A&A 407, 453–460 (2003) Astronomy DOI: 10.1051/0004-6361:20030915 & c ESO 2003 Astrophysics X-ray and optical observations of BL Lac objects: 3C 66A (B0219+428) and ON 325 (B1215+303) M. Perri1,2, E. Massaro1,3, P. Giommi2, M. Capalbi2, R. Nesci1, G. Tagliaferri4, G. Ghisellini4, M. Ravasio4, and H. R. Miller5 1 Dipartimento di Fisica, Universit`a “La Sapienza”, Piazzale A. Moro 2, 00185 Roma, Italy 2 ASI Science Data Center, c/o ESA-ESRIN, via G. Galilei, 00044 Frascati, Italy 3 Istituto di Astrofisica Spaziale e Fisica Cosmica, CNR, via del Fosso del Cavaliere 100, 00133 Roma, Italy 4 Osservatorio Astronomico di Brera, via Bianchi 46, 22055 Merate, Italy 5 Department of Physics and Astronomy, Georgia State University, Atlanta GA, USA Received 24 February 2003 / Accepted 10 June 2003 Abstract. We report the results of new observations of the two BL Lac objects 3C 66A and ON 325 performed with BeppoSAX and of nearly simultaneous photometric measurements in the bandpasses from I to B. 3C 66A was observed in January 1999 +0.3 12 2 1 and in July 2001: its X-ray flux was F(2 10 keV) = 2.0 0.5 10− erg cm− s− in January 1999 and F(2 10 keV) = (2.6 12 2 1 − − × − ± 0.6) 10− erg cm− s− in July 2001. In both observations the X-ray spectrum can be fitted by a single power law with an × energy index 1.2 0.1, steeper than that measured in the optical, equal to 0.86 0.05. The BeppoSAX observation of ON 325 ± +3 ± 13 2 1 was performed in December 1998 when its X-ray flux was F(2 10 keV) = 8 4 10− erg cm− s− . The X-ray spectrum cannot be fitted by a single power law, but it is found to be well represented− by a broken− × power law with a break energy at around 4 keV and energy indices equal to 1.48 0.12 and 0.57 0.63. Both sources are characterised by a synchrotron-dominated emission at energies lower than a few keV,± with a sharp− flattening± toward higher energies due to the inverse Compton component. We obtained satisfactory modelling of the overall spectral energy distributions, from the optical to low energy X-rays, with log- parabolic laws. Key words. radiation mechanisms: non-thermal – galaxies: active – BL Lacertae objects: individual: 3C 66A, ON 325 – X-rays: galaxies 1. Introduction IC emissivity has a quadratic dependence on the electron den- sity. In the ERC models, instead, the S and IC components are BL Lac objects have been classified into two main groups less connected and the IC emission depends only linearly on the on the basis of their Spectral Energy Distribution (SED) electron density. To investigate the possible behaviour we car- by Padovani & Giommi (1995): LBL (Low-energy peaked ried out nearly simultaneous optical and X-ray observations of BL Lacs) if the maximum of the synchrotron (S) peak is in the a sample of BL Lacs. The best suited objects are those with the range 1012–1014 Hz, and HBL (High-energy peaked BL Lacs) S peak in the near IR/optical band and the tail of this emission if it lies above 1016 Hz. Compton scattered photons by relativis- in the X-ray band where it mixes with the harder IC compo- tic electrons emitting the synchrotron radiation have energies nent. The BeppoSAX satellite (Boella et al. 1997a) was the best in the keV and GeV TeV ranges for LBL and HBL sources, observatory for these observations because its Narrow Field respectively. An open− problem is the origin of the seed pho- Instruments (NFIs) cover a wide spectral energy range (0.1– tons upscattered to high energies: they could either be the 200 keV) with a good sensitivity. Wide band X-ray observa- same radiated locally by synchrotron (SSC model, Synchro- tions combined with nearly simultaneous measurements in the Self Compton) or those originated in the surrounding regions optical are particularly useful to establish the spectral distribu- (ERC model, External Radiation Compton) – see Sikora & tion around the synchrotron peak. Madejski (2001) for a review. These two models predict differ- ent correlations between the optical and X-ray luminosities and The selected sources are generally observed in active states different changes of the SED. In particular, in the SSC scenario, detected either in the optical during intense monitoring pro- since the source of photons of the Inverse Compton (IC) emis- grams or in the X-rays (see the review by Tagliaferri et al. sion are those produced through the synchrotron radia- 2002). The BL Lacs whose observations were triggered by op- tion, the S and IC components are strongly linked and the tical flares were ON 231 (Tagliaferri et al. 2000), S5 0716+714 (Giommi et al. 1999; Tagliaferri et al. 2003), BL Lac (Ravasio Send offprint requests to: M. Perri, e-mail: [email protected] et al. 2002), OJ 287 and MS 1458+22 (Massaro et al. 2003a). Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20030915 454 M. Perri et al.: X-ray and optical observations of 3C 66A and ON 325 Table 1. BeppoSAX observation log and exposure times of 3C 66A and ON 325. Source Date Start UT End UT LECS Exp. (s) MECS Exp. (s) 3C 66A 1999/01/31 02:37 17:11 8 492 26 441 2001/07/29-30 11:48 16:23 19 840 50 156 ON 325 1998/12/23 06:26 23:59 12 545 31 383 For some of them we detected both the S and IC compo- the results of the EXOSAT data observations are presented by nents. Two other sources, PKS 2005-489 (Tagliaferri et al. Giommi et al. (1990) who reported flux changes in the en- 2001) and Mkn 421 belonging to the HBL class, were ob- ergy band 0.05–2.0 keV of a factor of about five. The spectral served because of an enhanced X-ray activity and showed only shape, however, was not determined because of the contami- the S component well extended into the X-ray energy band. In nation in the ME instrument by a nearby source. Ueda et al. this paper we present the results of BeppoSAX observations (2001) measured with ASCA a flux in the 2–10 keV interval 12 2 1 of the two BL Lac objects 3C 66A (B0219+428, J0222+4302) of 1.1 10− erg cm− s− . and ON 325 (B1215+303, J1217+3007). × The optical counterpart of the radio source 3C 66A was first identified by Wills & Wills (1974); Miller et al. (1978) derived 2. Observations and data reduction aredshiftz = 0.444 from a broad emission feature identified with a Mg II line. The classification as a BL Lac object is also 2.1. X-ray data supported by the high optical polarisation (16%, Mead et al. 3C 66A was observed twice by BeppoSAX on January 31, 1990) and by the brightness variability with an amplitude of 1999 for about 26 ks and on July 29–30, 2001 for about 50 ks. about 2 mag (Folsom et al. 1976; Miller & McGimsey 1978). ON 325 was observed on December 23, 1998 for about 31 ks. The first X-ray observations of 3C 66A were obtained with Although on both occasions 3C 66A was not very bright the IPC instrument aboard the Einstein Observatory in 1979 in the X-rays, it was nevertheless very well detected in the and 1980 and are summarised by Maccagni et al. (1983). LECS (Parmar et al. 1997) and MECS (Boella et al. 1997b) In July-August 1979 the source was very bright and reached images. Also ON 325 was clearly detected by these two instru- 11 2 1 a flux in the 0.2–4 keV range of about 10− erg cm− s− , ments. The presence of nearby confusing sources and a rather more than one order of magnitude higher than that measured low count rate, did not allow us the use of PDS (Frontera et al. in February 1979 and July 1980. EXOSAT observed 3C 66A 1997) data and therefore the useful energy ranges were lim- in 1986 on three occasions from January 6 to February 1. ited to about 10 keV. The journal of these pointings with the The optical behaviour of 3C 66A has been regularly net exposure times for the two NFIs used is given in Table 1. monitored only since the end of 1994, mainly by the Standard procedures and selection criteria were applied to the OJ 94 collaboration. This large database, described in Takalo data to avoid the South Atlantic Anomaly, solar, bright Earth et al. (1999), is directly accessible at the URL address and particle contamination using the SAXDAS v. 2.0.0 pack- http://www.astro.utu.fi/oj94/pub/pubdata.shtml. age. Data analysis was performed using the software available The EGRET source 3EG J0222+4253, whose error box in the XANADU Package (XIMAGE, XRONOS, XSPEC). contains 3C 66A, is indicated in the 3EG catalogue (Hartman The images of ON 325 in the LECS and MECS showed a et al. 1999) as its possible γ-ray counterpart. This high energy pointlike source well separated from the nearby Seyfert galaxy source is also positionally consistent with the millisecond Mkn 766: the events for spectral analysis were selected within pulsar PSR J0218+4232. Kuiper et al. (2000), on the basis circular regions, centred at the source position and with radii of a Maximum Likelihood Ratio analysis of the γ-ray field, of 40.