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Register a new userThe WEBT BL Lac Campaign 2000
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We present UBVRI light curves of BL Lacertae from May 2000 to January 2001, obtained by 24 telescopes in 11 countries. More than 15000 observations were performed in that period, which was the extension of the Whole Earth Blazar Telescope (WEBT) campaign originally planned for July-August 2000. Rapid flux oscillations are present all the time, involving variations up to a few tenths of mag on hour time scales, and witnessing an intense intraday activity of this source. Colour indexes have been derived by coupling the highest precision B and R data taken by the same instrument within 20 min and after subtracting the host galaxy contribution from the fluxes. The 620 indexes obtained show that the optical spectrum is weakly sensitive to the long-term trend, while it strictly follows the short-term flux behaviour, becoming bluer when the brightness increases. Thus, spectral changes are not related to the host galaxy contribution, but they are an intrinsic feature of fast flares. We suggest that the achromatic mechanism causing the long-term flux base-level modulation can be envisaged in a variation of the relativistic Doppler beaming factor, and that this variation is likely due to a change of the viewing angle. Discrete correlation function (DCF) analysis reveals the existence of a characteristic time scale of variability of about 7 h in the light curve of the core WEBT campaign, while no measurable time delay between variations in the B and R bands is found.
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We present two BeppoSAX observations of BL Lacertae as part of a multiwavelength radio-to-TeV campaign. During the first observation we observe a faint Compton spectrum, while during the second, we detect a synchrotron spectrum with the highest [2-10] keV flux ever measured; above 10 keV an inverse Compton component begin to dominate. The synchrotron flux is very variable with time scales of 1 hr. We describe four different SED shifting the synchrotron peak both in frequency and flux intensity and we sketch a scenario in which a blob moves along a jet and can be located in or outside the BLR. This implies different radiative mechanism at work, SSC or external Compton, producing different high energy spectra.
A multifrequency campaign on the BL Lac object PG 1553+113 was organized by the Whole Earth Blazar Telescope (WEBT) in 2013 April-August, involving 19 optical, two near-IR, and three radio telescopes. The aim was to study the source behaviour at low energies during and around the high-energy observations by the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes in April-July. We also analyse the UV and X-ray data acquired by the Swift and XMM-Newton satellites in the same period. The WEBT and satellite observations allow us to detail the synchrotron emission bump in the source spectral energy distribution (SED). In the optical we found a general bluer-when-brighter trend. The X-ray spectrum remained stable during 2013, but a comparison with previous observations suggests that it becomes harder when the X-ray flux increases. The long XMM-Newton exposure reveals a curved X-ray spectrum. In the SED, the XMM-Newton data show a hard near-UV spectrum, while Swift data display a softer shape that is confirmed by previous HST-COS and IUE observations. Polynomial fits to the optical-X-ray SED show that the synchrotron peak likely lies in the 4-30 eV energy range, with a general shift towards higher frequencies for increasing X-ray brightness. However, the UV and X-ray spectra do not connect smoothly. Possible interpretations include: i) orientation effects, ii) additional absorption, iii) multiple emission components, and iv) a peculiar energy distribution of relativistic electrons. We discuss the first possibility in terms of an inhomogeneous helical jet model.
Using the 1.56m telescope at the Shanghai Observatory (ShAO), China, we monitored two sources, BL Lac object S5 0716+714 and Flat Spectrum Radio Quasar (FSRQ) 3C 273. For S5 0716+714, we report 4969 sets of CCD (Charge-coupled Device) photometrical optical observations (1369 for V band, 1861 for R band and 1739 for I band) in the monitoring time from Dec.4, 2000 to Apr.5, 2014. For 3C 273, we report 460 observations (138 for V band, 146 for R band and 176 for I band) in the monitoring time from Mar. 28, 2006 to Apr. 9, 2014. The observations provide us with a large amount of data to analyze the short-term and long-term optical variabilities. Based on the variable timescales, we can estimate the central black hole mass and the Doppler factor. An abundance of multi-band observations can help us to analyze the relations between the brightness and spectrum. We use Gaussian fitting to analyze the intra-day light curves and obtain the intra-day variability (IDV) timescales. We use the discrete correlation function (DCF) method and Jurkevich method to analyze the quasi-periodic variability. Based on the VRI observations, we use the linear fitting to analyze the relations between brightness and spectrum. The two sources both show IDV properties for S5 0716+714. The timescales are in the range from 17.3 minutes to 4.82 hours; for 3C273, the timescale is 35.6 minutes. Based on the periodic analysis methods, we find the periods P(V) = 24.24 days, P(R)=24.12 days, P(I)=24.82 days for S5 0716+714, and P = 12.99, 21.76 yr for 3C273. The two sources displayed the bluer-when-brighter spectral evolution properties. S5 0716+714 and 3C 273 are frequently studied objects. The violent optical variability and IDV may come from the jet. Gaussian fitting can be used to analyze IDVs. The relations between brightness (flux density) and spectrum are strongly influenced by the frequency.
We report on our further analysis of the expanded and revised sample of potential BL Lac objects (the 2BL) optically identified from two catalogues of blue-selected (UV excess) point sources, the 2dF and 6dF QSO Redshift Surveys (2QZ and 6QZ). The 2BL comprises 52 objects with no apparent proper motion, over the magnitude range 16.0 < bj< 20.0. Follow-up high signal-to-noise spectra of 36 2BL objects and NIR imaging of 18 objects, together with data for 19 2BL objects found in the Sloan Digital Sky survey (SDSS), show 17 objects to be stellar, while a further 16 objects have evidence of weak, broad emission features, although for at least one of these the continuum level has clearly varied. Classification of three objects remains uncertain,with NIR results indicating a marked reduction in flux as compared to SDSS optical magnitudes. Seven objects have neither high signal-to-noise spectra nor NIR imaging. Deep radio observations of 26 2BL objects at the VLA resulted in only three further radio-detections, however none of the three is classed as a featureless continuum object. Seven 2BL objects with a radio detection are confirmed as candidate BL Lac objects while one extragalactic (z=0.494) continuum object is undetected at radio frequencies. One further radio-undetected object is also a potential BL Lac candidate. However it would appear that there is no significant population of radio-quiet BL Lac objects.
We report the results of a phase-referencing study aimed at uncovering precession of the VLBI jet of BL Lac. The observations were conducted at 8, 15, 22, and 43 GHz and consist of seven epochs spanning about two years. We investigated the change in the absolute position of BL Lacs radio core by means of phase-referencing with two nearby sources, 2151+431 and 2207+374. The shift in the position of the core perpendicular to the jet is a signature of precession. However, the periodic variations with an amplitude of ~0.15 mas and a period of 1 year can be attributed to seasonal weather variations. We also detect a trend in position of the core on the scale of ~0.1 mas over two years.
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