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The optical microvariability and spectral changes of the BL Lacertae object S5 0716+714

We monitored the BL Lac object S5 0716+714 in the optical band during October 2008, December 2008 and February 2009 with a best temporal resolution of about 5 minutes in the BVRI bands. Four fast flares were observed with amplitudes ranging from 0.3 to 0.75 mag. The source remained active during the whole monitoring campaign, showing microvariability in all days except for one. The overall variability amplitudes are delta B ~ 0.89 mag, delta V ~ 0.80 mag, delta R ~ 0.73 mag and delta I ~0.51 mag. Typical timescales of microvariability range from 2 to 8 hours. The overall V - R color index ranges from 0.37 to 0.59. Strong bluer- when-brighter chromatism was found on internight timescales. However, different spectral behavior was found on intranight timescales. A possible time lag of ~ 11 mins between B and I bands was found on one night. The shock-in-jet model and geometric effects can be applied to explain the source's intranight behavior.


Introduction
Blazars represent an extreme subclass of active galactic nuclei. They are characterized by rapid and strong variability throughout the entire electromagnetic wavebands, high and variable polarization(>3%) from radio to optical wavelengths. In the unified model of AGNs, blazars make an angle of less than 10 • from the line of sight (Urry & Padovani 1995). For low-energy peaked (or radio-selected) blazars the continuum from radio through the UV or soft X-rays is mainly contributed by synchrotron radiation, while a second hump in the spectrum at higher energies usually peaks in the GeV γ-ray band. Blazars exhibit variability on different timescales, from years down to hours or less (See Fan et al. 2005). Understanding blazar variability is one of the major issues of active galactic nuclei studies. There may be different behavior on different timescales. Periodicity may be found on the long term, as is the case of OJ 287 (Sillanpaa et al. 1988) and other blazars (Fan et al. , 2007. The periodicity of OJ 287 can be explained by a binary black hole model, with the secondary black hole passing through the accretion disk of the primary black hole twice per revolution (Sillanpaa et. al 1988;Lehto & Valtonen 1996). On shorter timescales, 3C 66A was claimed to have an optical period of ∼ 65 days during its bright state (Lainela et al. 1999), while Mkn 501 may have displayed a period of 23 days in high energy data (Osone 2006). Recently, analyses of X-ray data have yielded excellent evidence for a quasi-period of about an hour for 3C 273 (Espaillat et al. 2008) and very good evidence for near periods of ∼ 16 days for AO 0235+164 and ∼ 420 days for 1ES 2321+419 (Rani et al. 2009). This may be due to a shock wave moving along a helical path in the relativistic jet (e.g., Marscher 1996) or the unstability in the accretion disk  for Mkn 501. Different viewing angles may also result in different level of brightness, with the source being dimmer at larger viewing angles. BL Lacertae shows variability on both intranight and internight timescales with different spectral behaviors.
It shows an intranight strongly chromatic trend and an internight midly chromatic trend, -4indicating two different components operating in the engine (Villata et al. 2004;Hu et al. 2006). Understanding the shortest variability timescale is of special importance.
Brightness changes of up to a few tenths of a magnitude over the course of a night or less is known as intra-night optical variability (INOV) (Wagner & Witzel 1995) or the so-called microvariability. It can bring new insight into the understanding of the nature of blazars, probing into the innermost structure down to microparsecs, putting constraints on the size of the source and its physical environments.
Microvariability was first discovered in the sixties by Matthews and Sandage (1963), who found 3C 48 changed 0 m .04 in the V band in 15 minutes. But their results were not taken seriously and were considered due to instrumental errors. However, with the development of CCD, microvariability was confirmed to be the intrinsic nature of active galactic nuclei, especially for blazars (e.g., Miller et al. 1989). From a complete sample of BL Lac objects (Stickel et al. 1991), Heidt and Wagner (1996) found 80% exhibited microvariability, proving it to be the nature of BL Lac objects. Romero et al. (1999) detected microvariability in 60% of their selected sample of 23 southern AGNs.
Microvariability became extensively observed since the eighties. Up to now, the reasons for it are still unclear. Many different models have been proposed to explain this phenomenon.
It conceivably may be due to eclipses if there is a binary black hole system . The interaction of relativistic shock waves and inhomogeneous jet can also cause microvariability in the jet (Maraschi et al. 1989;Qian et al. 1991;Marscher 1992;Marscher 1996). Other models like instabilities and perturbations in the accretion disk can explain some of the microvariability phenomena (e.g., Mangalam & Wiita 1993;Wiita 1996;Fan et al. 2008). In order to understand the radiation mechanism and the structure of the radiating region, long-term and multiwavelength observations are needed. The BL Lac object S5 0716+714 is one the brightest BL Lac objects noted for its microvariability. Its high duty cycle means that it is always in an active state (Wagner & Witzel 1995). It -5has been the target of many monitoring programs (e.g., Wagner et al. 1996;Qian et al. 2001;Raiteri et al. 2003). Montagni et al. (2006) reported the fastest variability rate of 0.1-0.12mag/hr. Five major outbursts have been observed so far, occurring at the beginning of 1995, in late 1997, in the fall of 2001, in March 2004 and at the beginning of 2007 (Raiteri et al. 2003;Foschini et al. 2006;Gupta et al. 2008). These five outbursts indicate a long-term variability timescale of ∼ 3.0±0.3 years (e.g., Raiteri et al. 2003). Correlated radio/optical variability has been observed for the source. In a 4-week monitoring program, Quirrenbach et al. (1991) discovered a period change of 1 day to 7 days in both radio and optical bands. Heidt & Wagner (1996) reported a period of 4 days in the optical band while Qian, Tao & Fan (2002) derived a 10-day period from their 5.3 yr optical monitoring.
Recently Gupta et al. (2009) have found good evidence for quasi-periods in five of the 20 best quality nightly data sets of Montagni et al. (2006); these ranged from ∼ 23 to ∼ 75 minutes.
The spectral change of S5 0716+714 has been observed extensively (e.g., Ghisellini et al. 1997;Raiteri et al. 2003;Villata et al. 2004;Gu et al. 2006;Wu et al. 2005). Different behaviors have been reported. Raiteri et al. (2003) reported different chromatism in different timescales in their 8-year monitoring program. Sometimes the source was bluer when brighter, sometimes the opposite, sometimes no spectral change was seen despite changes in brightness. Ghisellini et al. (1997) and Wu et al. (2005) found the source exhibited a bluer-when-brighter chromatism when it was in fast flares. However, Stalin et al. (2006) found even if the source was in fast flare, there was no color change with brightness. In this paper we concentrate on the microvariability and spectral changes of S5 0716+714. We monitored the source from October 25 to 30 2008, December 23 to 29 2008 and February 3 to 10 2009. The temporal resolution was around 5 to 8 minutes in the four optical bands (BVRI ). Because of the high temporal resolution, we can provide high quality data with accurate results.
-6 -The paper is organized as follows: Section 2 describes observations and data reduction procedures. Section 3 presents the results. Discussions are reported in Section 4. A summary is given is Section 5.

Observations and Data Reductions
Our photometric observations were carried out at the 85-cm telescope which is located at the Xinglong Station of the National Astronomical Observatories of China (NAOC). This telescope is equipped with a standard Johnson-Cousin-Bessel multicolor CCD photometric system built on the primary focus (Zhou et al. 2008). The PI1024 BFT camera has 1024 × 1024 square pixels, a field of view of 16 ′ .5 × 16 ′ .5 at a focal ratio of 3.27 (f = 2780mm) with a scale of 0.96 arcsec per pixel.
After flat-fielding, bias and dark corrections, aperture photometry was performed using the apphot task of IRAF. The instrumental magnitudes of the source and comparison stars were then collected and then processed in order to obtain the standard magnitudes of S5 0716+71 and the relevant errors (Note that data in the I band were not calibrated due to large photometric errors). Two standard stars in the blazar field, 5 and 6 in the finding chart of Villata et al. (1988), were compared to check that any reported variations were intrinsic to the blazar and that each standard star was non-variable. The magnitude difference between the two stars in the B, V, R, I bands are 0 m .09, 0 m .08, 0 m .08 and 0 m .12 respectively. The observational log is presented in Tables 1 − 4 with columns being Julian date, magnitude and uncertainty. We first determined the differential instrumental magnitude of the blazar, star 5 and star 6. We determined observational scatter from the blazar − star 5 (σ (BL − Star 5)) and star 5 − star 6 (σ (Star 5 − Star 6)). The rms errors are calculated from the two stars using the formula: where m i = (m 5 − m 6 ) i is the differential magnitude of stars 5 and 6 whilem = m 5 − m 6 is the differential magnitude averaged over the entire dataset, and N is the number of observations on a given night. The variability of the source is then investigated by means of the variability parameter, C, introduced by Romero et al. (1999). This parameter is defined as: C = σ (BL−5) σ (5−6) . If C > 2.576, the source is variable at 99% confidence level. The object is decided to be variable only when C ≥ 2.576 at least in two bands if it is monitored in three or more bands (Jang & Miller 1997). The intranight variability is calculated (Heidt & Wagner 1996): where A max and A min are the maximum and minimum values of each light curve and σ is the same as what is described above. The results are given in Table 5 for filters B, V, R and I. Column (1) is the universal date of observation, column (2) the band, column (3) the intranight variability amplitude, column (4) the number of data points, column (5) the value of the C parameter (V/N indicates whether the source is variable or not), column (6) the rms errors, column (7) the Pearson correlation coefficients of the V -R color index vs.
R magnitude.
Our monitoring program of S5 0716+714 was divided into three periods. The first period was from October 25 to 30 2008, the second from December 23 to 29 2008 and the third from February 3 to 10 2009. As a result of weather conditions, 14 nights of data were obtained. In the first period of our observations, only filters V and R were used. In the second and third periods longer observation times allowed us to usually use four filters: B, V, R and I. The typical exposure times for BVRI measurements were 100s, 70s, 50s and 30s respectively. The readout time is about 10s, resulting in a temporal resolution of around 5 minutes for the best weather conditions. Because of the dense sampling and high quality -8 -(photometric error ∼ 0.004 mag), the data set can provide highly reliable results.

Light Curves
The overall light curves in bands V and R are displayed in Fig Clear intranight variations can be seen during the whole monitoring campaign. We detected significant brightness changes in a large fraction of nights: only in 1 night (marked -9with an asterisk in Table 5)

Spectral Variability
Based on our observations of high temporal resolution, the spectral variability with brightness was investigated in this section. Magnitudes have been dereddened by using a galactic extinction coefficient A B = 0.132, A V =0.102, A R =0.082 (Schlegel et al. 1998). We observed on all nights, with variability amplitude ranging from ∼ 0.04 -0.2 (see Table 5).
On the first night of observations of period 1, JD 2454765, the variability amplitude in the The trend that the source displayed stronger BWB chromatism with increasing variability amplitude is still noticeable in period 2 but not as obvious as in period 1 and again the color change does not seem to be related to brightness. On the whole, we find no consistent relation between color vs magnitude and also color vs variability amplitude on intranight timescale. BWB chromatisim and achromatisim can be found when the source is bright or faint, showing large variability amplitude or small variability amplitude. This suggests that even on intranight timescales, different mechanisms may be at work.

Discussions
The microvariability of S5 0716 + 714 has been observed by many authors. Some Time lags between different optical bands have also been reported. Stalin et al. (2006) found possible time lags of ∼ 6 and ∼ 13 mins between the V and R bands in their 2 nights of observations of the source. Qian et al. (2000) reported similar results in which they found a time lag of ∼ 6 mins between the V and I bands. From densely sampling data, Villata et al. (2000) presented a time lag with a strict upper limit of ∼ 10 mins between the B and I band. In this work, we tried to find out the time lag between B band and I band using the discrete cross-correlation function, DCF, suggested by Edelson & Krolik (1988) for unequally spaced data. It was performed only on the day with high-quality data (error ∼ 0.003 and ∼ 0.005 for B and I band respectively) and dense sampling rate (temporal resolution ∼ 5 minutes), which is JD 2454826, the day with with a hint of periodicity in its light curve. Fig. 16 presents the results of the calculations. The dashed line indicates the centroid which was calculated using the method proposed by Peterson (2001) and we found the barycenter using data points located near the peak value, DCF peak , specifically, those greater than 0.8DCF peak . The expression is: We found a time lag of ∼ 11 minutes, with the B band leading I band. However, given the sampling rate of ∼ 5 minutes and the negligible difference between the DCF values at -20 minutes and 0 minutes, this cannot be considered a convincing measurement of a lag. This result is consistent with Villata et al. (2000), who presented a strict upper limit of ∼ 10 minutes to a possible delay between B -and I -band variations using high-quality, densely sampled data on a single night. This result is expected in the shock-in-jet model but not in most disc-based variability models (e.g. Wiita 2006).
-14 -In order to quantitatively analyze possible periods, we performed structure function (SF) on JD 2454826, the only day with a lightcurve that shows a hint of a period. SF, discussed fully by Simonetti el al. (1985), is a common tool to search for periodicities and timescales of variation . It identified a timescale of 259 mins, 287 mins, 280 mins and 263 mins and a period of 482 mins, 497 mins, 498 mins and 491 mins were found for B, V, R and I band respectively. All these results are consistent with visual inspection. The results are shown in Fig. 17. Of course, since only one "cycle" is present that night, the presence of a period is no more than speculative; such a possible period does not appear to extend to the previous night, for which there is also data. Only observations made with multiple ground based telescopes at different longitudes (or space based telescopes) can convincingly find intranight periods that substantially exceed ∼ 1 hr.
The spectral variability of blazars has been investigated by many authors (e.g., Ghisellini et al. 1997;Fan & Lin 1999;Romero et al. 2000;Raiteri et al. 2003;Villata et al. 2000Villata et al. , 2004. Raiteri et al. (2003) found different spectral behavior for the S5 0716 + 714 in short timescales. Sometimes the source was bluer when brighter, sometimes the opposite and sometimes no spectral change was seen. Stalin et al. (2006) found no clear evidence of color variation with brightness in either their internight or intranight monitoring of the source for a fortnight. Ghisellini et al. (1997) and Wu et al. (2005) reported a BWB trend during fast flares but this trend is insensitive in the long-term. However, Wu et al. (2007) noticed that the source is bluer when brighter on both intranight and internight timescales but this trend was not present in the long-term data.
In our monitoring campaign, the source also showed different spectral change behavior.
On the long term, the source showed a bluer-when-brighter behavior. Unlike previous studies which reported consistent trends on spectral behavior on intranight timescales (e.g. Ghisellini et al. 1997, Stalin et al. 2006, Wu et al. 2005, no consistent spectral behavior is -15found among all those nights displaying microvariability in our present study. The source displayed BWB chromatism when it was either bright or dim, or when showing large or small variability amplitudes. Achromatism was also found when the source was in the same conditions, suggesting different mechanisms for microvariability. The BWB behavior is consistent with shock-in-jet model (Wagner et al. 1995). induced by a helical structure to the jet (e.g. Wagner & Witzel 1995). Microlensing predicts a strictly symmetric lightcurve, which cannot explain the concave shape of the second halves of the light curves in our case. Therefore a lighthouse effect is the most probable mechanism for explaining periodic components in blazar lightcurves as this type of variation is likely to be achromatic (e.g., Camenzind & Krockenberger 1992;Gopal-Krishna & Wiita -16 -1992).

Summary
We monitored S5 0716+714 during the period October 2008    R(bottom left) and R(bottom right) bands. Crosses represent the differential magnitude between the source and star 5 while plus signs represent the differential magnitude between star 5 and star 6 shifted by an arbitrary offset. I (bottom left) bands. Crosses represent the differential magnitude between the source and star 5 while plus signs represent the differential magnitude between star 5 and star 6 shifted by an arbitrary offset. R(bottom left) and I (bottom right) bands. Crosses represent the differential magnitude between the source and star 5 while plus signs represent the differential magnitude between star 5 and star 6. bands. Crosses represent the differential magnitude between the source and star 5 while plus signs represent the differential magnitude between star 5 and star 6. V (top right) and R(bottom left) bands. Crosses represent the differential magnitude between the source and star 5 while plus signs represent the differential magnitude between star 5 and star 6. V (top right) and R(bottom left) bands. Crosses represent the differential magnitude between the source and star 5 while plus signs represent the differential magnitude between star 5 and star 6.