BROAD Lyα EMISSION FROM THREE NEARBY BL LACERTAE OBJECTS
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We present far-UV HST/COS spectra of four nearby BL Lac Objects. BL Lac spectra are dominated by a smooth, power-law continuum which arises in a relativistic jet. However, the spectra are not necessarily featureless; weak, broad- and/or narrow-line emission is sometimes seen in high-quality optical spectra. We present detections of Lya emission in HST/COS spectra of Mrk421 (z=0.030) and PKS2005-489 (z=0.071) as well as an archival HST/GHRS observation of Mrk501 (z=0.0337). Archival HST/STIS observations of PKS2155-304 (z=0.116) show no Lya emission to a very low upper limit. Using the assumption that the broad-line region (BLR) clouds are asymmetrically placed around the AGN, we use these measured Lya emission features to constrain either the relativistic Gamma values for the ionizing continuum produced by the jet (in the ionization-bounded case) or the mass of warm gas (in the density-bounded case). While realistic Gamma values can be obtained for all four cases, the values for Mrk421 and PKS2155-304 are high enough to suggest that covering factors of broad-line-region clouds of ~1-2% might be required to provide consistency with earlier values of Doppler boosting and viewing angles suggested for this class of BL Lacs. This discrepancy also exists in the case of M87, where the amount of Doppler boosting in our direction is expected to be minimal, again suggestive of a small covering factor of broad-line-region clouds. If, as these small covering factors might suggest, the assumptions of a density-bounded model could be more correct, then the observed Lya luminosities require that BL\,Lac/FRI nuclei possess very little warm gas (10^-4 to 10^-5 Msun) as suggested by Guilbert, Fabian & McCray (1983). If these clouds are in pressure balance with a hotter (~10^6 K) gas, the BLR contains too little mass to power the AGN by accretion alone.Keywords:
BL Lac object
It is well known that there are only two low-frequency-peaked BL Lac objects (LBLs: BL Lacertae and S5 0716+714) and one flat spectrum radio quasar (FSRQ: 3C 279) among more than 30 active galactic nuclei (AGNs) with detected TeV emissions. We study the spectral energy distribution (SED) of a famous LBL OJ 287, whose light curve has a 12-y period. Using a homogeneous one-zone synchrotron + synchrotron-self Compton model, we model the quasi-simultaneous broad-band SED of OJ 287. With some reasonable assumptions, we extrapolate the model to the high state of OJ 287 and predict its γ-ray emissions. Taking into account the absorption of γ-ray by the extragalactic background light (EBL), we find that the TeV emission of OJ 287 in high state is slightly higher than the sensitivity of H.E.S.S. The study on SEDs of OJ 287 has implications to unveil the origin of jet activity during its 12-y period and the properties of EBL.
BL Lac object
Superluminal motion
Spectral energy distribution
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The BL Lac objects come in two subclasses: the "low-frequency-peaked BL Lac objects" in which the synchrotron peak falls in the IR-optical range and the "high-frequency-peaked BL Lac objects" where it falls in the UV-X-ray bands. The observation results of high-frequency peaked BL Lac object: Mkn421 (z = 0.031), Mkn501 (z = 0.034), Mkn 180 (z = 0.045) and low-frequency peaked BL Lac object OJ 287 (z=0.306) are presented with integral spectra, images and spectral energy distributions combined with those in all broad energy range. Mkn 180 and OJ 287 blazars were recently detected by SHALON Cherenkov telescopic system.
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In the late stages of type I supernovae evolution (t greater than 50 days) the spectrum is without doubt an emission line spectrum, but in the early stages it is difficult to tell if an emission line spectrum is observed, or a continuum spectrum modified by the overlaying absorption formed in the outer layers. The mechanism of formation of the observed spectra, the physical conditions in the shell, different possible sources of heating of the shell, identification of the lines, and theoretical computations of the line intensities are discussed. An attempt is made to interpret the set of spectra assuming that a spectrum of widely broadened emission lines truncated by an overlaying absorption is seen. The absorption spectrum is first studied. An identification for the emission lines is proposed, and their intensity on the observed spectra is tentatively estimated. These identifications have to be justified by the theoretical computations of a model of the shell.
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We present the first medium-resolution (R ∼ 600) mid-infrared (8–13.3 μm) spectra of 11 Be stars. A large number of lines are observed and identified in these spectra, including, as an example, 39 hydrogen recombination lines in the spectrum of γ Cas. In the majority of our spectra, all of the observed lines are attributable to hydrogen recombination. Two of the sources, β Lyr and MWC 349, also show emission from other species. Both of these objects show evidence of [Ne II] emission, and β Lyr also shows evidence of He I emission. We tabulate the effective line strength and line widths for the observed lines and briefly discuss the physical implications of the observed line series. We also use a simple model of free-free emission to characterize the disks around these sources.
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The results of optical photometric monitoring of three BL Lac objects over a time interval of about four years are presented. The sources are three classical radio-selected BL Lac objects, BL Lac, OJ 287 and PKS 0735+178. During our observation OJ 287 was in the stage of a large periodic outburst which consisted of at least two peaks. Almost all the observations obtained over consecutive nights detected intranight variations. In 1995 and 1996 BL Lac kept in faint states, with fewer and smaller rapid flares and fluctuations. On the contrary, in late 1997 BL Lac was at the stage of a large outburst, accompanied with much more large amplitude rapid flares and fluctuations. PKS 0735+178 was almost at its faint end from 1994 to early 1998. Over this time interval, the intraday variations and microvariations in PKS 0735+178 were rare and the amplitude was very small, except a rapid darkening of mag on 24 January 1995. Previous work by Webb et al. (1988); Wagner et al. (1996); Pian et al. (1997) also showed the same behaviour of variability as BL Lac and PKS 0735+178 in BL Lac, S5 0716+714, PKS 2155-304, respectively. We propose that the motion of orientation of the relativistic jet in a BL Lac object be responsible for these variability behaviours.
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BL Lac object
Very-long-baseline interferometry
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The Mα and Mβ emission spectra and the MIV and MV absorption spectra have been studied for the rare earth elements. It is conclusively shown that the complicated multiplet structure observed in the emission spectra is not real emission structure but is, instead, produced by sample self-absorption. This is demonstrated by observing the emission spectra over wide variations in take-off angle and bombarding-electron energies and finally by comparing the detailed structure of both the emission and absorption spectra. The MIV and MV absorption structure completely overlaps the Mα and Mβ emission lines which are each found to have but one intensity maximum when obtained under conditions of minimum self-absorption. Some of these spectra have never been shown previously, while others have been studied in detail by several investigators. Points of agreement and disagreement with previous work are mentioned and the wavelengths of the emission lines and absorption edges are listed for all of the lanthanides. It is concluded that 4f→3d electron transitions are reversible in these elements.
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The p2 3P2 level in the arc spectra of Zn, Cd and Hg has long been considered missing; the accepted explanation has been that transitions involving the level are excessively weakened by auto-ionization. By observation of the arc spectra of these elements in the range 1700-2350 Å, the level in question has been located in Zn and Cd and possibly in Hg. With increasing atomic number auto-ionization increases very greatly, so that one of two diffuse lines in CdI has previously been considered a band of Cd2. Intensity measurements on this line indicate a natural-broadening profile, and yield a value for the auto-ionization probability.
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Abstract We present medium-resolution (R ~ 600) mid-infrared (8-13.3 μ m) spectra of γ Cas and a coadded spectrum of nine Be stars. A large number of lines have been observed and identified in these spectra, including 39 hydrogen recombination lines in the spectrum of γ Cas. In the majority of our spectra, all of the observed lines are attributable to hydrogen recombination. Two additional sources, β Lyr and MWC 349, show [Ne II] emission and β Lyr also displays [He I] emission. We tabulate the effective line strength and line widths for the observed lines, and briefly discuss the physical implications of the observed line series. We also use a simple model of free-free emission to characterize the disks around these sources.
Line (geometry)
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