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Register a new userOptical variability of the BL Lacertae object GC 0109+224. Multiband behaviour and time scales from a 7-years monitoring campaign
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We present the most continuous data base of optical $BVR_{c}I_{c}$ observations ever published on the BL Lacertae object GC 0109+224, collected mainly by the robotic telescope of the Perugia University Observatory in the period November 1994-February 2002. These observations have been complemented by data from the Torino Observatory, collected in the period July 1995-January 1999, and Mt. Maidanak Observatory (December 2000). GC 0109+224 showed rapid optical variations and six major outbursts were observed at the beginning and end of 1996, in fall 1998, at the beginning and at the end of 2000, and at the beginning of 2002. Fast and large-amplitude drops characterized its flux behaviour. The $R_c$ magnitude ranged from 13.3 (16.16 mJy) to 16.46 (0.8 mJy), with a mean value of 14.9 (3.38 mJy). In the periods where we collected multi-filter observations, we analyzed colour and spectral indexes, and the variability patterns during some flares. The long-term behaviour seems approximatively achromatic, but during some isolated outbursts we found evidence of the typical loop-like hysteresis behaviour, suggesting that rapid optical variability is dominated by non-thermal cooling of a single emitting particle population. We performed also a statistical analysis of the data, through the discrete correlation function (DCF), the structure function (SF), and the Lomb-Scargle periodogram, to identify characteristic times scales, from days to months, in the light curves, and to quantify the mode of variability. We also include the reconstruction of the historical light curve and a photometric calibration of comparison stars, to favour further extensive optical monitoring of this interesting blazar.
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We report results of multiband optical monitoring of two well known blazars, S5 0716+714 and BL Lacertae, carried out in 1996 and 2000-01 with an aim to study optical variations on time scales from minutes to hours and longer.The light curves were derived relative to comparison stars present on the CCD frames. Night to night flux variations of >0.1 mag were observed in S5 0716+714 during a campaign of ~2 weeks in 1996.A good correlation between the lightcurves in different optical bands was found for both inter-night and intra-night observations. Two prominent events of intra-night optical variability were detected in S5 0716+714.Each of these rapidly varying segments of the lightcurves trace an exponential flux profile whose rate of variation is the same in both cases. Our long-term monitoring data of S5 0716+714 showed a distinct flare around JD 2451875 which can be identified in the BVRI bands.This flare coincides with the brightest phase recorded during 1994-2001 in the long-term lightcurves reported by Raiteri et al.(2003). No evidence for the bluer when brighter trend was noticed on inter-night and intra-night time scales. On the other hand, our nearly simultaneous multiband observations of BL Lacertae in October 2001 showed flux variations that were not achromatic. This blazar was found to become bluer when brighter on intra-night time scales and there is a hint of the same trend on inter-night time scales. Based on five nights of observations during a week, BL Lacertae showed a peak night-to-night variability of ~0.6 mag in B. Thus, we found that the present observations of the two blazars, reveal a contrasting behaviour in terms of the dependence of spectral hardening with increasing brightness, at least on intra-night, and possibly also on inter-night, time scales.
Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while radio light curves extend back to 1978. Many of these data are presented here for the first time. The long-term trend shown by the optical light curves seems to vary with a characteristic time scale of about 3.3 years, while a longer period of 5.5-6 years seems to characterize the radio long-term variations. In general, optical colour indices are only weakly correlated with brightness. The radio flux behaviour at different frequencies is similar, but the flux variation amplitude decreases with increasing wavelength. The radio spectral index varies with brightness (harder when brighter), but the radio fluxes seem to be the sum of two different-spectrum contributions: a steady base level and a harder-spectrum variable component. Once the base level is removed, the radio variations appear as essentially achromatic, similarly to the optical behaviour. Flux variations at the higher radio frequencies lead the lower-frequency ones with week-month time scales. The behaviour of the optical and radio light curves is quite different, the broad radio outbursts not corresponding in time to the faster optical ones and the cross-correlation analysis indicating only weak correlation with long time lags. However, minor radio flux enhancements simultaneous with the major optical flares can be recognized, which may imply that the mechanism producing the strong flux increases in the optical band also marginally affects the radio one.
About twenty years of radio observations in five bands (from 4.8 to 37 GHz) of the BL Lac object GC 0109+224 (S2 0109+22, RGB J0112+227), are presented and analysed together with the optical data. Over the past ten years this blazar has exhibited enhanced activity. There is only weak correlation between radio and optical flares delays, usually protracted on longer timescales in the radio with respect to the optical. In some cases no radio flare counterpart was observed for the optical outbursts. The radio variability, characterised by peaks superposition, shows hints of some characteristic timescales (around the 3-4 years), and a fluctuation mode between the flickering and the shot noise. The reconstructed spectral energy distribution, poorly monitored at high energies, is preliminarily parameterised with a synchrotron-self Compton description. The smooth synchrotron continuum, peaked in the near-IR-optical bands, strengthens the hypothesis that this source could be an intermediate blazar. Moreover the intense flux in millimetre bands, and the optical and X-ray brightness, might suggest a possible detectable gamma-ray emission.
We monitored BL Lacertae simultaneously in the optical B, V, R and I bands for 13 nights during the period 2012-2016. The variations were well correlated in all bands and the source showed significant intraday variability (IDV). We also studied its optical flux and colour behaviour, and searched for inter-band time lags. A strong bluer-when-brighter chromatism was found on the intra-night time-scale. The spectral changes are not sensitive to the host galaxy contribution. Cross-correlation analysis revealed possible time delay of about 10 min between variations in the V and R bands. We interpreted the observed flares in terms of the model consisting of individual synchrotron pulses.
We present the results of the first systematic long-term multi-color optical monitoring of the BL Lacertae object 1ES 0806+52.4. The monitoring was performed in multiple passbands with a 60/90 cm Schmidt telescope from December 2005 to February 2011. The overall brightness of this object decreased from 2005 December to 2008 December, and regained after that. A sharp outburst probably occurred around the end of our monitoring program. Overlapped on the long-term trend are some short-term small-amplitude oscillations. No intra-night variability was found in the object, which is in accord with the historical observations before 2005. By investigating the color behavior, we found strong bluer-when-brighter chromatism for the long-term variability of 1ES 0806+52.4. The total amplitudes at the c, i and o bands are 1.18, 1.12, and 1.02 mags, respectively. The amplitudes tend to increase toward shorter wavelength, which may be the major cause of bluer-when-brighter. Such bluer-when-brighter is also found in other blazars like S5 0716+714, OJ 287, etc. The hard X-ray data collected from the Swift/BAT archive was correlated with our optical data. No positive result was found, the reason of which may be that the hard X-ray flux is a combination of the synchrotron and inverse Compton emission but with different timescales and cadences under the leptonic Synchrotron-Self-Compton (SSC) model.
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