Morphology and orientation of radio-loud Broad Absorption Line quasars (original) (raw)
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The parsec-scale structure of radio-loud broad absorption line quasars
Astronomy & Astrophysics, 2013
Context. Broad absorption line quasars (BAL QSOs) belong to a class of objects not well-understood as yet. Their UV spectra show BALs in the blue wings of the UV resonance lines, owing to ionized gas with outflow velocities up to 0.2 c. They can have radio emission that is difficult to characterize and that needs to be studied at various wavelengths and resolutions. Aims. We aim to study the pc-scale properties of their synchrotron emission and, in particular, to determine their core properties. Methods. We performed observations in the Very Long Baseline Interferometry (VLBI) technique, using both the European VLBI Network (EVN) at 5 GHz, and the Very Long Baseline Array (VLBA) at 5 and 8.4 GHz to map the pc-scale structure of the brightest radio-loud objects of our sample, allowing a proper morphological interpretation. Results. A variety of morphologies have been found: 9 BAL QSOs on a total of 11 observed sources have a resolved structure. Core-jet, double, and symmetric objects are present, suggesting different orientations. In some cases the sources can be young GPS or CSS. The projected linear size of the sources, also considering observations from our previous work for the same objects, can vary from tens of pc to hundreds of kpc. In some cases, a diffuse emission can be supposed from the missing flux-density with respect to previous lower resolution observations. Finally, the magnetic field strength does not significantly differ from the values found in the literature for radio sources with similar sizes. Conclusions. These results are not easily interpreted with the youth scenario for BAL QSOs, in which they are generally compact objects still expelling a dust cocoon. The variety of orientations, morphologies, and extensions found are presumably related to different possible angles for the BAL producing outflows, with respect to the jet axis. Moreover, the phenomenon could be present in various phases of the QSO evolution.
Investigating the radio-loud phase of broad absorption line quasars
Astronomy & Astrophysics, 2014
Context. Broad absorption lines (BALs) are present in the spectra of ⇠20% of quasars (QSOs); this indicates fast outflows (up to 0.2c) that intercept the observer's line of sight. These QSOs can be distinguished again into radio-loud (RL) BAL QSOs and radio-quiet (RQ) BAL QSOs. The first are very rare, even four times less common than RQ BAL QSOs. The reason for this is still unclear and leaves open questions about the nature of the BAL-producing outflows and their connection with the radio jet.
Astrophysics and Space Science Proceedings, 2010
Broad Absorption Lines (BALs) seem to be the most extreme manifestations of quasar (QSO) outflows. Two main scenarios have been proposed to explain the nature of BAL QSOs. They may be a physically distinct population (e.g., newborn or recently refuelled QSOs) or present in all QSOs but intercepted by only a fraction of the lines of sight to the QSOs. Our previous observations of a sample of 15 radio BAL QSOs show that they have convex radio spectra typical of Giga-Hertz Peaked-Spectrum (GPS) sources. We have selected a well-defined sample of radio bright BAL QSOs from the Sloan Digital Sky Survey -Data Release 5. Here we present preliminary results on radio continuum observations in full polarisation of this sample, taken with the 100-m Effelsberg radiotelescope at 2.7, 4.8, 8.4 and 10.5 GHz. The aim is to describe the radio spectra and polarisation characteristics of these radio bright BAL QSOs and compare them with our previous results from the study of a radio fainter sample of BAL QSOs and with the properties of normal QSOs where the BAL phenomenon is not seen.
Astronomy & Astrophysics, 2012
Context. The origin of broad-absorption-line quasars (BAL QSOs) is still an open issue. Accounting for ∼ 20% of the QSO population, these objects present broad absorption lines in their optical spectra generated from outflows with velocities up to 0.2 c. In this work we present the results of a multi-frequency study of a well-defined radio-loud BAL QSO sample, and a comparison sample of radio-loud non-BAL QSOs, both selected from the Sloan Digital Sky Survey (SDSS). Aims. We aim to test which of the currently-popular models for the BAL phenomenon -'orientation' or 'evolutionary' -best accounts for the radio properties of BAL quasars. We also consider a third model in which BALs are due to polar jets driven by radiation pressure. Methods. Observations from 1.4 to 43 GHz have been obtained with the VLA and Effelsberg telescopes, and data from 74 to 408 MHz have been compiled from the literature. The spectral indices give clues to the orientation of these objects, while the determination of the peak frequency can constrain their age, and test the evolutionary scenario, in which BAL QSOs are young QSOs. The fractional polarisation and the rotation measure in part reflect the local magnetic field strength and particle density. Results. The fractions of resolved sources in the BAL and non-BAL QSO samples are similar (16% vs 12%). The resolved sources in the two samples have similar linear sizes (20 to 400 kpc) and morphology. There is weak evidence that the fraction of variable sources amongst BAL QSOs is lower. The fractions of candidate GHz-peaked sources are similar in the two samples (36±12% vs 23±8%), suggesting that BAL QSOs are not generally younger than non-BAL QSOs. BAL and non-BAL QSOs show a large range of spectral indices, including flat-spectrum and steep-spectrum sources, consistent with a broad range of orientations. There is weak evidence (91% confidence) that the spectral indices of the BAL QSOs are steeper than those of non-BAL QSOs, mildly favouring edge-on orientations. At a higher level of significance (≥97%), the spectra of BAL QSOs are not flatter than those of non-BAL QSOs, which suggests that a polar orientation is not preferred. The distributions of fractional polarisation in the two samples are similar, median values 1-3%. The distributions of rotation measure are also similar, the only outlier being the BAL QSO 1624+37, with an extreme rest-frame Rotation Measure (from the literature) of −18350±570 rad m −2 .
Radio spectra and polarization properties of radio-loud broad absorption-line quasars
Monthly Notices of the Royal Astronomical Society, 2008
We present multifrequency observations of a sample of 15 radio-emitting broad absorption-line quasars (BAL QSOs), covering a spectral range between 74 MHz and 43 GHz. They mostly display convex radio spectra which typically peak at about 1-5 GHz (in the observer's rest frame), flatten at MHz frequencies, probably due to synchrotron self-absorption, and become steeper at high frequencies, i.e. ν 20 GHz. Very Large Array (VLA) 22-GHz maps (HPBW ∼80 mas) show unresolved or very compact sources, with linear projected sizes of ≤1 kpc. About two-thirds of the sample looks unpolarized or weakly polarized at 8.4 GHz, frequency in which reasonable upper limits could be obtained for polarized intensity. Statistical comparisons have been made between the spectral index distributions of samples of BAL and non-BAL QSOs, both in the observed and in the rest frame, finding steeper spectra among non-BAL QSOs. However, constraining this comparison to compact sources results in no significant differences between both distributions. This comparison is consistent with BAL QSOs not being oriented along a particular line of sight. In addition, our analysis of the spectral shape, variability and polarization properties shows that radio BAL QSOs share several properties common to young radio sources like compact steep spectrum or gigahertz peaked spectrum sources.
Are radio-loud Broad Absorption Line Quasars young sources?
Astronomische Nachrichten, 2009
For a long time, radio-loud Broad Absorption Line Quasars (BAL QSOs) were thought to be extremely rare objects. The absorbing troughs seen in their optical spectra are due to strong winds which probably have their origin within the inner region of the AGN, as a result of the accretion processes. Their radio emission constitutes an additional diagnostic tool which is successfully contributing new perspectives and raise new questions, with the aim to enrich our understanding of the BAL phenomenon. In this contribution, we introduce a first characterisation of the radio-loud BAL QSO population. Radio continuum spectra have been collected for a sample of 15 objects, which we present together with their radio polarisation properties. VLA maps in A configuration confirm the compactness of these objects at different frequencies up to 43 GHz, yielding projected linear sizes below 1 kpc. We note that many of their radio properties are common to the population of young radio-sources, like Compact Steep Spectrum (CSS) or Gigahertz-Peaked Spectrum (GPS) sources.
On the absorption of X-ray bright broad absorption line quasars
Arxiv preprint arXiv:0809.0832, 2008
Abstract: Most X-ray studies of BALQSOs found significant (N_H~ 10^{22-24} cm^{-2}) intrinsic column densities of gas absorbing an underlying typical power-law continuum emission, in agreement with expectations from radiatively driven accretion disk wind ...
The ultraviolet spectra of radio-loud and radio-quiet quasars
The Astronomical Journal, 1993
The rest-frame ultraviolet spectral properties of matched samples of radio-loud, radio-moderate, and radio-quiet quasars are investigated, using quasars drawn from the Large Bright QSO Survey. We confirm the absence of spectral differences between radio-loud and radio-quiet quasars at rest-frame wavelengths longward of 1600 Á, as reported by previous authors. However, at shorter wavelengths we find a significant difference: radio-loud quasars have narrower (96% confidence), higher equivalent-width (97% confidence) Lyman-a and C IV emission lines. We further investigate quasars which are radio quiet, but have radio-to-optical flux ratios at the upper extreme of the radio-quiet population. Broad absorption line quasars are overabundant by a factor of-10 in this radio-moderate population; the overabundance is significant at a 99.99% confidence level.
Restarting radio activity and dust emission in radio-loud broad absorption line quasars
Astronomy & Astrophysics, 2015
Context. Broad absorption line quasars (BAL QSOs) are objects that show absorption from relativistic outflows that have velocities up to 0.2c. In about 15% of quasars, these manifest as absorption troughs on the blue side of UV emission lines, such as C and Mg . The launching mechanism and duration of these outflows is not clear yet. Aims. In this work, we complement the information collected in the cm band for our previously presented sample of radio loud BAL QSOs (25 objects with redshifts 1.7 < z < 3.6) with new observations in the m and mm bands. Our aim is to verify the presence of old, extended radio components in the MHz range and probe the emission of dust (linked to star formation) in the mm domain. Methods. We observed 5 sources from our sample, which already presented hints of low-frequency emission, with the GMRT at 235 and 610 MHz. Another 17 sources (more than half the sample) were observed with bolometer cameras at IRAM-30 m (MAMBO2, 250 GHz) and APEX (LABOCA and SABOCA, 350 and 850 GHz, respectively). Results. All sources observed with the GMRT present extended emission on a scale of tens of kpc. In some cases these measurements allow us to identify a second component in the SED at frequencies below 1.4 GHz, beyond the one already studied in the GHz domain. In the mm band, only one source shows emission clearly ascribable to dust, detached from the synchrotron tail. Upper limits were obtained for the remaining targets. Conclusions. These findings confirm that BAL QSOs can also be present in old radio sources or even in restarting ones where favourable conditions for the outflow launching or acceleration are present. A suggestion that these outflows could be precursors of the jet comes from the possibility that ∼70% of our sample is in a GigaHertz Peaked Spectrum (GPS) or Compact Steep Spectrum (CSS)+GPS phase. This would confirm the idea proposed by other authors that these outflows could be recollimated to form the jet. Compared with previous works in the literature, dust emission seems to be weaker than what is expected in "normal" QSOs (both radio loud and radio quiet ones), suggesting that a feedback mechanism could inhibit star formation in radio-loud BAL QSOs.