The Optical Microvariability and Spectral Changes of the BL Lacertae Object S

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1 J. Astrophys. Astr. (2011) 32, c Indian Academy of Sciences The Optical Microvariability and Spectral Changes of the BL Lacertae Object S H. Poon 1,, J.H.Fan 2 & J.N.Fu 1 1 Astronomy Department, Beijing Normal University, Beijing , China. 2 Center for Astrophysics, Guangzhou University, Guangzhou , China. china_108@yahoo.com Abstract. The BL Lac object S was monitored in the optical band during October 2008, December 2008 and February We achieved a best temporal resolution of about 5 min in the BVRI bands. The source was active during the whole monitoring campaign. It showed microvariability in 13 days out of 14 days of observation. Four fast flares were observed with amplitudes ranging from mag. The overall amplitude changes during the whole campaign are B 0. m 89, V 0. m 80, R 0. m 73, and I 0. m 51. On internight time scales, strong bluer-when-brighter chromatism was found while different spectral behaviours were found on intranight time scales. Key words. BL lacertae objects: individual (S ) galaxies: active galaxies: photometry. 1. Introduction Since the development of CCD in the 80s, microvariability of blazars has been widely observed. Both external and internal reasons are found for this phenomenon. The external ones include interstellar scintillation, micro-lensing or geometric effects. The internal ones include shocks moving down the relativistic jet or instabilities in the accretion disk (Wagner & Witzel 1995). Studying microvariability can bring new insight into the understanding of the nature of blazars, probing into the innermost structure down to microparsecs, and putting constraints on the size of the source and its physical environments. In order to understand microvariability better, we observed the BL Lac object S in three epochs from October 2008 to February Observations and results The observations were carried out using a 85 cm telescope 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. 2009). The PI1024 BFT camera has square pixels, a field of view of at a focal ratio of 3.27 ( f = 2780 mm) with a scale of 0.96 arcsec per pixel. 97

2 98 H. Poon et al. Figure 1. 10, Light curves of S in the VR bands from October 25, 2008 to February The overall light curves in V and R are shown in Fig. 1. The source was very active, showing microvariability on all nights of observation except for one. Two nights (JD and JD ) were observed with regular light curves. A symmetric light curve (Fig. 2) was observed for the former case while a light curve with a hint of periodicity (Fig. 3) was observed for the latter. The light curves of JD showed a correlation between colour and brightness, with a correlation coefficient of r = So the brightness change may be due to an intrinsic reason. The light curves of JD had correlation coefficient r = This indicates an external reason for the change. It is possible that lighthouse effect may be the reason.

3 Spectral Changes of the BL Lacertae Object 99 (a) (b) Figure 2. Intranight differential light curves on JD in V (a), R (b) andi (c) bands. Crosses represent the differential magnitude between the source and the comparison star, while plus signs represent the differential magnitude between the comparison and check star shifted by an arbitrary offset. The colour magnitude correlation on JD on V R (d).

4 100 H. Poon et al. (c) (d) Figure 2. (Continued).

5 Spectral Changes of the BL Lacertae Object 101 (a) (b) Figure 3. Intranight differential light curves on JD in B (a), V (b), R (c) andi (d) bands. Crosses represent the differential magnitude between the source and the comparison star, while plus signs represent the differential magnitude between the comparison and the check star.

6 102 H. Poon et al. (c) (d) Figure 3. (Continued).

7 Spectral Changes of the BL Lacertae Object Conclusions The source was in a very active state during the whole observation campaign. Two nights showed regular light curves, one was symmetric while the other exhibited a hint of periodicity. References Poon, H., Fan, J. H., Fu, J. N. 2009, Astrophys. J. Suppl. Ser., 185, 511. Wagner, S. J., Witzel, A. 1995, Annu. Rev. Astron. Astrophys., 33, 163. Zhou, A. Y., Jiang, X. J., Zhang, Y. P., Wei, J. Y. 2009, Res. Astron. Astrophys., 9, 349.