Transient Survey Research Articles

A case study of ACV variables discovered in the Zwicky Transient Facility survey

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.

Early Activity in Comet C/2014 UN271 Bernardinelli–Bernstein as Observed by TESS

Abstract We used long duration observations from the Transient Exoplanet Survey Satellite (TESS) to investigate the behavior of comet C/2014 UN271 Bernardinelli–Bernstein at large heliocentric distances. By combining data from sector 03 (976 30 minute exposures from 2018), and sectors 29 and 30 (3585 and 3410 10 minute exposures, respectively, from 2020), we produced deep coadded images of the comet. A comparison of these results with similarly processed images of inactive Kuiper Belt objects and asteroids reveals that the comet was already exhibiting coma at heliocentric distances 23.8 and 21.2 au, making this one of the most distant comets for which preperihelion activity has been directly detected. A simple syndyne analysis of asymmetries in the coma suggests that activity probably started several years prior to these observations, and likely arose from emission in roughly the sunward direction. The images were used to produce photometric lightcurves, though no rotational variability was detected. We used NEOWISE observations from 2020 November 26–28 to place an upper limit of 2 × 1028 molecules s−1 (3σ) on the CO production rate.

The ZTF-BTS Type Ia supernovae luminosity function is consistent with a single progenitor channel for the explosions

ABSTRACT We construct the Type Ia supernovae (SNe Ia) luminosity function (LF) using the Zwicky Transient Facility Bright Transient Survey (BTS) catalogue. While this magnitude-limited survey has an unprecedented number of objects, it suffers from large distance uncertainties and lacks an estimation of host extinction. We bypass these issues by calculating the intrinsic luminosities from the shape parameters of the light curve’s g and r bands, with the luminosities calibrated from the well observed SNe Ia sample of the Carnegie Supernova Project, allowing us to construct, for the first time, the intrinsic LF of SNe Ia. We then use a novel tight relation between the colour stretch and the synthesized 56Ni mass, MNi56, to determine the MNi56 distribution of SNe Ia. We find that the LFs are unimodal, with their peaks in line with previous results, but have a much lower rate of dim events and luminous events. We show that the features on top of the unimodal LF-derived distributions are all compatible with statistical noise, consistent with a single progenitor channel for the explosions. We further derive, for the first time, the SNe Ia distribution of host galaxy extinction, and find a mean selective extinction of E(B − V) ≈ 0.1 and a non-negligible fraction with large, $\gt 1\, \text{mag}$, extinction in the optical bands. The high extinction is typical for luminous SNe, supporting their young population origin.

The Zwicky Transient Facility Type Ia supernova survey: first data release and results

ABSTRACT Type Ia supernovae (SNe Ia) in the nearby Hubble flow are excellent distance indicators in cosmology. The Zwicky Transient Facility (ZTF) has observed a large sample of SNe from an untargeted, rolling survey, reaching 20.8, 20.6, and 20.3 mag in g r, and i band, respectively. With an FoV of 47 deg2, ZTF discovered > 3000 SNe Ia in a little over 2.5 yr. Here, we report on the sample of 761 spectroscopically classified SNe Ia from the first year of operations (DR1). The sample has a median redshift $\bar{z} =$ 0.057, nearly a factor of 2 higher than the current low-z sample. Our sample has a total of 934 spectra, of which 632 were obtained with the robotic SEDm on Palomar P60. We assess the potential for precision cosmology for a total of 305 SNe with redshifts from host galaxy spectra. The sample is already comparable in size to the entire combined literature low-z anchor sample. The median first detection is 13.5 d before maximum light, about 10 d earlier than the median in the literature. Furthermore, six SNe from our sample are at DL < 80 Mpc, for which host galaxy distances can be obtained in the JAMES WEBB SPACE TELESCOPE era, such that we have calibrator and Hubble flow SNe observed with the same instrument. In the entire duration of ZTF-I, we have observed nearly 50 SNe for which we can obtain calibrator distances, key for per cent level distance scale measurements.

Three-dimensional transient electromagnetic response of multiple grounded-wire sources

Multiple grounded-wire sources combination are proposed to improve the signal strength and resolution of the transient electromagnetic method. Multi-source excitation from the different direction was executed to identify the conductive body in the subsurface and achieve refined detection of the target body. The vector finite element method based on the rational Krylov subspace projection algorithm was used to calculate transient response of the target bodies. The voltage response calculated by our algorithm was compared with that calculated by the integral equation method and the results agreed well with them. We designed different grounded-wire combinations, including a single source, pairs of sources, and groups of three and four sources, and analyzed induced EMF and the secondary field response of one conductive body. It indicates that multi-source transient method can obtain stronger signals, which is beneficial to the discovery of deep weak anomalies. Finally, we established two adjacent conductive block bodies and compared their electromagnetic response for the different source combination. The results showed that multiple-source transient electromagnetic response anomaly amplitude and lateral resolution are superior to those of a single source. Multi-channel diagram for multiple sources have two obvious peaks, especially for three sources and four sources, but the induced EMF from a single source does not differentiate between the two conductive bodies in the horizontal direction. Multiple grounded-wire source combinations are used not only for the surface transient electromagnetic method but can also be extended to the grounded electrical-source airborne transient electromagnetic survey.

Strongly lensed candidates from the HSC transient survey

We present a lensed quasar search based on the variability of lens systems in the Hyper Suprime-Cam (HSC) transient survey. Starting from 101 353 variable objects with i-band photometry in the HSC transient survey, we used a variability-based lens search method measuring the spatial extent in difference images to select potential lensed quasar candidates. We adopted conservative constraints in this variability selection and obtained 83 657 variable objects as possible lens candidates. We then ran CHITAH, a lens search algorithm based on the image configuration, on those 83 657 variable objects, and 2130 variable objects were identified as potential lensed objects. We visually inspected the 2130 variable objects, and seven of them are our final lensed quasar candidates. Additionally, we found one lensed galaxy candidate as a serendipitous discovery. Among the eight final lensed candidates, one is the only known quadruply lensed quasar in the survey field, HSCJ095921+020638. None of the other seven lensed candidates have been previously classified as a lens nor a lensed candidate. Three of the five final candidates with available Hubble Space Telescope (HST) images, including HSCJ095921+020638, show clues of a lensed feature in the HST images. We show that a tightening of our variability selection criteria might result in the loss of possible lensed quasar candidates, especially the lensed quasars with faint brightness or narrow separation, without efficiently eliminating the non-lensed objects; CHITAH is therefore important as an advanced examination to improve the lens search efficiency through the object configuration. The recovery of HSCJ095921+020638 proves the effectiveness of the variability-based lens search method, and this lens search method can be used in other cadenced imaging surveys, such as the upcoming Rubin Observatory Legacy Survey of Space and Time.

A low-energy explosion yields the underluminous Type IIP SN 2020cxd

Context. We present our observations and analysis of SN 2020cxd, a low-luminosity (LL), long-lived Type IIP supernova (SN). This object is a clear outlier in the magnitude-limited SN sample recently presented by the Zwicky Transient Facility’s (ZTF) Bright Transient Survey. Aims. We demonstrate that SN 2020cxd is an additional member of the group of LL SNe and we discuss the rarity of LL SNe in the context of the ZTF survey. We consider how further studies of these faintest members of the core-collapse (CC) SN family might help improve the general understanding of the underlying initial mass function for stars that explode. Methods. We used optical light curves (LCs) from the ZTF in the gri bands and several epochs of ultraviolet data from the Neil Gehrels Swift observatory as well as a sequence of optical spectra. We constructed the colour curves and a bolometric LC. Then we compared the evolution of the ejecta velocity and black-body temperature for LL SNe as well as for typical Type II SNe. Furthermore, we adopted a Monte Carlo code that fits semi-analytic models to the LC of SN 2020cxd, which allows for the estimation of the physical parameters. Using our late-time nebular spectra, we also make a comparison against SN II spectral synthesis models from the literature to constrain the progenitor properties of SN 2020cxd. Results. The LCs of SN 2020cxd show a great similarity with those of LL SNe IIP in terms of luminosity, timescale, and colours. Also, the spectral evolution of SN 2020cxd is that of a Type IIP SN. The spectra show prominent and narrow P-Cygni lines, indicating low expansion velocities. This is one of the faintest LL SNe observed, with an absolute plateau magnitude of Mr = −14.5 mag and also one with the longest plateau lengths, with a duration of 118 days. Finally, the velocities measured from the nebular emission lines are among the lowest ever seen in a SN, with an intrinsic full width at half maximum value of 478 km s−1. The underluminous late-time exponential LC tail indicates that the mass of 56Ni ejected during the explosion is much smaller than the average of normal SNe IIP, we estimate M56Ni = 0.003 M⊙. The Monte Carlo fitting of the bolometric LC suggests that the progenitor of SN 2020cxd had a radius of R0 = 1.3 × 1013 cm, kinetic energy of Ekin = 4.3 × 1050 erg, and ejecta mass of Mej = 9.5 M⊙. From the bolometric LC, we estimated the total radiated energy Erad = 1.52 × 1048 erg. Using our late-time nebular spectra, we compared these results against SN II spectral synthesis models to constrain the progenitor zero-age main sequence mass and found that it is likely to be ≲15 M⊙. Conclusions. SN 2020cxd is a LL Type IIP SN. The inferred progenitor parameters and the features observed in the nebular spectrum favour a low-energy, Ni-poor, iron CC SN from a low-mass (∼12 M⊙) red supergiant.

On-sky demonstration of precision photometry with Bayer color filter arrays

Consumer-level digital single-lens reflex (DSLR) cameras are typically not used in professional astronomy because of the systematic errors present in the data as a result of the strong intra- and interpixel variations associated with each of the three different colors (RGB) of the Bayer color filter array. Nevertheless, because the cost of DSLRs compared with traditional astronomical CCDs is so much lower, they represent a potentially underexplored area of scientific quality astronomical imaging, especially in the area of wide-field transient surveys. We demonstrate an algorithm that can achieve ≈ 1 % level photometry in each of the RGB color channels from a stellar source and discuss the application of this algorithm to a ground-based transiting exoplanet survey. The algorithm primarily takes advantage of the large number of stellar sources available for statistical averaging within a single image, using a “lucky point-spread function” approach to identify sources in the image that exhibit systematic errors consistent with a chosen target from the same image. The selection of the appropriate“lucky” reference stars is accomplished through a comparison of the stellar image morphology as it appears on the Bayer array and the reference stars. These references are linearly combined to form a synthetic comparison star that can be used for differential photometry with the target. One key to the algorithm is that all data are retained at the individual pixel-level until the final differential comparison, which helps to alleviate systematic effects that might otherwise cancel each other out during the flux-summing process. We demonstrate the algorithm on HD 339461, a mV = 8.93 G0-type star on which we achieve single-percent level photometry that approaches the fundamental noise floor possible from a single camera.

The JCMT Transient Survey: Four-year Summary of Monitoring the Submillimeter Variability of Protostars

Abstract We present the four-year survey results of monthly submillimeter monitoring of eight nearby (<500 pc) star-forming regions by the JCMT Transient Survey. We apply the Lomb–Scargle Periodogram technique to search for and characterize variability on 295 submillimeter peaks brighter than 0.14 Jy beam−1, including 22 disk sources (Class II), 83 protostars (Class 0/I), and 190 starless sources. We uncover 18 secular variables, all of them protostars. No single-epoch burst or drop events and no inherently stochastic sources are observed. We classify the secular variables by their timescales into three groups: Periodic, Curved, and Linear. For the Curved and Periodic cases, the detectable fractional amplitude, with respect to mean peak brightness, is ∼4% for sources brighter than ∼0.5 Jy beam−1. Limiting our sample to only these bright sources, the observed variable fraction is 37% (16 out of 43). Considering source evolution, we find a similar fraction of bright variables for both Class 0 and Class I. Using an empirically motivated conversion from submillimeter variability to variation in mass accretion rate, six sources (7% of our full sample) are predicted to have years-long accretion events during which the excess mass accreted reaches more than 40% above the total quiescently accreted mass: two previously known eruptive Class I sources, V1647 Ori and EC 53 (V371 Ser), and four Class 0 sources, HOPS 356, HOPS 373, HOPS 383, and West 40. Considering the full protostellar ensemble, the importance of episodic accretion on few years timescale is negligible—only a few percent of the assembled mass. However, given that this accretion is dominated by events on the order of the observing time window, it remains uncertain as to whether the importance of episodic events will continue to rise with decades-long monitoring.

Finite-element modeling of time-domain controlled-source electromagnetic survey with weighted Laguerre polynomials

To facilitate the modeling of time-domain controlled-source electromagnetic surveys, we have developed an efficient finite-element method with weighted Laguerre polynomials, which indicates a much lower computational complexity than conventional time integration methods. Our method allows sampling the field at arbitrary time steps, and its accuracy is determined by the number of polynomials, instead of the time sampling interval. An analysis is performed regarding the optimization of the polynomial number to be used and the criterion of selecting the time scale factor. We conduct two numerical tests in marine and land survey environments to demonstrate the superiority of our method over the existing backward Euler time integration method. Our method is expected to facilitate the modeling of transient electromagnetic surveys in the geophysical regime.

Using geophysical survey results in the inference of aquifer vulnerability measures

ABSTRACTThe physical parameter derived from the inversion of electromagnetic surveys, the distribution of subsurface conductivity, is interesting in itself only in very few instances. In most cases, the conductivity distribution will have to be interpreted in terms of the target properties of the survey, for example: a geological interpretation of lithology; a hydrogeological interpretation of hydraulic conductivity; a biohazard/geotechnical interpretation of polluted/not‐polluted ground; and/or an archaeological interpretation of manmade/natural finds. The parameters of interest in these categories are often called derived products, indicating that the parameter of interest is not the same as the parameter whose distribution is found in the inversion process of the geophysical data. The interpretation process can be done in a wide variety of ways; from a predominantly cognitive approach based on professional experience, to an application of rigorous quantitative relations found from scientific endeavours.In most practical situations, the number of locations with independently measured information on the derived product is considerably smaller than the number of geophysics locations. It is precisely this sparsity of primary information on the derived product that encourages the use of geophysical inversion results as a sort of qualified interpolator through a formulation of a correlation between a geophysical parameter and the parameter characterising the derived product. In this paper, a general, quantitative approach to deriving the parameter of interest is presented using statistical analytic measures and an advanced use of an interpolation method that takes uncertainties into account. The approach is demonstrated in a field example from Ølgod, Denmark, where the cumulated clay thickness in the upper 30 m is estimated using a combination of borehole drilling records and an airborne transient survey.

Deep co-added sky from Catalina Sky Survey images

ABSTRACT A number of synoptic sky surveys are underway or being planned. Typically, they are done with small telescopes and relatively short exposure times. A search for transient or variable sources involves comparison with deeper baseline images, ideally obtained through the same telescope and camera. With that in mind, we have stacked images from the 0.68 m Schmidt telescope on Mt. Bigelow taken over 10 yr as part of the Catalina Sky Survey. In order to generate deep reference images for the Catalina Real-time Transient Survey (CRTS), close to 0.8 million images over 8000 fields and covering over 27 000 sq. deg have gone into the deep stack that goes up to 3 mag deeper than individual images. CRTS system does not use a filter in imaging; hence, there is no standard passband in which the optical magnitude is measured. We estimate depth by comparing these wide-band unfiltered co-added images with images in the g band and find that the image depth ranges from 22.0 to 24.2 across the sky, with a 200-image stack attaining an equivalent AB magnitude sensitivity of 22.8. We compared various state-of-the-art software packages for co-adding astronomical images and have used swarp for the stacking. We describe here the details of the process adopted. This methodology may be useful in other panoramic imaging applications, and to other surveys as well. The stacked images are available through a server at Inter-University Centre for Astronomy and Astrophysics.

SNIascore: Deep-learning Classification of Low-resolution Supernova Spectra

Abstract We present SNIascore, a deep-learning-based method for spectroscopic classification of thermonuclear supernovae (SNe Ia) based on very low-resolution (R ∼ 100) data. The goal of SNIascore is the fully automated classification of SNe Ia with a very low false-positive rate (FPR) so that human intervention can be greatly reduced in large-scale SN classification efforts, such as that undertaken by the public Zwicky Transient Facility (ZTF) Bright Transient Survey (BTS). We utilize a recurrent neural network architecture with a combination of bidirectional long short-term memory and gated recurrent unit layers. SNIascore achieves a <0.6% FPR while classifying up to 90% of the low-resolution SN Ia spectra obtained by the BTS. SNIascore simultaneously performs binary classification and predicts the redshifts of secure SNe Ia via regression (with a typical uncertainty of <0.005 in the range from z = 0.01 to z = 0.12). For the magnitude-limited ZTF BTS survey (≈70% SNe Ia), deploying SNIascore reduces the amount of spectra in need of human classification or confirmation by ≈60%. Furthermore, SNIascore allows SN Ia classifications to be automatically announced in real time to the public immediately following a finished observation during the night.

Gaia Early Data Release 3

Context. Since July 2014, the Gaia mission has been engaged in a high-spatial-resolution, time-resolved, precise, accurate astrometric, and photometric survey of the entire sky. Aims. We present the Gaia Science Alerts project, which has been in operation since 1 June 2016. We describe the system which has been developed to enable the discovery and publication of transient photometric events as seen by Gaia. Methods. We outline the data handling, timings, and performances, and we describe the transient detection algorithms and filtering procedures needed to manage the high false alarm rate. We identify two classes of events: (1) sources which are new to Gaia and (2) Gaia sources which have undergone a significant brightening or fading. Validation of the Gaia transit astrometry and photometry was performed, followed by testing of the source environment to minimise contamination from Solar System objects, bright stars, and fainter near-neighbours. Results. We show that the Gaia Science Alerts project suffers from very low contamination, that is there are very few false-positives. We find that the external completeness for supernovae, CE = 0.46, is dominated by the Gaia scanning law and the requirement of detections from both fields-of-view. Where we have two or more scans the internal completeness is CI = 0.79 at 3 arcsec or larger from the centres of galaxies, but it drops closer in, especially within 1 arcsec. Conclusions. The per-transit photometry for Gaia transients is precise to 1% at G = 13, and 3% at G = 19. The per-transit astrometry is accurate to 55 mas when compared to Gaia DR2. The Gaia Science Alerts project is one of the most homogeneous and productive transient surveys in operation, and it is the only survey which covers the whole sky at high spatial resolution (subarcsecond), including the Galactic plane and bulge.

Detection of Galactic and Extragalactic Millimeter-wavelength Transient Sources with SPT-3G

Abstract High angular resolution cosmic microwave background experiments provide a unique opportunity to conduct a survey of time-variable sources at millimeter wavelengths, a population that has primarily been understood through follow-up measurements of detections in other bands. Here we report the first results of an astronomical transient survey with the South Pole Telescope (SPT) using the SPT-3G camera to observe 1500 deg2 of the southern sky. The observations took place from 2020 March to November in three bands centered at 95, 150, and 220 GHz. This survey yielded the detection of 15 transient events from sources not previously detected by the SPT. The majority are associated with variable stars of different types, expanding the number of such detected flares by more than a factor of two. The stellar flares are unpolarized and bright, in some cases exceeding 1 Jy, and have durations from a few minutes to several hours. Another population of detected events last for 2–3 weeks and appear to be extragalactic in origin. Though data availability at other wavelengths is limited, we find evidence for concurrent optical activity for two of the stellar flares. Future data from SPT-3G and forthcoming instruments will provide real-time detection of millimeter-wave transients on timescales of minutes to months.

New SU UMa-type star ZTF18abdlzhd in the Zwicky Transient Facility data

We carried out a search for unknown dwarf novae in a public data release of the Zwicky Transient Facility survey and suspected that the object ZTF18abdlzhd is a SU UMa-type star. Performed multicolor CCD observations permit us to follow its fading from an outburst in August and an entire superoutburst in October 2020. The duration of the superoutburst is 13 days. We detected superhumps with period P = 0.06918(3) d that are characteristic of UGSU type stars.

SN 2018ijp: the explosion of a stripped-envelope star within a dense H-rich shell?

In this paper, we discuss the outcomes of the follow-up campaign of SN 2018ijp, discovered as part of the Zwicky Transient Facility survey for optical transients. Its first spectrum shows similarities to broad-lined Type Ic supernovae around maximum light, whereas later spectra display strong signatures of interaction between rapidly expanding ejecta and a dense H-rich circumstellar medium, coinciding with a second peak in the photometric evolution of the transient. This evolution, along with the results of modeling of the first light-curve peak, suggests a scenario where a stripped star exploded within a dense circumstellar medium. The two main phases in the evolution of the transient could be interpreted as a first phase dominated by radioactive decays, and a later interaction-dominated phase where the ejecta collide with a pre-existing shell. We therefore discuss SN 2018jp within the context of a massive star depleted of its outer layers exploding within a dense H-rich circumstellar medium.

An 86 GHz Search for Pulsars in the Galactic Center with the Atacama Large Millimeter / submillimeter Array

Abstract We report on the first pulsar and transient survey of the Galactic Center (GC) with the Atacama Large Millimeter/submillimeter Array (ALMA). The observations were conducted during the Global Millimeter VLBI Array campaign in 2017 and 2018. We carry out searches using time series of both total intensity and other polarization components in the form of Stokes parameters. We incorporate acceleration and its derivative in the pulsar search, and also search in segments of the entire observation to compensate for potential orbital motion of the pulsar. While no new pulsar is found, our observations yield the polarization profile of the GC magnetar PSR J1745−2900 at millimeter wavelength for the first time, which turns out to be nearly 100% linearly polarized. Additionally, we estimate the survey sensitivity placed by both system and red noise, and evaluate its capability of finding pulsars in orbital motion with either Sgr A* or a binary companion. We show that the survey is sensitive to only the most luminous pulsars in the known population and future observations with ALMA in Band-1 will deliver significantly deeper survey sensitivity on the GC pulsar population.

The AARTFAAC 60 MHz transients survey

ABSTRACT We report the experimental set-up and overall results of the AARTFAAC wide-field radio survey, which consists of observing the sky within 50° of Zenith, with a bandwidth of 3.2 MHz, at a cadence of 1 s, for 545 h. This yielded nearly 4 million snapshots, two per second, of on average 4800 square degrees and a sensitivity of around 60 Jy. We find two populations of transient events, one originating from PSR B0950+08 and one from strong ionospheric lensing events, as well as a single strong candidate for an extragalactic transient, with a peak flux density of 80 ± 30 Jy and a dispersion measure of $73\pm 3\, \mathrm{~pc~cm^{-3}}$. We also set a strong upper limit of 1.1 all-sky per day to the rate of any other populations of fast, bright transients. Lastly, we constrain some previously detected types of transient sources by comparing our detections and limits with other low-frequency radio transient surveys.

A study of the long-term activity of five intermediate polars with accretion discs

ABSTRACT Intermediate polars (IPs) are cataclysmic variables with mildly magnetized white dwarfs (WDs). This analysis of the long-term optical activity of five examples of IPs with accretion discs used data from the Catalina Real-time Transient Survey, Digital Access to a Sky Century @ Harvard (DASCH) and the American Association of Variable Star Observers (AAVSO). It is shown that each of these IPs had their most preferred value of absolute magnitude Mopt, even if it significantly varied on the superorbital time-scale. The values of Mopt of these IPs were in the zone of thermal-viscous instability (TVI) of the disc most of the time. The properties of a series of outbursts of V426 Oph can be explained by an intermittently operating TVI. The activity of TV Col and DW Cnc is interpreted as caused by a gradually variable mass inflow rate from the secondary to a cool disc. The mass transfer rate from the secondary varied on a well-determined time-scale. It is shown that Mopt of EI UMa, close to the peaks of outbursts of non-magnetic dwarf novae, fluctuated on the time-scale of days; it also produced short flares, ascribed to the bursts of matter from the donor. HY Leo, with a presumably cool disc, fluctuated between its high and low states. A temporary brightening from an extended low state is ascribed to a short, intense burst of matter from the donor to the remaining cool disc or torus.