Fermi Blazars in the Zwicky Transient Facility Survey: Properties of Large Optical Variations

Context. Magnetic chemically peculiar (mCP) stars exhibit complex atmospheres that allow the investigation of the interplay of atomic diffusion, magnetic fields, and stellar rotation. A non-uniform surface distribution of chemical elements and the non-alignment of the rotational and magnetic axes result in the variability of several observables. Photometrically variable mCP stars are referred to as α2 Canum Venaticorum (ACV) variables. Aims. The present work presents a case study of known variables from the Zwicky Transient Facility (ZTF) survey, with the aim of investigating the survey’s suitability for the detection and study of new ACV variables. Methods. Using suitable selection criteria based on the known characteristics of ACV variables, candidate ACV stars were selected from the ZTF Catalog of Periodic Variable Stars. All light curves were inspected in detail to select the most promising candidates. Where available, low-resolution spectra from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) were employed to classify the stars on the MK system and confirm their status as mCP stars. Results. We have identified 86 new promising ACV star candidates. Fifteen of these stars have LAMOST spectra available that, in all cases, confirm them as classical mCP stars, which highlights the viability of our approach. We present astrophysical parameters for all sample stars that can be sorted into four subgroups characterized by distinct light curve shapes. Anti-phase variations in different photometric passbands, in particular, are a unique characteristic of a subset of ACV stars readily usable for their identification. The availability of data in three different passbands (g, r, and i) is a major advantage of the ZTF survey. Conclusions. On the basis of our experience with other photometric surveys and the analysis of light curves, we conclude that the ZTF is well suited to the search for, and the analysis of, ACV variables that are not considered in the available ZTF variable star catalogs. Further work will focus on the development and refinement of a search algorithm to correctly identify these stars in ZTF data and, subsequently, in massive photometric time series databases in general.

Study of the polarization behaviour in blazars is a powerful tool to discern the role of the magnetic field in the variable emission process in their relativistic jets. We present here the results of our systematic investigation on the correlation between optical flux and polarization variations for eight flat-spectrum radio quasars on various time-scales using data from the Steward Observatory that covers a period of ∼10 years. On long time-scales (∼several months), from a total of 79 observing cycles, we found a significant positive correlation between optical flux and optical polarization degree (PD) in 34 observing cycles, a negative correlation in three cycles, and no correlation in 42 cycles. On short time-scales (∼few days), in 47 out of a total of 55 epochs we found a positive correlation between optical flux and PD, while for the remaining eight epochs an anticorrelation was detected between the two quantities. Moreover, we noticed a significant positive correlation between optical and γ-ray fluxes in 14 epochs, and a negative correlation between the two in one epoch. While the observed optical flux changes fit the shock-in-jet model well, the observed changes in PD are not explainable by changes in the power-law spectral index of the relativistic electrons in the jet. Instead, the observed varied correlations between optical flux and PD could be due to multizone emission regions or the enhanced flux coinciding with the emergence of a new emission knot with its magnetic field either aligned or misaligned with the large-scale magnetic field.

We investigate the temporal and colour variability of 897 blazars, comprising 455 BL Lacertae objects (BL Lacs) and 442 Flat Spectrum Radio Quasars (FSRQs), selected from the Roma-BZCAT catalogue, using the multiband light curves from the Zwicky Transient Facility (ZTF DR6) survey. Assessing the colour variability characteristics over ∼2 yr time-scales, we found that 18.5 per cent (84 out of 455) BL Lacs showed a stronger bluer-when-brighter (BWB) trend, whereas 9.0 per cent (41 out of 455) showed a redder-when-brighter (RWB) trend. The majority (70 per cent) of the BL Lacs showing RWB are host galaxy dominated. For the FSRQ subclass, 10.2 per cent (45 out of 442) objects showed a strong BWB trend and 17.6 per cent (78 out of 442) showed a strong RWB trend. Hence, we find that BL Lacs more commonly follow a BWB trend than do FSRQs. This can be attributed to the more dominant jet emission in the case of BL Lacs and the contribution of thermal emission from the accretion disc for FSRQs. In analysing the colour behaviour on shorter time windows, we find many blazars evince shorter partial trends of BWB or RWB nature (or occasionally both). Some of such complex colour behaviours observed in the colour–magnitude diagrams of the blazars may result from transitions between the jet-dominated state and the disc-dominated state and vice versa.

The short GRB 170817A, detected by the Fermi Gamma-ray Burst Monitor, orbiting satellites and ground-based telescopes, was the electromagnetic counterpart of a gravitational-wave transient (GW170817) from a binary neutron star merger. After this merger, the γ-ray light curve exhibited a faint peak at ∼1.7 s and the X-ray, optical, and radio light curves displayed extended emission that increased in brightness up to ∼160 days. In this paper, we show that the X-ray, optical and radio fluxes are consistent with the synchrotron forward-shock model viewed off-axis when the matter in the outflow is parameterized through a power-law velocity distribution. We discuss the origin of the γ-ray peak in terms of internal and external shocks. We show that the γ-ray flux might be consistent with a synchrotron self-Compton reverse-shock model observed at high latitudes. Comparing the best-fit values obtained after describing the γ-ray, X-ray, optical, and radio fluxes with our model, we find that the afterglow and γ-ray emission occurred in different regions and also find evidence to propose that the progenitor environment was entrained with magnetic fields; therefore we argue for the presence of magnetic field amplification in the binary neutron star merger.

Violent multi-wavelength variabilities are observed in γ-ray-selected blazars. We present an analysis of long-term light curves for eight bright blazars to explore the co-variation pattern in the γ-ray and radio bands. We extract their γ-ray light curves and spectra with data observed by the Fermi Large Area Telescope (LAT) since 2008. We find diverse co-variation patterns between the γ-ray and radio (at 43 GHz) fluxes in these sources. The γ-ray and radio fluxes of 3C 454.3 and PKS 1633+382 are correlated without any time lag, suggesting that they are from the same radiation region. Similar correlation is also observed in 3C 273 and PKS 1222+216, but the radio flux lags behind the γ-ray flux by approximately ∼160 d and ∼290 d, respectively. This likely suggests that their γ-ray emission regions are located the upstream of their radio cores at 43 GHz. The γ-ray and radio fluxes of the other four blazars are not correlated, implying that the γ-ray and radio emission may be from different regions in their jets. The γ-ray light curves of the eight blazars can be decomposed into some components with long-timescale variability and some fast spike flares. We propose that they may be attributed to the central engine activity and the magnetic reconnection process or turbulence in the local emission region, respectively.

The Zwicky Transient Facility (ZTF) survey generates real-time alerts for optical transients, variables, and moving objects discovered in its wide-field survey. We describe the ZTF alert stream distribution and processing (filtering) system. The system uses existing open-source technologies developed in industry: Kafka, a real-time streaming platform, and Avro, a binary serialization format. The technologies used in this system provide a number of advantages for the ZTF use case, including (1) built-in replication, scalability, and stream rewind for the distribution mechanism; (2) structured messages with strictly enforced schemas and dynamic typing for fast parsing; and (3) a Python-based stream processing interface that is similar to batch for a familiar and user-friendly plug-in filter system, all in a modular, primarily containerized system. The production deployment has successfully supported streaming up to 1.2 million alerts or roughly 70 GB of data per night, with each alert available to a consumer within about 10 s of alert candidate production. Data transfer rates of about 80,000 alerts/minute have been observed. In this paper, we discuss this alert distribution and processing system, the design motivations for the technology choices for the framework, performance in production, and how this system may be generally suitable for other alert stream use cases, including the upcoming Large Synoptic Survey Telescope.

The discovery rate of fast radio bursts (FRBs) is increasing dramatically thanks to new radio facilities. Meanwhile, wide-field instruments such as the 47 deg2 Zwicky Transient Facility (ZTF) survey the optical sky to study transient and variable sources. We present serendipitous ZTF observations of the Canadian Hydrogen Intensity Mapping Experiment (CHIME) repeating source FRB 180916.J0158+65 that was localized to a spiral galaxy 149 Mpc away and is the first FRB suggesting periodic modulation in its activity. While 147 ZTF exposures corresponded to expected high-activity periods of this FRB, no single ZTF exposure was at the same time as a CHIME detection. No >3σ optical source was found at the FRB location in 683 ZTF exposures, totaling 5.69 hr of integration time. We combined ZTF upper limits and expected repetitions from FRB 180916.J0158+65 in a statistical framework using a Weibull distribution, agnostic of periodic modulation priors. The analysis yielded a constraint on the ratio between the optical and radio fluences of η ≲ 200, corresponding to an optical energy E opt ≲ 3 × 1046 erg for a fiducial 10 Jy ms FRB (90% confidence). A deeper (but less statistically robust) constraint of η ≲ 3 can be placed assuming a rate of and 1.2 ± 1.1 FRB occurring during exposures taken in high-activity windows. The constraint can be improved with shorter per-image exposures and longer integration time, or observing FRBs at higher Galactic latitudes. This work demonstrated how current surveys can statistically constrain multiwavelength counterparts to FRBs even without deliberately scheduled simultaneous radio observation.

Active galactic nuclei (AGNs) are known to exhibit optical/ultraviolet variability and most of them can be well modelled by the damped random walks. Physical processes that are not related to the accretion disc, such as tidal disruption events (TDEs) or moving foreground dusty clouds, can cause flare-like and eclipse-like features in the optical light curve. Both long-term and high-cadence monitoring are needed to identify such features. By combining the Sloan Digital Sky Survey (SDSS), Panoramic Survey Telescope and Rapid Response System, and the Zwicky Transient Facility (ZTF) survey, we are able to identify a rare sample (11) out of the SDSS quasar catalogue ($\sim\!\! 83\, 000$). These quasars exhibit more or less constant brightness but show rapid optical variation in the epochs of ZTF Public Data Release 2. To investigate the possible origins of these flare/eclipse-like variabilities, we propose the second-epoch spectroscopic observations with the Gran Telescopio CANARIAS. We find that the change in accretion rate plays a significant role in these quasar variabilities. Among them, we identify two changing-look AGN candidates: SDSS J1427+2930 and SDSS J1420+3757. The luminosity change of the former may be caused by the enhanced supermassive black hole’s accretion or the TDE, while the latter is more related to the change in the accretion rate.

Despite numerous studies, the origin of the γ-ray emission from blazars is still debated, in particular whether it is produced by leptonic or hadronic processes. In this study, we are testing the leptonic scenario for the flat-spectrum radio quasar 3C 279, assuming that the γ-ray emission is generated by inverse Compton scattering of external target photons from the broad-line region (IC-BLR scenario). For this purpose we use a 10 yr data set of the source consisting of the optical spectroscopy data from the Steward Observatory blazar monitoring program and Fermi Large Area Telescope γ-ray data. We search for a possible correlation between the Compton dominance and the emission-line luminosity using the discrete correlation function analysis. As a result, we find no significant correlation between these two quantities at any time lag value, while the emission-line luminosity displays a moderate correlation with the γ-ray flux at a zero time lag. We also reveal that the optical synchrotron continuum flux shows a pronounced correlation with the γ-ray flux, and therefore we interpret these results within the leptonic IC-BLR scenario where the variations in Compton dominance are primarily induced by changes in the magnetic field, rather than in the emission-line luminosity.

Supernovae that are strongly gravitationally lensed (gLSNe) by elliptical galaxies are powerful probes of astrophysics and cosmology that will be discovered systematically by wide-field, high-cadence imaging surveys such as the Zwicky Transient Facility (ZTF) and the Large Synoptic Survey Telescope (LSST). Here we use pixel-level simulations that include observing strategy, target selection, supernova properties, and dust to forecast the rates and properties of gLSNe that ZTF and LSST will find. Applying the resolution-insensitive discovery strategy of Goldstein et al., we forecast that ZTF (LSST) can discover 0.02 (0.79) 91bg-like, 0.17 (5.92) 91T-like, 1.22 (47.84) Type Ia, 2.76 (88.51) Type IIP, 0.31 (12.78) Type IIL, and 0.36 (15.43) Type Ib/c gLSNe per year, with uncertainties dominated by uncertainties in the supernova rate. We also forecast that the surveys can discover at least 3.75 (209.32) Type IIn gLSNe per year, for a total of at least 8.60 (380.60) gLSNe per year under fiducial observing strategies. ZTF gLSNe have a median z s = 0.9, z l = 0.35, , Δt max = 10 days, min(θ) = 0.″25, and N img = 4. LSST gLSNe are less compact and less magnified, with a median z s = 1.0, z l = 0.4, , Δt max = 25 days, min(θ) = 0.″6, and N img = 2. We develop a model of the supernova–host galaxy connection and find that the vast majority of gLSN host galaxies will be multiply imaged, enabling detailed constraints on lens models with sufficiently deep high-resolution imaging taken after the supernova has faded. We release the results of our simulations as catalogs at http://portal.nersc.gov/project/astro250/glsne/.

In March 2017, the Intermediate Palomar Transient Factory (iPTF) ceased operations. This Symposium was an appropriate opportunity to review the scientific returns from iPTF and its predecessor survey, the Palomar Transient Factory (PTF), and to summarise the lessons learned. Succeeding iPTF on the Palomar Observatory 48-inch Schmidt telescope is the Zwicky Transient Facility (ZTF), a new survey with an order of magnitude faster survey speed that is now being commissioned. I described the design and scientific rationale for ZTF. ZTF is prototyping new alert stream technologies being explored by the Large Synoptic Survey Telescope (LSST) to distribute millions of transient alerts per night to downstream science users. I described the design of the alert system and discussed it in the context of the wider LSST and community broker ecosystem.

The Advanced LIGO and Virgo experiments are poised to detect gravitational waves (GWs) directly for the first time this decade. The ultimate prize will be joint observation of a compact binary merger in both gravitational and electromagnetic channels. However, GW sky locations that are uncertain by hundreds of square degrees will pose a challenge. I describe a real-time detection pipeline and a rapid Bayesian parameter estimation code that will make it possible to search promptly for optical counterparts in Advanced LIGO. Having analyzed a comprehensive population of simulated GW sources, we describe the sky localization accuracy that the GW detector network will achieve as each detector comes online and progresses toward design sensitivity. Next, in preparation for the optical search with the intermediate Palomar Transient Factory (iPTF), we have developed a unique capability to detect optical afterglows of gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Burst Monitor (GBM). Its comparable error regions offer a close parallel to the Advanced LIGO problem, but Fermi's unique access to MeV-GeV photons and its near all-sky coverage may allow us to look at optical afterglows in a relatively unexplored part of the GRB parameter space. We present the discovery and broadband follow-up observations (X-ray, UV, optical, millimeter, and radio) of eight GBM-IPTF afterglows. Two of the bursts (GRB 130702A / iPTF13bxl and GRB 140606B / iPTF14bfu) are at low redshift (z=0.145 and z = 0.384, respectively), are sub-luminous with respect to "standard" cosmological bursts, and have spectroscopically confirmed broad-line type Ic supernovae. These two bursts are possibly consistent with mildly relativistic shocks breaking out from the progenitor envelopes rather than the standard mechanism of internal shocks within an ultra-relativistic jet. On a technical level, the GBM--IPTF effort is a prototype for locating and observing optical counterparts of GW events in Advanced LIGO with the Zwicky Transient Facility.

BL Lacertae, the prototype of the BL Lacertae (BL Lac) category of blazars, underwent a giant γ-ray flare in April 2021. The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (hereafter Fermi-LAT) observed a peak γ-ray (0.1−500 GeV) flux of ∼2 × 10−5 ph cm−2 s−1 within a single orbit on 2021 April 27, which is historically the brightest γ-ray flux ever detected from the source. Here, we report, for the first time, the detection of significant minute-timescale GeV γ-ray flux variability in the BL Lac subclass of blazars by the Fermi-LAT. We resolved the source variability down to two-minute binned timescales with a flux halving time of ∼1 min, which is the shortest GeV variability timescale ever observed from blazars. The detected variability timescale is much shorter than the light-crossing time (∼14 min) across the central black hole of BL Lac, indicating a very compact γ-ray emission site within the outflowing jet. Such a compact emitting region requires the bulk Lorentz factor of the jet to be larger than 16 so that the jet power is not super Eddington. We found a minimum Doppler factor δmin of 15 using the δ function approximation for the γγ opacity constraint. For a conical jet geometry, considering Γ = δmin, the observed short variability timescale for BL Lac suggests that the very compact emission region lies at a distance of about 8.62 × 1014 cm from its central engine.

We report the discovery of six active galactic nuclei (AGNs) caught “turning on” during the first nine months of the Zwicky Transient Facility (ZTF) survey. The host galaxies were classified as low-ionization nuclear emission-line region galaxies (LINERs) by weak narrow forbidden line emission in their archival SDSS spectra, and detected by ZTF as nuclear transients. In five of the cases, we found via follow-up spectroscopy that they had transformed into broad-line AGNs, reminiscent of the changing-look LINER iPTF16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up Hubble Space Telescope ultraviolet and ground-based optical spectra revealed the transformation into a narrow-line Seyfert 1 with strong [Fe vii, x, xiv] and He ii λ 4686 coronal lines. Swift monitoring observations of this source reveal bright UV emission that tracks the optical flare, accompanied by a luminous soft X-ray flare that peaks ∼60 days later. Spitzer follow-up observations also detect a luminous mid-infrared flare, implying a large covering fraction of dust. Archival light curves of the entire sample from CRTS, ATLAS, and ASAS-SN constrain the onset of the optical nuclear flaring from a prolonged quiescent state. Here we present the systematic selection and follow-up of this new class of changing-look LINERs, compare their properties to previously reported changing-look Seyfert galaxies, and conclude that they are a unique class of transients well-suited to test the uncertain physical processes associated with the LINER accretion state.

We describe ZStreak, a semi-real-time pipeline specialized in detecting small, fast-moving, near-Earth asteroids (NEAs), which is currently operating on the data from the newly commissioned Zwicky Transient Facility (ZTF) survey. Based on a prototype originally developed by Waszczak et al. () for the Palomar Transient Factory (PTF), the predecessor of ZTF, ZStreak features an improved machine-learning model that can cope with the 10× data rate increment between PTF and ZTF. Since its first discovery on 2018 February 5 (2018 CL), ZTF/ZStreak has discovered 45 confirmed new NEAs over a total of 232 observable nights until 2018 December 31. Most of the discoveries are small NEAs, with diameters less than ∼100 m. By analyzing the discovery circumstances, we find that objects having the first to last detection time interval under 2 hr are at risk of being lost. We will further improve real-time follow-up capabilities, and work on suppressing false positives using deep learning.