Intranight Variability Research Articles

Intranight optical variability of radio-quiet and radio lobe-dominated quasars

We present results of a programme of multi-epoch, intra-night optical monitoring of a sample of non-blazar type AGN, which includes seven radio-quiet QSOs (RQQs) and an equal number of radio-loud, lobe-dominated quasars (LDQs), covering a redshift range from about 0.2 to 2.0. These two sets of optically bright and intrinsically luminous QSOs are well matched in the redshift-optical luminosity ($z - M_B$) plane. Our CCD monitoring covered a total of 61 nights with an average of 6.1 hours of densely sampled monitoring of just a single QSO per night, thereby achieving a typical detection threshold of $\sim 1$% variation over the night. Unambiguous detection of intra-night variability(INOV) amplitudes in the range 1$-$3% on day-like or shorter time scales were thus made for both RQQs and LDQs. Based on these clear detections of INOV, we estimate duty cycles of 17% and 9% for RQQs and LDQs, respectively; inclusion of the two cases of probable variations of LDQs would raise the duty cycle to 15% for LDQs. The similarity in the duty cycle and amplitude of INOV for the RQQs and LDQs suggests, firstly, that the radio loudness alone does not guarantee an enhanced INOV in QSOs and,secondly, that as in LDQs, relativistic jets may also be present in RQQs. We argue that, as compared to BL Lacs, the conspicuously milder, rarer and possibly slower INOV of RQQs and LDQs, can in fact be readily understood in terms of their having optical synchrotron jets which are modestly misaligned from us,but are otherwise intrinsically as relativistic and active as the jets in BL Lacs.

Optical variability properties of high luminosity AGN classes

We present the results of a comparative study of the intranight optical variability (INOV) characteristics of radio-loud and radioquiet quasars, which involves a systematic intra-night optical monitoring of seven sets of high luminosity AGNs covering the redshift rangez ≃ 0.2 toz ≃ 2.2. The sample, matched in the optical luminosity—redshift(M B—z) plane, consists of seven radio-quiet quasars (RQQs), eight radio lobedominated quasars (LDQs), five radio core-dominated quasars (CDQs) and six BL Lac objects (BLs). Systematic CCD observations, aided by a careful data analysis procedure, have allowed us to detect INOV with amplitudes as low as about 1%. Present observations cover a total of 113 nights (720 hours) with only a single quasar monitored as continuously as possible on a given night. Considering the cases of only unambiguous detections of INOV we have estimated duty cycles (DCs) of 17%, 12%, 20% and 61% for RQQs, LDQs, CDQs, and BLs, respectively. The much lower amplitude and DC of ESfOV shown by RQQs compared to BLs may be understood in terms of their having optical synchrotron jets which are modestly misdirected from us. From our fairly extensive dataset, no general trend of a correlation between the INOV amplitude and the apparent optical brightness of the quasar is noticed. This suggests that the physical mechanisms of INOV and long term optical variability (LTOV) do not have a one-to-one relationship and different factors are involved. Also, the absence of a clear negative correlation between the INOV and LTOV characteristics of blazars of our sample points toward an inconspicuous contribution of accretion disk fluctuations to the observed INOV. The INOV duty cycle of the AGNs observed in this program suggests that INOV is associated predominantly with the highly polarized optical emission components. We also report new VLA imaging of two RQQs (1029 + 329 & 1252 + 020) in our sample which has yielded a 5 GHz detection in one of them (1252 + 020;S 5GHZ ≃ 1 mJy).

Clear Evidence for Intranight Optical Variability in Radio-quiet Quasars

We present new clues to the problem of the radio loudness dichotomy arising from an extensive search for intranight optical variability in seven sets of optically luminous radio-quiet quasars and (radio-loud) BL Lacertae objects, which are matched in optical luminosity and redshift. Our monitoring of radio-quiet quasars has for the first time clearly detected such intranight variability, with peak-to-peak amplitudes ~1%, occurring with a duty cycle of ~. The matched BL Lac objects have both higher variability amplitudes and duty cycles when observed in the same fashion. We show that the much less pronounced intranight variability of the radio-quiet quasars relative to BL Lac objects can be understood in terms of a modest misalignment of the jets in radio-quiet quasars from the line of sight. We thus infer that relativistic particle jets may well also emerge from radio-quiet quasars, but while traversing the short optical-emitting distances, they could be snuffed out, possibly through inverse Compton losses in the nuclear region.

Multi-band optical micro-variability observations of BL Lacertae

We have observed BL Lacertae in the B, R and I bands for 2 nights in July, 1999, and 3 nights in July, 2001. The observations resulted in almost evenly sampled light curves, with an average sampling interval of ~5 min. The source is significantly variable in all bands. On average, the variability amplitude increases from ~5% in the I band, to ~5.5% in the R and ~6.5% in the B band light curves. The rising and decaying time scales are comparable within each band, but they increase from the B, to R and I band light curves. The optical power spectrum shows a red noise component with a slope of ~ -2. Cross-correlation analysis shows that in most cases the delay between the variations in the B and I band light curves is less than ~ 0.4 hrs. The cross-correlation functions are asymmetric, implying complex delays of the I band variations with respect to the B band variations. Furthermore, in one case we find that the I band variations are significantly delayed (by ~0.2 hrs) with respect to the B band variations. We also detect significant spectral variations: the spectrum becomes steeper as the flux increases, and the flattest spectral index corresponds to the maximum B band flux. Our results imply that the fast, intra-night variations of the source correspond to perturbations of different regions in the jet which cause localized injections of relativistic particles on time scales much sorter that the average sampling interval of the light curves. The variations are controlled by the cooling and light crossing time scales, which are probably comparable.

UBVRI simultaneous observations of the nucleus of Seyfert galaxy NGC 5548 in 1993–1999

An ongoing program on photometric and spectral monitoring of some bright Seyfert galaxies has been carried out at the Crimean Astrophysical Observatory since 1989. Results of photometric observations of NGC 5548 obtained with the 1.25 m telescope are reported in this paper; it focuses on the analysis of intranight variations. During 44 observational nights in 1993{1999 in each spectral band of the Johnson UBVRI system, 672 mea- surements have been performed simultaneously through the round aperture (diameter 15 00 ) using dierential photometry techniques. The estimated accuracy of each measurement is about 0.01 mag. The peak amplitude Rmax = Fmax=Fmin =2 :99 for the whole light curve was observed in the U band, while the minimum amplitude Fmax=Fmin =1 :37 occurred in the I band during the full observation period. UBVRI observations and good sampled data of international monitoring campaigns of NGC 5548, were used to calculate Structure Functions. A comparison is made of the characteristics of the long and short time scale variations of NGC 5548 with those of NGC 4151, NGC 7469 and NGC 1275. In order to examine the intranight variations of the nucleus of NGC 5548, standard deviations (SD) of the nightly averaged flux F , and a measure of intranight variability { SD=F were calculated for each night. Using this parameter, a probability characteristics is introduced, and duty cycles (the fraction of time when the galaxy is variable), characterizing the eciency of the central energy source, were eval- uated. It is concluded that intranight variability is really transient in character and manifests itself with dierent probabilities for dierent galaxies.

Optical Monitoring of S5 0716+714 from 1994 to 2000

We present multicolor optical (Johnson BV and Cousins RI) monitoring results of 5.3 yr for S5 0716+714. The four-band light curves showed a high duty cycle of variability. We studied its intranight variability and the quasi-periodic oscillations and found that the variability can be explained by a relativistic jet model.

Characteristics of the continuum variability of the nucleus of NGC 4151 in the optical from 1989-1996

The Structure Function (SF) analysis of variable fluxes of the Seyfert galaxy NGC 4151 obtained during its extraordinary brightening in 1989{1996 in the UBVRI bands is reported. More than 1500 measurements were made in each spectral band. The logarithmic slopes of SFs for all samples of flux observations are found to be dierent for intranight and extranight flux variations: 0.81{0.96 and 0.44{0.60, respectively. The dependence of the SF slopes on the phase of nucleus activity was revealed. The SF slopes increased for intranight variations in accordance with the nucleus brightening and decreased for yearly variations. The processes causing the flux variations evolved during the nucleus brightening in dierent directions: for intranight variations the process evolved from flicker{noise to extreme shot{noise; for yearly variations { vice versa. The obtained data provide evidence in favor of the supposition that intranight and extranight variations are caused by dierent sources. As a result, the variable source of the NGC 4151 nucleus during the active phase in the UBVRI bands exhibited characteristics of radio loud AGNs. The highest values of logarithmic slopes of SF for variations on a time scale of years were in the range 0:8 b 1:1, and for 10{150 day flares they were as high as 2.2. There was flattening of the optical spectrum with increasing nucleus brightness. The brightness amplitude of the nucleus in the U band reduced to an aperture D =5 00 over 7.3 years increased by a factor of more than 50. The brightness in the U band varied by a factor of more than 10 per year. The obtained characteristics of the variable source acting in the NGC 4151 nucleus from 1990{1996 do not contradict the model where the increasing nucleus brightening is caused by clouds of synchrotron radiation ejected from the nucleus during its active period.

Multiband Optical Polarimetry of the BL Lacertae Object PKS 2155−304: Intranight and Long‐Term Variability

The polarized and total flux of the BL Lac object PKS 2155-304 were monitored intensively and simultaneously in the optical UBVRI bands with the Turin photopolarimeter at the CASLEO 2.15 m telescope during four campaigns in 1998 June, August, and November and 1999 August. The effective observation time amounted to ~47 hr. PKS 2155-304 showed a linear polarization percentage (P) usually ranging between 3% and 7% and a polarization position angle (P.A.) mainly between 70° and 120°. The highest temporal resolution of our observations, 15 minutes, is unprecedented for polarimetric monitoring of this source and has allowed us to detect amplitude variations of the linear polarization percentage from 6% to 7.5% in timescales of hours. In some nights the polarization percentage seems to increase toward shorter wavelengths; however, the polarized spectrum does not vary significantly with time. The most remarkable variability event occurred on 1998 June 18, when the degree of linear polarization decreased by more than a factor 2 in 1 day in all bands, while the P.A. rotated by 90°. This is consistent with the presence of two emission components, of different polarization degree and position angle. Intranight variability of P and P.A. can be interpreted with small-amplitude physical or geometrical changes within the jet. Measurements of the circular polarization over time intervals of days set upper limits of 0.2%. Simultaneous photometry taken with the Turin Photopolarimeter and with a CCD camera at Cordoba Astronomical Observatory did not show light variations correlated with those of the linearly polarized flux.

The spontaneous K-complex during stage 2 sleep: is it the ‘forerunner’ of delta waves?

The hypothesis that K-complexes (KCs) contribute to the process of synchronization leading to Slow-Wave Sleep (SWS) was evaluated by measuring their dynamic evolution across sleep cycles and before transitions to rapid eye movement (REM) or to SWS. KC density and inter-KC intervals respectively decreased and increased across the sleep cycles, revealing linear trends. Comparisons among transitions from stage 2 to SWS or to REM sleep showed a prevalence of KCs before the shift to SWS as compared to REM. Changes in KC density before the shift to SWS were fitted by a linear regression, at variance with the transition to REM sleep. Intra-night variations of KCs, paralleling the well-known decrease of slow waves across sleep cycles, and intracycle variations before shifting to SWS, both converge to indicate that KCs can be considered as the forerunner of delta waves.

Nereid Has Complex Large-Amplitude Photometric Variability

We report on 224 photometric measurements of Nereid, a small outer satellite of Neptune with a 360-day orbit of high eccentricity (0.751). Our photometry covers 64 nights from 1987 to 1997 and is primarily in the V-band, although we also have 20 measurements in the U, B, R, and I bands. (1) Nereid displays large-amplitude brightness variations with a total amplitude of 1.83 mag on time scales ranging from a few hours to roughly a year. (2) During the 12 days of the Voyager encounter with Neptune, Nereid did not display any short-term variations; however, large-amplitude long-term variations could easily be hidden by the large phase effects and the short duration of observation. (3) Nereid's variability is caused by high contrast albedo features, i.e., a dark hemisphere, along with rotational modulation. (4) The character of the brightness variations changed around 1991 from fast and large-amplitude to comparatively slow and low-amplitude. This demonstrates that the direction and magnitude of Nereid's rotational angular momentum vector is changing on time scales comparable to its orbital period. (5) Large changes in the magnitude and direction of Nereid's angular momentum vector are predicted to arise from chaotic rotation during every periapse passage provided that Nereid is more than ∼1% nonspherical and is spinning slowly. The match between prediction and observation could be taken as strong evidence for chaotic rotation of Nereid. However, the intranight variability cannot be readily explained by chaotic rotation. (6) The colors of Nereid are U-V=0.84±0.05, B-V=0.71±0.04, V-R=0.44±0.03, V-I=0.72±0.05, and V-K=1.6, indicating a nearly flat reflectance spectrum from 0.36 to 2.2 μm. We identify asteroids, inner satellites, and centaurs with similar spectra. (7) Nereid is likely either an inner moon of Neptune kicked to its current orbit or a captured Kuiper Belt object or centaur, with the latter possibility being strongly preferred.

Rapid optical variability in radio-quiet QSOs

ABSTRA C T We report results of the observations at the Vainu Bappu Observatory and the Uttar Pradesh State Observatory of eight radio-quiet quasi-stellar objects (RQQSOs) during 1996‐99. This is a part of our ongoing programme to search for intranight optical variability in RQQSOs. Additional evidence for very rapid variability in three of the five optically bright and very luminous RQQSOs we had observed earlier, 10492006, 14441408 and 16301377, was found. Of the three newly observed RQQSOs, the data for 00431039 are too noisy to allow conclusions about variability to be drawn, but 07481294 and 08241098 show strong hints of microvariability. We also present a summary of the results from our entire programme to date, which includes observations of 16 radio-quiet QSOs and one radio-weak QSO, and compare the general properties of rapid variability in radio-quiet versus radio-loud AGN as determined from our work and that of several other groups. Observations of this kind are likely to play a key role in understanding the relative contributions of accretion discs and relativistic plasma jets to rapid optical fluctuations of AGN.

ITAL]UBVRI[/ITAL] Light Curves of the Seyfert Galaxy NGC 7469 During 1990–1998: Microvariability

Observations of the nuclear region of the Seyfert galaxy NGC 7469 obtained at Crimean Astrophysical Observatory with the 1.25 m telescope are presented. During 64 nights on nine observational runs between 1990 September 24 and 1998 October 22 in each spectral band of the Johnson UBVRI system, about 1500 measurements have been performed simultaneously through the round aperture 20'' in diameter using differential photometry techniques. The estimated accuracy of each measurement is about 0.01 mag. During the observing period 1990–1996 the mean luminosity of the nucleus was almost constant; only overlapping brightness fluctuations were observed. The mean luminosity level has been raised in 1996 October. The peak amplitude (maximum flux/minimum flux) Fmax/Fmin = 2.09 on the light curves was observed in the U band, while the minimum amplitude Fmax/Fmin = 1.32 was in the I band for the entire observation period. Using structure function (SF) analysis, the following conclusions have been made: (1) Long-term variability is caused by the same processes in the optical, because the slope b of the SF is approximately equal for all wave bands, except for the I band the slope is appreciably distinguished from the others. This would be an indication of the presence of an independent IR energy source in NGC 7469. (2) Considering the same time interval (from 6 minutes to 2 hr) for intranight variability on SFs at different wave bands, one can conclude that flicker noise causes variations observed on the light at the UV region (U and B bands), while at the near-IR region the light is formed by mixed shot noise and flicker noise—the greater the wavelength, the more the contribution of shot noise processes. (3) On intranight light curves of the NGC 7469 there exist rapid flares with durations ~25 minutes at U band, ~55 minutes at B, V bands, and ~2 hr at R, I bands—a typical timescale of intranight variability increasing with the increasing wavelength. In order to examine the intranight variations of the nucleus of NGC 7469, standard deviations (SDs) of the nightly averaged flux, F, and a measure of intranight variability, SD/F, were calculated for each night of observations. The ratios P = Ni/Ntot of number of nights with SD/F ≥ 1%, 2%, 3%, and so forth (Ni) to all 64 observational nights (Ntot) were expressed as a percentage of to detect variability at a given level; they were plotted versus the appropriate parameter SD/F. It is interesting that a to detect intranight variability at a level given by a parameter SD/F may be fitted well by a probability curve for a given galaxy. The homogeneity of data obtained with the same telescope using the same technique has made statistical comparisons possible among different galaxies and different wave bands. Therefore, analogous curves for the Seyfert galaxies NGC 1275 and NGC 4151 were plotted and were averaged by bands. Probability curves for the two galaxies of the same type (Seyfert 1; NGC 4151 and NGC 7469) coincided well. The for the galaxy NGC 1275 of BL Lacertae type shows more nights with variations of amplitude less than 7%. The to observe intranight variations with an amplitude of more than 7% is approximately equal for both types of objects. An excess of nights with intranight variability parameter SD/F in the range of ~4%–8% is a common characteristic of the two Seyfert 1 galaxies (NGC 4151 and NGC 7469) and of the BL Lac object (NGC 1275) studied in this paper. This excess may be connected with a specific type of intranight galaxy variability. All curves are fitted best by the second-order exponential decay. As a result, one can conclude that intranight variability is really transient in character and has manifested itself with different probabilities for different galaxies. The efficiency of the energy source for every galaxy to produce intranight variability on a given level (duty cycle of the central energy engine) was estimated using these curves. For a given threshold of detection estimated as an error of a given technique (for instance, 1% for observations presented in this paper), one can conclude that duty cycle of NGC 1275 is about 100%. Vice versa, NGC 4151 has a more quiet energy source: only ~60% of nights show intranight variability on a level greater than 1%. The efficiency of the energy source producing the near-IR radiation of NGC 7469 is more than 0.9, while at the optical bands it is equal to ~0.7 ± 0.1. It may be an indication of the presence of an independent IR energy source, too.

Intranight variability of NGC 1275 nucleus in optics

Abstract Intranight variability of NGC 1275 nucleus in optics was investigated using the photoelectric observations in 1982–1994. Analysis of these data shows that a process of mixed shot-noise and flicker-noise operated in the nucleus.

ITAL]UBVRI[/ITAL] Observations of the Nucleus of NGC 1275 from 1989 to 1994: Microvariability

Intranight variability of the NGC 1275 nucleus in the optical range has been studied using five series of photoelectric observations obtained over 37 nights from 1989 December 22 to 1994 December 29. The UBVRI monitoring on a single night continued for up to 4.5 hr. In total, 820 measurements were performed simultaneously in each of the UBVRI bands (3600 A ≤ λeff ≤ 8300 A) with the 1.25 m telescope at the Crimean Astrophysical Observatory. Flux densities (F) with their standard deviations (SD) and the measure of intranight variability (SD/F) were calculated for each night of observations. On 40% of the nights, intranight activity in the nucleus was seen at a level of variability SD/F ≥ 3% in all spectral bands. Independent activity in the nucleus in the I band (the spectral region around 8300 A) is suspected on about 10% of nights. The peak amplitude of the variability, Fmax/Fmin ~ 40%, was observed only once during the 37 nights. There was suspected low-level intranight variability in each of the UBVRI spectral bands for nights with SD/F ≤ 3%. The data we obtained lead us to argue that during 1989–1994 a process operated in the nucleus of the galaxy NGC 1275 that caused variations of the optical flux on a timescale of several hours. We obtain preliminary evidence that the causes of intranight and internight variations seem to be different. All of the data obtained are discussed from the point of view of current models of active galactic nuclei.

Photometric monitoring of three BL Lacertae objects in 1993-1998

The results of optical photometric (BV RI) monitoring of three BL Lac objects over a time interval of about four years are presented. The sources are three classical radio-selected BL Lac objects, BL Lac, OJ 287 and PKS 0735+178. During our observation OJ 287 was in the stage of a large periodic outburst which consisted of at least two peaks. Almost all the observations obtained over consecutive nights detected intranight variations. In 1995 and 1996 BL Lac kept in faint states, with fewer and smaller rapid flares and fluctuations. On the contrary, in late 1997 BL Lac was at the stage of a large outburst, accompanied with much more large amplitude rapid flares and fluctuations. PKS 0735+178 was almost at its faint end from 1994 to early 1998. Over this time interval, the intraday variations and microvariations in PKS 0735+178 were rare and the amplitude was very small, except a rapid darkening of similar to 0.4 mag on 24 January 1995. Previous work by Webb et al. (1988); Wagner et al. (1996); Plan et al. (1997) also showed the same behaviour of variability as BL Lac and PKS 0735+178 in BL Lac, S5 0716+714, PKS 2155 - 304, respectively. We propose that the motion of orientation of the relativistic jet in a BL Lac object be responsible for these variability behaviours.

Multi-colour optical monitoring of the intra-day variable blazar S5 0716+71

We report multi-colour optical monitoring of the intra-day variable blazar S5 0716+71 with a dense temporal coverage over a total duration of 4 weeks in February/March 1994. Four lters (BV Johnson and RI Cousins) were used on a majority of the nights. The light curves of the blazar were derived relative to two compar- ison stars present on the CCD frames. Intensity varia- tions of 0.05 mag from night-to-night were observed in half the cases and a good correlation between the light curves in the dierent passbands was noticed (as also for the observed intra-night variations). There seems to be no correlation between flux and optical spectral index of the blazar. In all, three prominent events of intra-night variability were detected, each with a rate of 0.1 mag per hour, lasting for at least 3 hours. Each of these flaring segments of the blazar light curves can be well tted with an exponential intensity prole whose rate of variation is essentially the same for all the passbands. No clear event of ultra-rapid fluctuations of the blazar on time scale 1 hour was detected in these multi-colour data. Present ob- servations have been carried out during a low brightness level of the blazar and they have no large amplitude vari- able events.

3.9. Intranight variability of NGC 1275 nucleus in optics implying its central activity

NGC 1275 is one of the enigmatic blazars. The nature of central sources of blazars is open to discussion. Variability measurement of continuum is a major touchstone in the understanding of the nature of central sources of AGNs. Below we examine the intranight optical variability of NGC 1275 nucleus using our observations in 1982-1994.

The Long‐Term Periodic Oscillation of the NGC 4151 Nucleus

The hypothesis that the luminosity of the nucleus of the Seyfert 1 galaxy NGC 4151 varies with a period of P0 ≈ 160.0101 minutes is revised. In 1987-1994 a series of photometric observations of the NGC 4151 has been carried out in the U and V filters at several instruments. In the total sample of 27 nights, the U data of 15 nights showed an appreciable intranight variability of the AGN flux. Being detrended and folded with the a priori period P0, those data exhibited significant P0 signal coinciding in phase with that found in previous studies. Nearly the same result is obtained for 39 nights in the V-filter. The average harmonic amplitudes of the P0 oscillation (for the total interval 1968-1994) are found to be ≈ 7 and ≈ 2 mmag in the U and V filters, respectively. The power spectrum was computed for the total data series 1968-1994 (in all 2771 measurements including X-ray data from Ariel 5, EXOSAT, and Ginga satellites). It reveals a prominent peak corresponding to a period of 160.0104 ± 0.0005 minutes (with the lower level of confidence 3.6 σ). The latter fairly well coincides with the P0 period discovered much earlier in the power spectrum of global oscillations of the Sun. This analysis thus strongly supports a cosmological interpretation of the ubiquitous oscillation with the P0 period.

Optical photometric monitoring of γ-ray loud blazars.

The results of the optical monitoring between November 1994 and November 1995 of twenty γ-ray loud blazars included in the Torino blazar monitoring program are presented. All data were taken with the REOSC astrometric telescope of the Torino Astronomical Observatory, equipped with a pixels charge-coupled device (CCD) camera. Observations have been carried out in the standard B, V (Johnson), and R (Cousins) bands. Source magnitudes are calculated with respect to reference stars in the same frame. For half blazar fields it was possible to perform a photometric calibration through the observation of Landolt's fields during photometric nights. In the other cases either photometric sequences from the literature were adopted or, when lacking, magnitudes were simply normalized to their minimum value. Most of the monitored objects show a more or less pronounced brightness variability on both short and long time scales. In a few cases also noticeable intranight variations were detected. Data simultaneous to pointings of the Compton Gamma Ray Observatory (CGRO) are present in our light curves: when the γ data are available they will provide a useful information in order to understand the possible correlations between the optical and γ-ray emissions. For one source (PKS 2254+074) we performed also photopolarimetry, deducing magnitudes, amount of polarization, and position angle in the UBVRI bands.

Intranight optical variability in optically selected QSOs

We report results from a continuing programme to search for intranight optical variability in one radio-moderate and six radio-quiet (but all optically bright and luminous) quasi-stellar objects, using the 2.34-m Vainu Bappu Telescope. Our data clearly show microvariability for the radio-moderate QSO, 0838+359, and provide strong indications of microvariability for two of the radio-quiet QSOs, 0946+301 and 1049-006, and possibly also for a third, 1206+459. Additional observations of these and other QSOs could provide a powerful means of discriminating among various theoretical mechanisms proposed for the energy source and, in particular, the origin of optical microvariability in active galactic nuclei. These apparent detections of microvariability in radio-quiet QSOs seem to favour models where flares on accretion discs are responsible for the microvariability.