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Astron. Astrophys. 353, 847-852 (2000)

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6. BL Lacertae (1ES 2200+42.0)

Several experiments extensively observed this source following the detection of a strong flare by EGRET in the [FORMULA]-ray regime (only several hours observation time) and simultaneously in the optical regime around July 19th, 1997 (MJD 50648) (Bloom et al. 1997; Madejski et al. 1999). Because this flare occurred during a full moon period, the TeV observations started only about ten days after the detection of the flare. Our observations yielded a flux upper limit of [FORMULA] ph cm-2 s-1 ([FORMULA]580 GeV). The CAT group obtained a similar upper limit: [FORMULA] ph cm-2 s-1 ([FORMULA]11% of the Crab) ([FORMULA]300 GeV) (Barrau, private communication). Earlier measurements at VHE energies were performed in 1995 with the Whipple telescope following a weak flare detected by EGRET with an integral [FORMULA]-ray flux of [FORMULA] ph cm-2 s-1 ([FORMULA]100 MeV), i.e. three times lower than the flux of the July, 1997 flare. The Whipple observations (40 h) yielded a flux upper limit of 5.3 10- 12 ph cm-2 s-1 ([FORMULA]350 GeV) (Catanese et al. 1997). The results from the three VHE experiments HEGRA, CAT, and Whipple are shown in Fig. 4.

Information about the X-ray activity between February 1996 and August 1998 in the energy region 2-12 keV is provided by the All Sky Monitor (ASM) on board the Rossi X-Ray Timing Explorer (RXTE) (Remillard 1997). We determined the ASM count rates from the "definitive" results obtained through analysis of the processed data by the RXTE ASM team at MIT; data have a dwell duration larger than 30 seconds and a flux fit with a reduced [FORMULA]-value below 1.5. The light curve is extracted using the "ftools 4.0" package. Unfortunately, the mean count rates are very low, i.e. typically around [FORMULA]-[FORMULA] Hz and the observation frequency is less 1 "run" of 90 seconds per hour, making it difficult to get information about the source activity on time scales as short as one day. So data are binned in 5 week bins, yielding flux estimates with acceptable statistical errors.

The light curve is shown in Fig. 3. A pronounced luminosity increase during June 1997 followed by a slow decrease during the following 5 months can be recognized. HEGRA data were taken in July/August 1997 when the source was still active. In 1998, Bl Lacertae was in a low X-ray activity. Note that EGRET was able to detect the source, and the TeV instruments were not. Intriguingly in the case of Mkn 501 the opposite happened: the source was bright in the TeV energy regime but could hardly be detected at MeV/GeV energies.

[FIGURE] Fig. 3. Light curves computed from data of the ASM/RXTE detector (1.3-12.1 keV) with a binning of five weeks. Observations range from February 1996 to August 1998.

[FIGURE] Fig. 4. Spectral energy distribution of the object BL Lacertae. Data are taken from (Perlman et al. 1996; Webb et al. 1998; Catanese et al. 1997); different measurements well before and after the 1995 and the 1997 flare (circles), measurements around the 1995 flare (squares) and measurements around the 1997 flare (triangles). HEGRA CT-system data are represented by filled symbols while others are represented by open ones. The black symbols indicate the raw limit, the grey ones the limit after deconvolution by the CIB absorption (LCDM model by Primack et al. 1999).

Model calculations by Madejski (1999) predict TeV flux significantly higher than the upper limits obtained by HEGRA (and also by CAT at the same period). We explain the non-detection at VHE energies by two reasons. First, the absorption by a CIB density as in Fig. 1 reduces the observable flux by about 50%. Secondly, the HEGRA measurements were taken roughly 10 days after the detection of the GeV-flare. Taking into account the strong variability observed in the two BL Lac objects Mkn 501 and Mkn 421, the VHE flux of BL Lacertae could have easily decreased by a factor of [FORMULA]5 in 10 days. Note that predictions from Boïttcher & Bloom models are below the VHE upper limits. So no model can be excluded or confirmed.

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© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000