X-Ray Observations Made by the Boston University Blazar Group

Note: All images, movies, and figures presented here are copyrighted by Alan Marscher, Svetlana Jorstad, et al. Any use of these images (other than viewing) requires written permission by the authors. Send such requests to Alan Marscher. Permission will normally be granted if suitable acknowledgment is made of the source.

Multifrequency Monitoring of the Brightness of Blazars and Relation of Flares to Events in the Relativistic Jets

We carry out (with collaborators Margo Aller, Ian McHardy, Dick Miller, Martin Gaskell, Markus Boettcher, and our students) a program of intensive monitoring of the quasars 3C273, 3C279, and PKS 1510-089 and the quasar-like object BL Lacertae with RXTE to verify whether the X-ray flares are related to flares at optical, near-infrared, and radio frequencies. This will allow us to understand better the physics of relativistic jets in regions close to the "central engines." The X-ray flux (brightness) of all these objects is highly variable. In PKS 1510-089 radio and X-ray variability are highly correlated. The same is true for the optical and X-ray variations in 3C 279 and BL Lac, and for the near-IR and X-ray variations in 3C 273.

The presence of correlations across wavebands and measurement of frequency-dependent time lags are extremely important for testing models for the high-energy emission in blazars. In order to establish the existence and determine the characteristics of such correlations, we currently monitor 3C 273, 3C 279, and BL Lac three times per week and PKS 1510-089 twice per week with RXTE, except for 8-week periods when they are too close to the sun in the sky. You can view our X-ray, radio, and optical light curves for 3C 279, PKS 1510-089, and BL Lac as well as an X-ray-only light curve of 3C 273. All are complete up to early 2007. If you need more information, please contact Alan Marscher.

X-Ray - Radio Correlations in the Radio Galaxies 3C 120 and 3C 111 (Collaborators: Margo Aller, Ian McHardy, Martin Gaskell, Dick Miller, & Jose-Luis Gomez)

If you want to study the connection between the black hole/accretion disk and the jet, you need an object that acts as a blazar at radio wavelengths and as a Seyfert galaxy at optical and X-ray frequencies. There is strong evidence that the X-rays of the radio galaxies 3C 120 and 3C 111 come mostly from the accretion disk or its immediate surroundings, close to the black hole. The main indication that this is the case is the presence of an X-ray emission line at a rest energy (i.e., after removing the redshift caused by the expansion of the universe) of about 6.4 keV, as seen in Seyfert galaxies. This is interpreted as iron K-shell fluorescence when an electron in the innermost shell is knocked out of the atom by an X-ray photon, with another electron in an outer shell jumping down to take its place. According to the favorite current model, it is caused by X-rays from the corona shining onto the accretion disk. (See the sketches on our research page.)

In both radio galaxies we find that, as in microquasars (binary systems containing a black hole of 10-20 solar masses and a giant-star companion), for some reason a decrease in the X-ray flux occurs as extra energy is shot down the jet, appearing some time later as a bright knot moving down the jet. The evidence is clear in 3C 120 and preliminary in 3C 111, since we have only seen five X-ray dip/superluminal knot ejection events thus far and the X-ray light curve does not have prolonged "plateaus" as in the case of 3C 120. Promising models involve changes in the structure of the magnetic field of the accretion disk. It is thought that energy flow into the jet requires that the magnetic field direction is mainly along the poles of the disk, whereas heating of the accretion disk is most efficient when the magnetic field is turbulent and chaotic. Perhaps when the magnetic field is aligned along the polar direction the heating of the disk and/or infall of gas toward the black hole is inhibited but flow of energy into the jet is enhanced.

For a detailed discussion of our 3C 120 data, go to the 3C 120 page, where you can download our paper that appeared in the June 2002 issue of Nature, read press releases from June 2002, and view animations. There you will find light curves updated through May 2007. For 3C 111, go to the page containing our current data for 3C 111.

Future Improvement: Better Optical Light Curves

The Liverpool Telescope, located on the Canary Island of La Palma, is now observing 3C 273, 3C 279, and PKS 1510-089 2-3 times per week at optical and near-IR wavelengths. We therefore expect to get better-sampled optical light curves for comparison with the X-ray variations. We welcome additional monitoring at other observatories, especially at optical and near-IR wavelengths. This is especially needed for 3C 120 and 3C 111, in which the optical emission is thought to track changes in the accretion disk itself, which should be too cool to produce X-rays. The leading theory for the X-ray production is that it is inverse Compton scattering by hot electrons in a "corona" that lies above the accretion disk. It is also possible that the corona is really the base of the jet before the flow speed accelerates up to near the speed of light. If you are interested and are not already a member of the project, please send e-mail to Alan Marscher at marscher@bu.edu. Current collaborators are listed on our optical page.

X-ray/Radio Imaging Program: Chandra and Very Large Array Observations of Relativistic Jets of Quasars on Kiloparsec Scales

Our program of X-ray observations with Chandra and radio observations with the Very Large Array includes 5 quasars (0827+243, 0923+392, 1222+216, 1317+520, and 2209+080) with arcsecond-scale radio jets. If the jets are relativistic out to kiloparsec scales, we would expect to see X-ray emission from inverse Compton scattering of Cosmic Microwave Background photons (IC-CMB), as appears to be the case in X-ray jets already observed in FR II galaxies and quasars. The ratio of X-ray to radio flux for IC-CMB should depend on the jet orientation, magnetic field direction, Lorentz factor, and redshift. Since we have selected a sample with jets that subtend a variety of angles to the line of sight, our observations test the IC-CMB X-ray emission model in quasar jets.

We have detected and imaged the X-ray jets of four of the above five objects, with 0923+392 (4C39.25) being the only exception. The results thus far (see, for example, the image of 0827+243 on our home page) indicate that two gamma-ray bright blazars with strongly twisted jets, 0827+243 and 1222+216, are indeed emitting X-rays via the IC-CMB process. (The two quasars with nearly straight jets - 1317+520 (also gamma-ray bright), and 2209+080 - may emit X-rays via the synchrotron process.) We conclude that the jets of these two gamma-ray blazars remain highly relativistic out to distances of hundred of kiloparsecs from the nucleus. (1 kpc = 3260 light-years.) The spectacular bends seen in the X-ray jets of these objects are an illusion caused by projection effects: the actual bends are modest - several degrees - but the jets point almost directly at us, so what appears to be a given length of the jet to us - 10 kpc, say - is really extended over a much larger distance - 100 to 200 kpc. The degree of bending that occurs over such a very long distance is then amplified because, in projection, it appears to occur over a much shorter distance.

We have published a paper (Jorstad, S.G., & Marscher, A.P. 2004, Astrophysical Journal, 614, 615-625) that analyzes the Chandra and VLA images of the quasar 0827+243 (image on our home page). We conclude that the jet does not decelerate until the bend, maintaining a velocity of 0.999 times the speed of light (Lorentz factor between 20 and 25) along the same direction from parsec scales out to hundreds of kiloparsecs from the nucleus Then at the bend it decelerates abruptly, perhaps because of interaction with a cloud. The kinetic power of the jet needs to be nearly 1048 ergs/s if it is composed of electrons and protons. It could be two orders of magnitude lower if it is composed of electrons and positrons.

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