Is BL Lacertae an “orphan” AGN?

Abstract

Aims. We have analysed optical spectra of BL Lacertae, the prototype of its blazar subclass, to verify the broad Hα emission line detected more than a decade ago and its possible flux variation. We used the spectroscopic information to investigate the question of the BL Lacertae parent population. Methods. Low-and high-resolution optical spectra of BL Lacertae were acquired with the DOLORES spectrograph at the 3.58 m telescopio nazionale Galileo (TNG) during four nights in [2007][2008], when the source was in a relatively faint state. In three cases we were able to fit the complex Hα spectral range with multiple line components and to measure both the broad Hα and several narrow emission line fluxes. Results. A critical comparison with previous results suggests that the broad Hα flux has increased by about 50% in ten years. This might be due to an addition of gas in the broad line region (BLR), or to a strengthening of the disc luminosity, but such flux changes are not unusual in Broad Lined active nuclei. We estimated the BL Lacertae black hole mass by means of its relation with the bulge luminosity, finding 4−6 × 10 8 M . The virial mass estimated from the spectroscopic data gives instead a value 20-30 times lower. An analysis of the disc and BLR properties in different AGNs suggests that this discrepancy is due to an underluminosity of the BL Lacertae BLR. Finally, we addressed the problem of the BL Lacertae parent population, comparing its isotropic quantities with those of other AGN classes. From the point of view of the narrow emission line spectrum, the source is located close to low-excitation radio galaxies. When one also considers its diffuse radio power, an association with FR I radio galaxies is severely questioned due to the lower radio luminosity (at a given line luminosity) of BL Lacertae. The narrow line and radio luminosities of BL Lacertae instead match those of a sample of miniature radio galaxies, which however do not show a BLR. Yet, if existing, "misaligned BL Lacertae" objects should have entered that sample. We also rule out the possibility that they were excluded because of a QSO optical appearance. Conclusions. The observational constraints suggest that BL Lacertae is caught in a short term transient stage, which does not leave a detectable evolutionary "trace" in the AGN population. We present a scenario that can account for the observed properties.

Another look at the BL Lacertae flux and spectral variability

“…As discussed in the present paper and in Raiteri et al (2009), the amount of this excess strongly depends on both the Galactic extinction and instrument calibration, but it is not easy to cancel it out completely. In any case, Capetti et al (2010) showed that after twelve years from the first detection of the Hα broad emission line by Vermeulen et al (1995) and Corbett et al (1996, see also Corbett et al 2000, the Hα line is still there, even more luminous than before. This suggests that a disc is also there to photoionise the broad line region.…”

“…We notice that the source redshift is z = 0.0686 (Vermeulen et al 1995) and hence the broad Hα emission line enters the tails of the R and I passbands. However, referring to Capetti et al (2010), one can estimate that its flux contribution is only a few thousandths of that of the continuum. Hence, the presence of the line cannot affect the variability in these bands, at least when the source brightness is at these levels.…”

“…They applied the inhomogeneous, rotating helical jet model by , see also Raiteri et al 1999Raiteri et al 2003;Ostorero et al 2004) to fit the SEDs, and suggested that the broad-band A&A 524, A43 (2010) spectral properties of BL Lacertae may result from the combination of two synchrotron emission components with their self inverse-Compton emission, plus a thermal component from the disc. Subsequently, Capetti et al (2010) analysed optical spectra acquired in the same period with the 3.56 m telescopio nazionale galileo (TNG). They found a broad Hα emission line, with luminosity of ∼4 × 10 41 erg s −1 and FWHM of ∼4600 km s −1 , even brighter than that found in 1995-1997 by Vermeulen et al (1995) and Corbett et al (1996Corbett et al ( , 2000.…”

Erratic Flaring of Bl Lac in 2012–2013: Multiwavelength Observations

“…BL Lac has broad H-α and H-β lines (Vermeulen et al 1995) that vary in flux. Capetti et al (2010) found that the broad H-α flux increased by 50% in ten years, which could indicate that the photoionizing radiation field from the accretion disk is variable or that there was an increase in the gas in the broad line region. In BL Lac, the broad line region may be underluminous compared to other AGN (Capetti et al 2010).…”

“…Capetti et al (2010) found that the broad H-α flux increased by 50% in ten years, which could indicate that the photoionizing radiation field from the accretion disk is variable or that there was an increase in the gas in the broad line region. In BL Lac, the broad line region may be underluminous compared to other AGN (Capetti et al 2010). The discovery of even weak broad emission lines in BL Lac means that in some ways, BL Lac itself is more similar to FSRQs than to less luminous BL Lac objects such as Mkn 421 and Mkn 501 (see discussion in Madejski et al 1999).…”