flares are necessary for constraining the occurrence of various emission line phenomena in stellar flares.

Optical and X-ray discovery of the changing-look AGN IRAS 23226-3843 showing extremely broad and double-peaked Balmer profiles

Aims. We detected a very strong X-ray decline in the galaxy IRAS 23226-3843 within the XMM-Newton slew survey in 2017. Subsequently, we carried out multi-band follow-up studies to investigate this fading galaxy in more detail. Methods. We took deep follow-up Swift, XMM-Newton, and NuSTAR observations in combination with optical SALT spectra of IRAS 23226-3843 in 2017. In addition, we reinspected optical, UV, and X-ray data that were taken in the past. Results. IRAS 23226-3843 decreased in X-rays by a factor of more than 30 with respect to ROSAT and Swift data taken 10 to 27 years before. The broadband XMM-Newton/NuSTAR spectrum is power-law dominated, with a contribution from photoionized emission from cold gas, likely the outer accretion disk or torus. The optical continuum decreased by 60% and the Balmer line intensities decreased by 50% between 1999 and 2017. The optical Seyfert spectral type changed simultaneously with the X-ray flux from a clear broad-line Seyfert 1 type in 1999 to a Seyfert 1.9 type in 2017. The Balmer line profiles in IRAS 23226-3843 are extremely broad. The profiles during the minimum state indicate that they originate in an accretion disk. The unusual flat Balmer decrement Hα/ Hβ with a value of 2 indicates a very high hydrogen density of nH > 1011 cm−3 at the center of the accretion disk. IRAS 23226-3843 shows unusually strong FeII blends with respect to the broad line widths, in contrast to what is known from Eigenvector 1 studies.