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Register a new userESO-VLT optical spectroscopy of BL Lac objects: I. new redshifts
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Authors
B. Sbarufatti
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We report redshift measurements for 12 BL Lacertae objects from a program aimed at obtaining high signal to noise (up to ~ 500) optical spectroscopy of a mixed sample of objects. The new observations, gathered with the 8 m ESO Very Large Telescope, allowed us to detect weak spectral features down to a line equivalent width as small as ~ 1 Angstrom. The new redshifts fall in the 0.2-1.3 interval. For nine objects we observe emission lines from the active nucleus. In the remaining three cases absorption lines from the host galaxy are found. For two objects we also detect absorption lines from intervening systems.
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We report on ESO Very Large Telescope optical spectroscopy of 42 BL Lacertae objects of unknown redshift. Nuclear emission lines were observed in 12 objects, while for another six we detected absorption features due to their host galaxy. The new high S/N spectra therefore allow us to measure the redshift of 18 sources. Five of the observed objects were reclassified either as stars or quasars, and one is of uncertain nature. For the remaining 18 the optical spectra appear without intrinsic features in spite of our ability to measure rather faint (EW nearly 0.1 Angstrom) spectral lines. For the latter sources a lower limit to the redshift was set exploiting the very fact that the absorption lines of the host galaxy are undetected on the observed spectra.
Context. The study of BL Lac objects (BLL) detected in gamma-rays gives insights on the acceleration mechanisms in play in such systems and is also a valuable tool to constrain the density of the extragalactic background light. As their spectra are dominated by the non-thermal emission of the jet and the spectral features are weak and narrow in the optical domain, measuring their redshift is challenging. However such a measure is fundamental as it allows a firm determination of the distance and luminosity of the source, and therefore a consistent model of its emission. Aims. Measurement of the redshift of BLL detected in gamma-rays and determination of global properties of their host galaxies. Methods. We observed a sample of eight BLL (KUV 00311-1938, PKS 0447-439, PKS 0301-243, BZB J0238-3116, BZB J0543-5532, BZB J0505+0415, BZB J0816-1311 and RBS 334) with the X-shooter spectrograph installed at the ESO Very Large Telescope in order to take advantage of its unprecedented wavelength coverage and of its resolution about 5 times higher than generally used in such studies. We extracted UVB to NIR spectra that we then corrected for telluric absorption and calibrated in flux. We systematically searched for spectral features. When possible, we determined the contribution of the host galaxy to the overall emission. Results. Of the eight BLL, we measured the redshift of five of them and determined lower limits for two through the detection of intervening systems. All seven of these objects have redshifts greater than 0.2. In two cases, we refuted redshift values reported in other publications. Through careful modelling, we determined the magnitude of the host galaxies. In two cases, the detection of emission lines allowed to provide hints on the overall properties of the gas in the host galaxies.
We performed an observational program with the X-ray satellite BeppoSAX to study objects with extreme synchrotron peak frequencies (nu_peak > 1 keV). Of the seven sources observed, four showed peak frequencies in the range 1-5 keV, while one (1ES 1426+428) displayed a flat power law spectrum (alpha= 0.92), locating its synchrotron peak at or above 100 keV. This is the third source of this type ever found, after Mkn 501 and 1ES 2344+514. Our data confirm the large nu_peak variability of this class of sources, compared with lower peaked objects. The high synchrotron peak energies, flagging the presence of high energy electrons, make the extreme BL Lacs also good candidates for TeV emission, and therefore good probes for the IR background.
Only BL Lac objects have been detected as extragalactic sources of very high energy (E > 300 GeV) gamma rays. Using the Whipple Observatory Gamma-ray Telescope, we have attempted to detect more BL Lacs using three approaches. First, we have conducted surveys of nearby BL Lacs, which led to the detections of Mrk 501 and 1ES 2344+514. Second, we have observed X-ray bright BL Lacs when the RXTE All-Sky Monitor identifies high state X-ray emission in an object, in order to efficiently detect extended high emission states. Third, we have conducted rapid observations of several BL Lacs and QSOs located close together in the sky to search for very high flux, short time-scale flare states such as have been seen from Mrk 421. We will present the results of a survey using the third observational technique.
The Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory has been shown to be sensitive to non-transient hard X-ray sources in our galaxy, down to flux levels of 100 mCrab for daily measurements, 3 mCrab for integrations over several years. We use the continuous BATSE database and the Earth Occultation technique to extract average flux values between 20 and 200 keV from complete radio- and X-ray- selected BL Lac samples over a 2 year period.
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