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Optical behaviour of BL Lacertae during 1997-2001: long-term, intraday and intrahour variability
Nuclear Physics B (Proc. Suppl.) 132 (2004) 205–208 www.elsevierphysics.com
Optical behaviour of BL Lacertae during 1997-2001: long-term, intraday and intrahour variability Maria G. Nikolashvilia and Omar M. Kurtanidzeab . a
Abastumani Observatory, 383762 Abastumani, Georgia
b
Astrophysikalisches Institute Potsdam, 14482 Potsdam and Landessternwarte Heidelberg-K¨ohigstuhl, 69117 Heidelberg, Germany
We present the results of optical observations of BL Lacertae from August 1997 to November 2001 carried out with ST-6 CCD Camera attached to the Newtonian focus of the 70-cm meniscus telescope of Abastumani Observatory. The long-term, intraday and intrahour variabilities of BL Lacertae were studied on the bases of 317 and 259 nights, respectively. The variability pattern shown by BL Lacertae is very complex. The maximum m amplitude of the long-term variability in the B band equals 3 . 0 (rms=0.03). The variation in the V and R bands m m are within 2 . 71 (0.02) and 2 . 53 (0.01), respectively. This means that variations are larger at shorter wavelength or the object become bluer in the active phase. It were also demonstrated that BL Lacertae shows intraday variability within 0.30 (0.02), while intrahour variability within 0.10 (0.01) magnitudes.
1. INTRODUCTION BL Lacertae is the prototype of one of the most extreme subclass of AGNs. It was discovered in 1929 by Guno Hoffmeister, who found it to vary by more than a factor two in one week and classified it as a short period variable star [1]. Since its identification as an extragalactic source it was the subject of numerous studies in many frequency bands. Historically, BL Lac is known to show a m ∼5 . 0 variation in the optical band with episodic outbursts [2–4]. The maximum variation in the m infrared K band is ∼3 . 0 [5]. During the summer 1997 outburst it showed very strong activity, including on intranight timescales [6,7]. The strong activity was also detected in the radio, X-ray and γ-ray bands [8–12]. 2. OBSERVATION AND DATA REDUCTION We are intensively monitoring BL Lacertae at Abastumani Observatory since August 1997, when it remained in a high state for more than two months. Rapid and large-amplitude flux variations characterized the source during this period. Here we present observations carried out from 0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.04.034
Table 1 List of observations and variability in R. Observational Periods Nights δm(R) Aug 1997 - May 1998 74 2.53 May 1998 - Aug 1998 39 1.10 Aug 1998 - Jan 1999 50 0.80 July 1999 - Aug 1999 78 0.90 Aug 1999 - June 2000 72 1.85 June 2000 - Nov 2000 69 1.40 May 2001 - Nov 2001 83 1.80
August 1997 to November 2001. All observations were carried out with 70-cm meniscus telescope and with the CCD Camera ST-6 attached to the Newtonian focus [13,14]. To study the long-term variability we observed BL Lacertae during 317 nights, collected 320 frames in each of the BVI bands and 457 frames in the R band. More than 18 000 frames were obtained in the R band during 259 nights to study intraday variability [15,16] and intrahour variability [17]. The duration of the observational runs varied from two hours to six hours. The exposure times varied from 60 to 180 sec depending on the brightness of the
206
M.G. Nikolashvili, O.M. Kurtanidze / Nuclear Physics B (Proc. Suppl.) 132 (2004) 205–208
Figure 1. The BVRI lightcurves of BL Lacertae from August 1997 to May 1998.
Figure 2. The BVRI lightcurves of BL Lacertae from May 1998 to August 1998.
object and the filter used. Instrumental magnitudes were obtained using DaophotII routines [18]. Magnitudes are calculated relative to comparison stars C and H having nearly the same colours as the object under study [19]. To eliminate the effects of seeing-induced spurious IDV and IHV [20] the apertures are taken to include the whole host galaxy.
equals 2 . 53 (0.01). The amplitudes of variation during other observing periods are presented in Table 1. The evidence for intraday and intrahour variabilities are found during many nights of observations. The typical intraday and intrahour varim ability amplitudes in the R band are within 0 . 30 m (0.02) and 0 . 10 (0.01) magnitudes, respectively. They are clearly demonstrated in the Fig. 8.
3. RESULTS AND CONCLUSION The results of the optical observations of BL Lac during the great summer 1997 outburst are presented by different blazar monitoring groups [21–24,7]. On the basis of observations of BL Lacertae during the period from August 1997 to November 2001 it was clearly demonstrated that variations are larger in the B band or that the object becomes bluer in the active phase [25–28]. This was also confirmed by other groups [7,24]. The long-term variability lightcurves obtained in the R band are presented in Figs. 1 through 7. The maximum variation was observed in the m B band during Aug 1997 and equals 3 . 0 (0.03), while the maximum amplitude in the R band
m
4. ACKNOWLEDGEMENTS O.M.K. thanks the SOC, LOC and Prof. Ed van den Heuvel for invaluable financial support. He gratefully acknowledges the hospitality and invaluable financial support of the Astrophysikalisches Institute Potsdam and thanks Dr. G.M. Richter for the kind collaboration of many years without which this program would never have been conducted. REFERENCES 1. G. Hoffmeister, et al., Ver¨ anderliche Sterne, (Leipzig, 1990). 2. J.R.Webb, et al., AJ, 95 374 (1988).
M.G. Nikolashvili, O.M. Kurtanidze / Nuclear Physics B (Proc. Suppl.) 132 (2004) 205–208
Figure 3. The R band lightcurve of BL Lacertae from May 2001 to November 2001.
3. J.H.Fan, et al., ApJ, 507 173 (1998). 4. M. Maesano, et al., A&A Suppl. Ser. 122 267 (1997). 5. J. H. Fan, O.M. Kurtanidze, et al., A&A Suppl. Ser. 133 217 (1998). 6. R. Nesci, et al., Pubbl. Osserv. Astron. Univ. de Perugia, 3, 189 (1998). 7. S.D. Clements, M. T. Carini, AJ, 119 534 (2001). 8. S.D. Bloom, et al., ApJ, 490 L145 (1997). 9. R.M. Sambruna, et al., ApJ, 515 140 (1999). 10. G.M. Madejski, et al., ApJ, 521 145 (1999). 11. S. Tanihata, et al., ApJ, 543 124 (2000). 12. M. B¨ottcher, S.D. Bloom, AJ, 119 469 (2000). 13. O.M. Kurtanidze, M.G. Nikolashvili, in Blazars with BeppoSax and Other Observatories, eds. P. Giommi and E. Massaro, ASI SP, 189 (2002). 14. O.M. Kurtanidze, M.G. Nikolashvili, in Proc. 8th Asia-Pacific RAM, Tokyo, July 2002, eds. S. Ikeuchi, J. Hearnshaw, and T. Hanawa, ASP Conf. Ser., Vol. 289, p. 389 (2003). 15. S.J. Wagner, A. Wintzel, Ann. Rev. Astron. Astrophys. 33, 163(1995). 16. S.J. Wagner, in Blazar Demographics and
207
Figure 4. The R band lightcurve of BL Lacertae from August 1998 to January 1999.
17. 18. 19. 20. 21. 22. 23. 24. 25.
26.
27.
28.
Physics, eds., P. Padovani and M. Urry, ASP CS, 227, 112 (2001). H.R. Miller, et al., Nature 337, 627(1989). P.B. Stetson, Publ. Astr. Soc. Pacific 99, 191(1987). P.S. Smith, et al., AJ, 90 1184 (1985). S.A. Cellone, et al., AJ, 121 90 (2000). J.R. Webb, et al., AJ, 115 2244 (1998). G. Sobrito, et al., Blazar Data, 1, no.5 (1999). R. Speziali, G. Natali, A&A Suppl. Ser. 339 382 (2000). J.H. Fan, et al., A&A, 369 758 (2001). M.G. Nikolashvili, O.M. Kurtanidze, in Blazar Monitoring Towards the Third Millenium, eds. Raiteri C., et al., Osservatorio Astronomico di Torino, 33 (1999). O.M. Kurtanidze, in Proc. IAU Symp. 204, eds. Harwitt M., et al., ASP CS 204, 155 (2001). O.M. Kurtanidze, M.G. Nikolashvili, in Proc. IAU Symp. 205, eds., Schilizzi L., et al., ASP CS 205, 82 (2001). R. Nesci, E. Maesano, et al., in Blazar Demographics and Physics, eds., P. Padovani and M. Urry, ASP CS, 227, 144 (2001).
208
M.G. Nikolashvili, O.M. Kurtanidze / Nuclear Physics B (Proc. Suppl.) 132 (2004) 205–208
Figure 5. The R band lightcurve of BL Lacertae from July 1999 to August 1999.
Figure 7. The R band lightcurve of BL Lacertae from June 2000 to November 2000.
Figure 6. The R band lightcurve of BL Lacertae from August 1999 to June 2000.
Figure 8. The R band intrahour variability lightcurve of BL Lacertae August 11, 2000.
Optical behaviour of BL Lacertae during 1997-2001: long-term, intraday and intrahour variability Maria G. Nikolashvilia and Omar M. Kurtanidzeab . a
Abastumani Observatory, 383762 Abastumani, Georgia
b
Astrophysikalisches Institute Potsdam, 14482 Potsdam and Landessternwarte Heidelberg-K¨ohigstuhl, 69117 Heidelberg, Germany
We present the results of optical observations of BL Lacertae from August 1997 to November 2001 carried out with ST-6 CCD Camera attached to the Newtonian focus of the 70-cm meniscus telescope of Abastumani Observatory. The long-term, intraday and intrahour variabilities of BL Lacertae were studied on the bases of 317 and 259 nights, respectively. The variability pattern shown by BL Lacertae is very complex. The maximum m amplitude of the long-term variability in the B band equals 3 . 0 (rms=0.03). The variation in the V and R bands m m are within 2 . 71 (0.02) and 2 . 53 (0.01), respectively. This means that variations are larger at shorter wavelength or the object become bluer in the active phase. It were also demonstrated that BL Lacertae shows intraday variability within 0.30 (0.02), while intrahour variability within 0.10 (0.01) magnitudes.
1. INTRODUCTION BL Lacertae is the prototype of one of the most extreme subclass of AGNs. It was discovered in 1929 by Guno Hoffmeister, who found it to vary by more than a factor two in one week and classified it as a short period variable star [1]. Since its identification as an extragalactic source it was the subject of numerous studies in many frequency bands. Historically, BL Lac is known to show a m ∼5 . 0 variation in the optical band with episodic outbursts [2–4]. The maximum variation in the m infrared K band is ∼3 . 0 [5]. During the summer 1997 outburst it showed very strong activity, including on intranight timescales [6,7]. The strong activity was also detected in the radio, X-ray and γ-ray bands [8–12]. 2. OBSERVATION AND DATA REDUCTION We are intensively monitoring BL Lacertae at Abastumani Observatory since August 1997, when it remained in a high state for more than two months. Rapid and large-amplitude flux variations characterized the source during this period. Here we present observations carried out from 0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.04.034
Table 1 List of observations and variability in R. Observational Periods Nights δm(R) Aug 1997 - May 1998 74 2.53 May 1998 - Aug 1998 39 1.10 Aug 1998 - Jan 1999 50 0.80 July 1999 - Aug 1999 78 0.90 Aug 1999 - June 2000 72 1.85 June 2000 - Nov 2000 69 1.40 May 2001 - Nov 2001 83 1.80
August 1997 to November 2001. All observations were carried out with 70-cm meniscus telescope and with the CCD Camera ST-6 attached to the Newtonian focus [13,14]. To study the long-term variability we observed BL Lacertae during 317 nights, collected 320 frames in each of the BVI bands and 457 frames in the R band. More than 18 000 frames were obtained in the R band during 259 nights to study intraday variability [15,16] and intrahour variability [17]. The duration of the observational runs varied from two hours to six hours. The exposure times varied from 60 to 180 sec depending on the brightness of the
206
M.G. Nikolashvili, O.M. Kurtanidze / Nuclear Physics B (Proc. Suppl.) 132 (2004) 205–208
Figure 1. The BVRI lightcurves of BL Lacertae from August 1997 to May 1998.
Figure 2. The BVRI lightcurves of BL Lacertae from May 1998 to August 1998.
object and the filter used. Instrumental magnitudes were obtained using DaophotII routines [18]. Magnitudes are calculated relative to comparison stars C and H having nearly the same colours as the object under study [19]. To eliminate the effects of seeing-induced spurious IDV and IHV [20] the apertures are taken to include the whole host galaxy.
equals 2 . 53 (0.01). The amplitudes of variation during other observing periods are presented in Table 1. The evidence for intraday and intrahour variabilities are found during many nights of observations. The typical intraday and intrahour varim ability amplitudes in the R band are within 0 . 30 m (0.02) and 0 . 10 (0.01) magnitudes, respectively. They are clearly demonstrated in the Fig. 8.
3. RESULTS AND CONCLUSION The results of the optical observations of BL Lac during the great summer 1997 outburst are presented by different blazar monitoring groups [21–24,7]. On the basis of observations of BL Lacertae during the period from August 1997 to November 2001 it was clearly demonstrated that variations are larger in the B band or that the object becomes bluer in the active phase [25–28]. This was also confirmed by other groups [7,24]. The long-term variability lightcurves obtained in the R band are presented in Figs. 1 through 7. The maximum variation was observed in the m B band during Aug 1997 and equals 3 . 0 (0.03), while the maximum amplitude in the R band
m
4. ACKNOWLEDGEMENTS O.M.K. thanks the SOC, LOC and Prof. Ed van den Heuvel for invaluable financial support. He gratefully acknowledges the hospitality and invaluable financial support of the Astrophysikalisches Institute Potsdam and thanks Dr. G.M. Richter for the kind collaboration of many years without which this program would never have been conducted. REFERENCES 1. G. Hoffmeister, et al., Ver¨ anderliche Sterne, (Leipzig, 1990). 2. J.R.Webb, et al., AJ, 95 374 (1988).
M.G. Nikolashvili, O.M. Kurtanidze / Nuclear Physics B (Proc. Suppl.) 132 (2004) 205–208
Figure 3. The R band lightcurve of BL Lacertae from May 2001 to November 2001.
3. J.H.Fan, et al., ApJ, 507 173 (1998). 4. M. Maesano, et al., A&A Suppl. Ser. 122 267 (1997). 5. J. H. Fan, O.M. Kurtanidze, et al., A&A Suppl. Ser. 133 217 (1998). 6. R. Nesci, et al., Pubbl. Osserv. Astron. Univ. de Perugia, 3, 189 (1998). 7. S.D. Clements, M. T. Carini, AJ, 119 534 (2001). 8. S.D. Bloom, et al., ApJ, 490 L145 (1997). 9. R.M. Sambruna, et al., ApJ, 515 140 (1999). 10. G.M. Madejski, et al., ApJ, 521 145 (1999). 11. S. Tanihata, et al., ApJ, 543 124 (2000). 12. M. B¨ottcher, S.D. Bloom, AJ, 119 469 (2000). 13. O.M. Kurtanidze, M.G. Nikolashvili, in Blazars with BeppoSax and Other Observatories, eds. P. Giommi and E. Massaro, ASI SP, 189 (2002). 14. O.M. Kurtanidze, M.G. Nikolashvili, in Proc. 8th Asia-Pacific RAM, Tokyo, July 2002, eds. S. Ikeuchi, J. Hearnshaw, and T. Hanawa, ASP Conf. Ser., Vol. 289, p. 389 (2003). 15. S.J. Wagner, A. Wintzel, Ann. Rev. Astron. Astrophys. 33, 163(1995). 16. S.J. Wagner, in Blazar Demographics and
207
Figure 4. The R band lightcurve of BL Lacertae from August 1998 to January 1999.
17. 18. 19. 20. 21. 22. 23. 24. 25.
26.
27.
28.
Physics, eds., P. Padovani and M. Urry, ASP CS, 227, 112 (2001). H.R. Miller, et al., Nature 337, 627(1989). P.B. Stetson, Publ. Astr. Soc. Pacific 99, 191(1987). P.S. Smith, et al., AJ, 90 1184 (1985). S.A. Cellone, et al., AJ, 121 90 (2000). J.R. Webb, et al., AJ, 115 2244 (1998). G. Sobrito, et al., Blazar Data, 1, no.5 (1999). R. Speziali, G. Natali, A&A Suppl. Ser. 339 382 (2000). J.H. Fan, et al., A&A, 369 758 (2001). M.G. Nikolashvili, O.M. Kurtanidze, in Blazar Monitoring Towards the Third Millenium, eds. Raiteri C., et al., Osservatorio Astronomico di Torino, 33 (1999). O.M. Kurtanidze, in Proc. IAU Symp. 204, eds. Harwitt M., et al., ASP CS 204, 155 (2001). O.M. Kurtanidze, M.G. Nikolashvili, in Proc. IAU Symp. 205, eds., Schilizzi L., et al., ASP CS 205, 82 (2001). R. Nesci, E. Maesano, et al., in Blazar Demographics and Physics, eds., P. Padovani and M. Urry, ASP CS, 227, 144 (2001).
208
M.G. Nikolashvili, O.M. Kurtanidze / Nuclear Physics B (Proc. Suppl.) 132 (2004) 205–208
Figure 5. The R band lightcurve of BL Lacertae from July 1999 to August 1999.
Figure 7. The R band lightcurve of BL Lacertae from June 2000 to November 2000.
Figure 6. The R band lightcurve of BL Lacertae from August 1999 to June 2000.
Figure 8. The R band intrahour variability lightcurve of BL Lacertae August 11, 2000.