Simultaneous UV, optical and radio monitoring of the BL Lac object OJ 287 in March 1993

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SIMULTANEOUS UV, OPTICAL AND RADIO MONITORING OF THE BL LAC OBJECT OJ 287 IN MARCH 1993 E. A. T. H. H. A. H.

Pian,’ R. Edelson,2 S. Wagner,3 J. Bregman,4 I. George,5 Treves,l W. Warnsteker,6 H. Bock,3 M. Carini,5 Courvoisier,7 M. Donahue,* Y. Efimov,9 A. Fili penko,1° Fink,’ 1 J. Heidt,3 A. Lawrence,12 L. Maraschi,l !i R. Miller,14 G. Pike,5 A. Quirrenbach,15 N. Shakhovskoy,9 Sillanp&%,16 M. Sitko,17 P. Smith18 L. Takalo,16 Ter&sranta,19 E. Valtaoja, l9 M. Ward20 and R. Warwick21

I SISXUISAS, Via Beirut 2-4, l-34014 Trieste, Italy 2 University of Iowa, Department of Physics and Astronomy, lowa City, IA 52242, U.S.A. 3 Landesstemwarte Heidelberg-Kiinigstuhl, Kiinigstuhl, 691 I7 Heidelberg, %~~~%ty of Michigan V S A. 5 LHEA, Code 660, NASA&FC, Greenbelt, MD 20771, U.S.A. 6 ESA IVE Observatory, P. 0. Box 50727, 28080 Madrid, Spain 7 Geneva Observatory,. 1290 Sauverny, Switzerland 8 Mt Wilson and Las Camoanas Observatorv ’ 9 Astrophysical Observatdty, Ukraine lo University of California, Berkeley, CA, U.S.A. I 1 MPE, Giessenbachstrasse, 85746 Garching bei Munchen, Germany I2 Queen Mary College, London, U.K. I3 Dipartimento di Fisica Vniversitat di Genova, V. Dodecaneso 33, 16146 Genova, Italy I4 University of Georgia, Athens, GA, U.S.A. I5 U.S. Naval Observatory. U.S.A. I6 Tuorla Observatory, Finland I7 Vniversint of Cincinnati. U.S.A. ’ l8 Steward bbservatory I9 MetsUhovi Radio Research Station, Finland 2o Oxford University, Oxford, U.K. 21 University of Leicester, Leicester, U.K.

ABSTRACT The BL Lacertae object OJ 287 was intensively monitored with the IUE satellite from 1993 March 15 to 20 in the 2000-3000 A wavelength region. The very low emission state of the source hampered the detection during part of the 23 performed exposures. The light curve at 2650 A constructed with the 11 best images exhibits a variable trend with a factor 2 2 enhancement of the flux in about 3 hours. Simultaneous ground based observations show an optical flux variability of smaller amplitude, but correlated with the UV light curve without any apparent lag. The radio light curve is nearly constant. The UV emission state is the weakest observed in 15 years for this flaring blazar, being a factor of 5 20 lower than the maximum recorded one. JASR 16:3-E

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INTRODUCTION The object is one of the best studied flat spectrum radio sources across the whole electromagnetic spectrum. Recent observations are reported at radio (/l/), mmand sub-mm (/2/), near-IR.-opticai (/3/; /4/) and X-ray (Rosat, /5/) wavelengths. An upper limit of 1.9 *lo-’ ph s -l cmB2 (E>lOO MeV) for the r-ray emission was determined by EGRET (/6/). The presence of strong beaming of the energy outflow along the observing direction is suggested by the large core-to-extended radio emission ratio and by the detection of superluminai motions (R>3.5 and ,&,,,, = 6.4 respectively, /7/, and references therein). At the UV frequencies, it is the most variable blazar after 3C 446 (/8/). Its activity peaked in April 1983 when a big flare was detected at optical and W wavelengths (/9/; /lo/; /ll/). The historicai optical light curve suggests that outbursts may occur with a 11.6 period (/12/). Since the last remarkable flare was detected in 1983, observations during the current years are of crucial importance. We present here simultaneous UV, optical and radio observations performed in several places during one of the faintest ever recorded states of the source. OBSERVATIONS IUE observed OJ 287 on 1993 March 15-20. The source was in an extremely weak state (in some cases below the iimit of detectability), then only the LWP camera was exposed. AR of the 23 images exhibit underexposed spectra, however some signal is visible in half of them. The IUE spectra have been extracted with the GEX procedure (/13/) and with the Slit Weighted Extraction Technique (so called Optimal Extraction, /14/).

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Fig. 1. Light curves of OJ 287 normaiised to the average: a) UV (2650 A); 6) opticai R band. The variability indices and x2 values are also reported. For both sets of extracted spectra the signal was integrated and averaged in the wavelength region 2600-2700 A, where the instrumental sensitivity is maximal and the scattered light due to the IUE babe anomaly is minor (/15/). The associated uncertainty was evaluated as in /16/. Comparing the results of the two extractions, no systematic difference is seen between them and an agreement

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Multi-Wavelength Monitoring of the BL Lac Object OJ 287

within ~20% is found, which is comparable to the intrinsic flux uncertainties. In Fig. la the GEX light curve normalized to the average = (0.32 f 0.06) mJy is reported. Relative photometry with the A filter was obtained nearly simultaneously to IUE at the Landesstemwarte Heidelberg (see Fig. lb). Radio data at 37 Ghz were obtained in March 1993 at the Metsiihovi Radio Research Station (Finland; see Fig. 2). Optical and radio observations have been already shortly described and discussed in /17/. --:

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Fig. 2. Radio light curve of OJ 287 at 37 GHz. DISCUSSION In the UV band it is difhcult to detect variability, since the errors are large (x2 = 1). The of a flux variation of 2 90% in -3 hours is present in the light curve at MJD = 9063.3 significance. Variability is much more significant in the optical range; the variability (which does not depend on errors, see Edelson 1992) is approximately equal or lower found in the UV.

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indication with a 2~ index 61’ than that

E. Pian cf al.

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Although the time coverage of optical and UV data is not regular and not strictly simultaneous, the light curves present a similar trend in the temporal fractions of common observation. The data are however too scarce and unevenly sampled to attempt a correlation analysis. The radio flux at 37 GHs decreases by 30 % in N 2 weeks, but it appears to be constant (~2.2 Jy) during the UV monitoring time. The object has been observed several times with the LWP and LWR cameras of IUE before our 1993 exposures. The historical light curve at 2650 A, based on JUESIPS extracted archival spectra, is shown in Fig. 3 together with the average flux obtained in March 1993. A general decrease seems to characterize the ultraviolet emission of OJ 287 in the IUE life period, with occasional flares of even a factor of 20 larger than the flux detected during the present monitoring. Based on the periodical 11.6 yr variability, an outburst is predicted for next July/August 1994. Recently, observations at mm, sub-mm and optical wavelengths showed indeed an increasing trend with respect to March 1993 (IAUC 5909, 5913, 5944, 5971). UV observations during the next months would be of vital importance and should be greatly encouraged. REFERENCES 1. D.C. Gabuzda and M.L. Sitko, Astron. J., 107, 884-891 (1994) 2. W.K. Gear, et al., Mon. Not. R. A&.

Sot., 26J 167-186 (1994)

3. R. Falomo, M. Bersanelli, P. Bouchet and E.G. Tanzi, Astron. J., 106, 11-27 (1993) 4. L.O. Takalo, A. Sillanp55, K. Nilsson, M. Kidger and J.A. De Diego, Astron. Astroph. Suppl., l& 115-120 (1994) 5. T. Boiler, E.J.A. Meurs, W. Brinkmann, Astroph. ,261, 57-77 (1992)

H.H. Fink, U. Zimmermann

and H.-M. Adorf, Astron.

6. C.E. Fichtel, et al. Astrophys. J. Suppf. , in press 7. G. Ghisellini, P. Padovani, A. Celotti and L. Maraschi, Astrophys. J., 407, 65-82 (1993) 8. R.A. Edelson, Astrophys. J., 4Jl, 516-528 (1992) 9. L. Maraschi; G. Tagliaferri,

E.G. Tanzi and A. Treves, Astrophys. J., 304, 637-645 (1986)

10. M.T. Carini, H.R. Miller, J.C. Noble and B.D. Goodrich, Astron. J., I&, 11. M. Moles, J. Garcia-Pelayo

15-27 (1992)

and J. Masegosa, Mon. Not. R. Astr. Sot., 211, 621-627 (1984)

12. L.O. Takalo, in: Multi-Wavelength Continuum Emission of AGN, ed. T.J.-L. A. Blecha, Kluwer, Dodrecht 1994, p. 409

Courvoisier and

13. C.M. Urry and G. Reichert, IUE NASA Newsletter, 34, 96-101 (1988) 14. A.L. Kinney, R.C. Bohiin and J.D. Neill, Publ. Astr. Sot. Pac., l& 15. P. Rodriguez Pascual and J. Fernley, ESA IUE NewsletterQ 16. R. Falomo, A. Treves, L. Chiappetti, 532-536 (1993)

694-711 (1991)

15-16 (1993)

L. Maraschi, E. Pian and E.G. Tanzi, Astrophys. J., 402,

17. A. Sillanpfi, L.O. Takalo, E. Valtaoja, H. Terisranta, Y.S. Efimov, N. Shakhovskoy, J. Heidt and H. Bock, in: Multi-Wavelength Continuum Emission of AGN, ed. T.J.-L. Courvoisier and A. Blecha, Kluwer, Dodrecht 1994, p. 404