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Off the straight and narrow: The kinky jets of Blazars

December 4, 2010

ResearchBlogging.orgGoogle any of the myriad artists impressions of blazars and relativistic jets available online and they all share one common feature: the jet streams out helically from the central accretion disc/black hole region in a long thin straight line that would make any Roman road builder green with envy. For example (image: University of Michigan):

Artists Impression of a Blazar

Yet this is actually a misrepresentation, as a new paper published in this month’s Astronomy and Astrophysics discusses. The paper, entitled Another look at the BL Lacertae flux and spectral variability and authored by a multinational group of astronomers led by Claudia Raiteri of the Osservatorio Astronomico di Torino suggests that the relativistic jet of the famous blazar BL Lacertae is anything but straight – in addition to the previously known spiral rotation, it contains a bend or ‘kink’.

The authors analysed the flux from the canonical blazar BL Lacertae (also known as BL Lac) over a period of a year covering most of 2008 in a wide variety of wavelengths (radio, near-IR and optical) using a wide variety of telescopes from the Whole Earth Blazar Telescope (WEBT) network including both ground and space-borne instruments (including the AGILE and Fermi (GLAST) satellites). The flux from BL Lac is well-known to be variable, but a major flare was observed near the start of the monitoring period (diagram: Raiteri et al.):

R-band composite light curve of BL Lacertae by the collaborating observatories from February 2008 to February2009

From these observations, they built a broad-band Spectral Energy Distribution (SED) of BL Lac which they then used to develop an enhanced model of the jet that includes for the first time a ‘kink’ to explain the observed emissions (diagram: Raiteri et al.):

Broad-band SEDs (dashed/dotted lines) of BL Lacertae in August 2008 (blue) and July 1997 (red). Solid lines represent model fits from Raiteri et al.

Using their model, the authors show that the observed SED and flux of the jet are explained in both a flared and non-flared state by a jet has has a bent section or ‘kink’ (Image: Raiteri et al.):

Sketch of the Raiteri et al. helical jet model during both the 2008 faint state and the 1997 outburst state. The different colours represent  different regions & emission: purple-blue – inner region, emitting the high-energy synchrotron plus self inverse-Compton component;  green-yellow-red – the outer zone, where the low-energy emission component is produced

In addition to the ‘kink’, as mentioned in the introduction to this post, the whole jet is rotating helically like a corkscrew, and is also constantly changing its alignment with respect to our line of sight (the angles in the above diagram have been exaggerated for clarity).

Another advantage of this new model is that it also confirms previous calculations by the same authors published last year (Raiteri et al. 2009) that reduced the number of emitting regions (“components”) in the jet that were required to explain the observed emission:

“Photons coming from the disc or broad line region could then enter the jet, and be inverse-Compton scattered, giving rise to other high-energy emission components that are sometimes invoked to account for the SED properties of blazars. In particular, the 1997 outburst state has previously been interpreted by Madejski et al. (1999) in terms of three emission components: synchrotron, synchrotron self-Compton, and Comptonisation of the broad emission line flux. Similar results were obtained by Bottcher & Bloom (2000) and by Ravasio et al. (2002).”

“Our ‘geometrical’€interpretation does not require these external-Compton emission components, which are not expected to contribute if the jet emission regions are parsecs away from the central black hole.”

This reappraisal of the physical structure of the jet is important, because, as the authors state:

“…the whole range of BL Lacertae multi-wavelength variability can be interpreted in terms of orientation effects. Although the rotating helical jet model we have adopted in the previous section is not a physically complete model, but more a phenomenological approach, it has the advantage of taking into account variations of the orientation of the emitting regions with respect to the line of sight, with consequent changes of the Doppler beaming factor. This is an aspect that is usually neglected by theoretical models of blazar emission,which explain flux and spectral changes uniquely in terms of energetic processes inside the jet.”

So what could cause the intricate structure of the jet? We know that there are three possible factors that are involved – curved magnetic fields in the region of the jet, the rotation of the parent black hole/accretion disc, and interaction of the particles in the jet with the surrounding medium.

The idea that relativistic jets (and the jet of BL Lac in particular) can possess helical structure caused by curved magnetic fields is not new. In 2008, a team led by Prof. Alan Marscher (who also contributed to this new paper) observed material in the jet of the same blazar, BL Lac, and observed that material close to the base of the jet appeared to follow a corkscrew-shaped path caused by twisting magnetic fields (image: Boston University).

Marscher et al. also noted that the radiation emitted by the moving material brightened when its rotating path was aimed almost directly toward Earth (due to doppler boosting), an effect Raiteri et al. have confirmed. From the original Marscher press release:

“[Theorists predicted] that material moving outward in this close-in acceleration region would follow a corkscrew-shaped path inside the bundle of twisted magnetic fields. They also predicted that light and other radiation emitted by the moving material would brighten when its rotating path was aimed most directly toward Earth. Marscher and his colleagues anticipated that there might also be a flare later when the material hits a stationary shock wave called the “core” some time after it has emerged from the acceleration region. “That behaviour is exactly what we saw,” Marscher said, when his team followed an outburst of radiation from BL Lac. In late 2005 and early 2006, the astronomers watched BL Lac with an international collection of ground- and space-based telescopes as a bright knot of condensed material was ejected outward through the jet. As the material sped out from the neighbourhood of the black hole, the VLBA could pinpoint its location, while other telescopes measured the properties of the radiation emitted from the knot.

Bright bursts of light, X rays, and gamma rays came when the knot was precisely at locations where the theories said such bursts would be seen. In addition, the property of the radio and light waves called polarization rotated as the knot wound its corkscrew path inside the tight throat of twisted magnetic fields.”

But why are the magnetic fields twisted? The answer is that the rotation of the black hole is distorting space itself, an effect known as frame-dragging. As space itself twists, any magnetic fields present also twist (image: NASA):

The big question raised by this paper is whither this ‘kink’ is unique to the jet of BL Lacertae or if its a common feature of other relativistic jets. And if it turns out the jets of AGN can be bent, what about the jets of their smaller galactic cousins, microquasars?

References:

Raiteri, C., Villata, M., Bruschini, L., Capetti, A., Kurtanidze, O., Larionov, V., Romano, P., Vercellone, S., Agudo, I., Aller, H., Aller, M., Arkharov, A., Bach, U., Berdyugin, A., Blinov, D., Böttcher, M., Buemi, C., Calcidese, P., Carosati, D., Casas, R., Chen, W., Coloma, J., Diltz, C., Di Paola, A., Dolci, M., Efimova, N., Forné, E., Gómez, J., Gurwell, M., Hakola, A., Hovatta, T., Hsiao, H., Jordan, B., Jorstad, S., Koptelova, E., Kurtanidze, S., Lähteenmäki, A., Larionova, E., Leto, P., Lindfors, E., Ligustri, R., Marscher, A., Morozova, D., Nikolashvili, M., Nilsson, K., Ros, J., Roustazadeh, P., Sadun, A., Sillanpää, A., Sainio, J., Takalo, L., Tornikoski, M., Trigilio, C., Troitsky, I., & Umana, G. (2010). Another look at the BL Lacertae flux and spectral variability Astronomy and Astrophysics, 524 DOI: 10.1051/0004-6361/201015191

Marscher, A., Jorstad, S., D’Arcangelo, F., Smith, P., Williams, G., Larionov, V., Oh, H., Olmstead, A., Aller, M., Aller, H., McHardy, I., Lähteenmäki, A., Tornikoski, M., Valtaoja, E., Hagen-Thorn, V., Kopatskaya, E., Gear, W., Tosti, G., Kurtanidze, O., Nikolashvili, M., Sigua, L., Miller, H., & Ryle, W. (2008). The inner jet of an active galactic nucleus as revealed by a radio-to-gamma-ray outburst Nature, 452 (7190), 966-969 DOI: 10.1038/nature06895

Raiteri, C., Villata, M., Capetti, A., Aller, M., Bach, U., Calcidese, P., Gurwell, M., Larionov, V., Ohlert, J., Nilsson, K., Strigachev, A., Agudo, I., Aller, H., Bachev, R., Benítez, E., Berdyugin, A., Böttcher, M., Buemi, C., Buttiglione, S., Carosati, D., Charlot, P., Chen, W., Dultzin, D., Forné, E., Fuhrmann, L., Gómez, J., Gupta, A., Heidt, J., Hiriart, D., Hsiao, W., Jelínek, M., Jorstad, S., Kimeridze, G., Konstantinova, T., Kopatskaya, E., Kostov, A., Kurtanidze, O., Lähteenmäki, A., Lanteri, L., Larionova, L., Leto, P., Latev, G., Le Campion, J., Lee, C., Ligustri, R., Lindfors, E., Marscher, A., Mihov, B., Nikolashvili, M., Nikolov, Y., Ovcharov, E., Principe, D., Pursimo, T., Ragozzine, B., Robb, R., Ros, J., Sadun, A., Sagar, R., Semkov, E., Sigua, L., Smart, R., Sorcia, M., Takalo, L., Tornikoski, M., Trigilio, C., Uckert, K., Umana, G., Valcheva, A., & Volvach, A. (2009). WEBT multiwavelength monitoring and XMM-Newton observations of
BL Lacertae in 2007–2008 Astronomy and Astrophysics, 507 (2), 769-779 DOI: 10.1051/0004-6361/200912953

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