Fuel for the black hole

First investigations of a galactic nucleus with the AMBER instrument of the Very Large Telescope Interferometer in Chile


An international research team led by Gerd Weigelt from the Max-Planck-Institut für Radioastronomie in Bonn reports on high-resolution studies of an active galactic nucleus in the near-infrared. The observations were carried out with the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). The use of near-infrared interferometry allowed the team to resolve a ring-shaped dust distribution (generally called "dust torus") in the inner region of the nucleus of the active galaxy NGC 3783. This dust torus probably represents the reservoir of gaseous and dusty material that "feeds" the hot gas disk ("accretion disk") and the supermassive black hole in the center of this galaxy. The resolved dust torus has an angular radius of only 0.7 milli-arcseconds on the sky, an angle that is 5 million times smaller than one degree. This angular radius corresponds to a radius of approximately 0.5 light years for a distance of 150 million light years. Studies of the physical properties of these dust tori are very important to improve our understanding of their structure and interaction with the accretion disk. To obtain these measurements, the light from up to three telescopes of the Very Large Telescope Interferometer was interferometrically combined. This method is able to achieve an angular resolution equivalent to the resolution of a telescope with a diameter of 130 Meters.

Figure 1: Artist's view of a dust torus surrounding the accretion disk and the central black hole in active galactic nuclei. Credit: NASA E/PO - Sonoma State University, Aurore Simonnet (http://epo.sonoma.edu/). (Click here for higher resolution)

Extreme physical processes occur in the innermost regions of galactic nuclei. Supermassive black holes were discovered in many galaxies. The masses of these black holes are often a millionfold larger than the mass of our sun. These central black holes are surrounded by hot and bright gaseous disks, called "accretion disks". The emitted radiation from these accretion disks is probably generated by infalling material. To maintain the high luminosity of the accretion disk, fresh material has to be permanently supplied. The dust tori (see Fig. 1) surrounding the accretion disks are most likely the reservoir of the material that flows through the accretion disk and finally "feeds" the growing black hole.

Observations of these dust tori are very challenging since their sizes are very small. A giant telescope with a mirror diameter of more than 100 Meters would be able to provide the required angular resolution, but unfortunately telescopes of this size will not be available in the near future. This raises the question: Is there an alternative approach that provides the high resolution required?

The solution is to simultaneously combine ("interfere") the light from two or more telescopes since these multi-telescope images, which are called interferograms, contain high-resolution information. In the reported NGC 3783 observations, the AMBER interferometry instrument was used to combine the infrared light from two or three telescopes of ESO's Very Large Telescope Interferometer (VLTI, see Fig. 2). This interferometric method is able to achieve an extreme angular resolution that is proportional to the distance between the telescopes. Since the largest distance between the four telescopes of the VLTI is 130 Meters, an angular resolution is obtained that is as high as the theoretical resolution of a telescope with a mirror diameter of 130 Meters - a resolution that is 15 times higher than the resolution of one of the VLTI telescopes, which have a mirror diameter of 8 Meters.

"The ESO VLTI provides us with a unique opportunity to improve our understanding of active galactic nuclei,", says Gerd Weigelt from the Max-Planck-Institut für Radioastronomie in Bonn. "It allows us to study fascinating physical processes with unprecedented resolution over a wide range of infrared wavelengths. This is needed to derive physical properties of these sources."

And Makoto Kishimoto emphasizes: "We hope to obtain more detailed information in the next few years by additional observations at shorter wavelengths, with longer baselines, and with higher spectral resolution. Most importantly, in a few years, two further interferometric VLTI instruments will be available, which can provide complementary information."

To resolve the nucleus of the active galaxy NGC 3783, the research team recorded thousands of two- and three-telescope interferograms with the VLTI. The telescope distances were in the range of 45 to 114 Meters. The evaluation of these interferograms allowed the team to derive the radius of the compact dust torus in NGC 3783. A very small angular torus radius of 0.74 milli-arcsecond was measured, which corresponds to a radius of 0.52 light years. These near-infrared radius measurements, together with previously obtained mid-infrared measurements, allowed the team to derive important physical parameters of the torus of NGC 3783.

"The high resolution of the VLTI is also important for studying many other types of astrophysical key objects", underlines Karl-Heinz Hofmann. "It is clear that infrared interferometry will revolutionize infrared astronomy in a similar way as radio interferometry has revolutionized radio astronomy."

Fig. 2: The Very Large Telescope Interferometer of the European Southern Observatory.Photo: Gerd Weigelt/MPIfR. (Click here for higher resolution).

The research team comprises scientists from the Universities of Florence, Grenoble, Nice, Santa Barbara, and from the MPI für Radioastronomie.

Original Paper:

VLTI/AMBER observations of the Seyfert nucleus of NGC 3783, G. Weigelt, K.-H. Hofmann, M. Kishimoto, S. Hönig, D. Schertl, A. Marconi,, F. Millour, R. Petrov, D. Fraix-Burnet, F. Malbet, K. Tristram and M. Vannier, Astronomy & Astrophysics (Volume 541, L9, 2012). DOI: 10.1051/0004-6361/201219213.

Parallel Press Releases:

Buco nero in HD , Media INAF 16/05/2012 (in Italian)


Further Information:

Max-Planck-Institut für Radioastronomie (MPIfR)

Infrared Astronomy Group at MPIfR

European Southern Observatory (ESO)

AMBER - Astronomical Multi-BEam combineR, ESO Homepage

Astronomical Multi-BEam combineR, Grenoble Homepage

Institute for Planetology and Astrophysics (IPAG), Grenoble, France

Laboratoire Lagrange, Nice, France

Observatoire de la Côte d'Azur, Nice, France

Arcetri Astrophysical Observatory, Firenze, Italy

Department of Physics, UCSB, U.S.A.


Contact:

Prof. Dr. Gerd Weigelt,
Head of Research group "Infrared Astronomy",
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49(0)228-525-243
E-mail: gweigelt@mpifr-bonn.mpg.de

Dr. Makoto Kishimoto,
Max-Planck-Institut für Radioastronomie:
Fon: +49(0)228-525-189
E-mail: mk@mpifr-bonn.mpg.de

Dr. Norbert Junkes,
Press and Public Outreach,
Max-Planck-Institut für Radioastronomie:
Fon: +49(0)228-525-399
E-mail: njunkes@mpifr-bonn.mpg.de

More about → Fuel for the black hole

The Art of Recycling Pulsars

Stellar Astrophysics helps to explain the behaviour of fast rotating neutron stars in binary systems

What happens to the spin of rapidly rotating neutron stars called millisecond pulsars when reaching the end of their mass-accretion phase? The formation of millisecond pulsars is the result of stellar cannibalism where matter flows from a donor star to an accreting pulsar in a binary system. During this process the pulsar emits X-rays while being spun up to amazingly high rotational speeds with periods of about 1 to 10 milliseconds. Astrophysicist Thomas Tauris has a joint appointment at both, Argelander-Institut für Astronomie and Max-Planck-Institut für Radioastronomie, based in Bonn, Germany. Through numerical calculations on the base of stellar evolution and accretion torques, he can show that millisecond pulsars loose about half of their rotational energy during the final stages of the mass-transfer process before the pulsar turns on its radio beam. This result is in agreement with current observations and the findings also explain why radio millisecond pulsars appear to be much older than the white dwarf remnants of their companion stars - and perhaps why no sub-millisecond radio pulsars exist at all.

The results are reported in the February 03 issue of the journal "Science".

Figure 1: An artist's impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA / Goddard Space Flight Center / Dana Berry (Click here for higher resolution)

Millisecond pulsars are strongly magnetized, old neutron stars in binary systems which have been spun up to high rotational frequencies by accumulation of mass and angular momentum from a companion star. Today we know of about 200 such pulsars with spin periods between 1.4-10 milliseconds. These are located in both the Galactic Disk and in Globular Clusters.

Since the first millisecond pulsar was detected in 1982 is has remained a challenge for theorists to explain their spin periods, magnetic fields and ages. As an example, there is the "turn-off" problem, i.e. what happens to the spin of the pulsar when the donor star terminates its mass-transfer process?

"We have now, for the first time, combined detailed numerical stellar evolution models with calculations of the braking torque acting on the spinning pulsar", says Thomas Tauris, the author of the present study. "The result is that the millisecond pulsars loose about half of their rotational energy in the so-called Roche-lobe decoupling phase." This phase is describing the termination of the mass transfer in the binary system. Hence, radio-emitting millisecond pulsars should spin slightly slower than their progenitors, X-ray emitting millisecond pulsars which are still accreting material from their donor star. This is exactly what the observational data seem to suggest. Furthermore, these new findings can help explain why some millisecond pulsars appear to have characteristic ages exceeding the age of the Universe and perhaps why no sub-millisecond radio pulsars exist.

The key feature of the new results is that it has now been demonstrated how the spinning pulsar is able to brake out of its so-called equilibrium spin. At this epoch the mass-transfer rate decreases which causes the magnetospheric radius of the pulsar to expand and thereby expelling the infalling matter like a propeller. This causes the pulsar to loose additional rotational energy and thus slow down its spin rate.

"Actually, without a solution to the "turn-off" problem we would expect the pulsars to even slow down to spin periods of 50-100 milliseconds during the Roche-lobe decoupling phase", concludes Thomas Tauris. "That would be in clear contradiction with observational evidence for the existence of millisecond pulsars."


This work has profited from a recent effort to bridge the Stellar Physics group at the Argelander-Institut für Astronomie at University of Bonn (led by Norbert Langer) with the Fundamental Physics in Radio Astronomy group at the Max-Planck-Institut für Radioastronomie (led by Michael Kramer). The stellar evolution models used for this work were made using a state-of-the-art code developed by Norbert Langer. A significant part of the observational data was supplied by the pulsar group. Michael Kramer and his colleagues are using the 100-m Effelsberg Radio Telescope to participate in several ongoing searches and discoveries of millisecond pulsars.

Thomas Tauris has been working at the Argelander-Institut für Astronomie and the Max-Planck-Institut für Radioastronomie as a visiting research professor since 2010. Some of his recent work on the recycling of millisecond pulsars has been published in the journal "Monthly Notices of the Royal Astronomical Society" in joint publications with Norbert Langer and Michael Kramer. On February 27 they host an international one-day workshop in Bonn on the "Formation and Evolution of Neutron stars".


Original Paper:

Spin-Down of Radio Millisecond Pulsars at Genesis, Thomas M. Tauris, Science Bd. 335, S. 561. DOI 10.1126/science.1216355.


Further Information:

Max Planck Institute for Radio Astronomy (MPIfR).

Argelander-Institut für Astronomie (AIfA), University of Bonn.

Fundamental Physics in Radio Astronomy.

Bonn Stellar Physics Group.

Formation of millisecond pulsars with CO white dwarf companions, Thomas M. Tauris, Norbert Langer, Michael Kramer, Preprint MNRAS.

Formation and Evolution of Neutron Stars, Workshop Bonn Feb 27, 2012.


Parallel Press Releases:

The discovery of deceleration , Max Planck Society News Release, 02 February 2012.

Sternenkreisel entpuppt sich als Vampir , Press Release, University of Bonn, February 03, 2012 (in German).

Contact:

Dr. Thomas M. Tauris,
Argelander-Institut für Astronomie, Univ. of Bonn
& Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49-73-3660
E-mail: tauris (at) astro.uni-bonn.de

Prof. Dr. Michael Kramer,
Director and Head of Research Group "Fundamental Physics in Radio Astronomy",
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49(0)228-525-278
E-mail: mkramer (at) mpifr-bonn.mpg.de

Dr. Norbert Junkes,
Press and Public Outreach,
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49-228-525-399
E-mail: njunkes (at) mpifr.de

More about → The Art of Recycling Pulsars