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

The main instrument that we use for X-ray observations of blazars during the Fermi era is the X-ray Telescope XRT on board the NASA Swift satellite. We use measurements at 0.3-10 keV of gamma-ray blazars provided by the Swift team in support of the Fermi mission. We also have had a number of monitoring and ToO Swift proposals approved since 2009. We use the Swift UVOT instrument to obtain UV measurements of the same objects. Analysis of these observations is presented in Willamson et al. 2014. In that paper, we identified quiescent and active states in our sample of blazars based on their gamma-ray behavior and derived gamma-ray, X-ray, and optical spectral indices. We constructed spectral energy distributions (SEDs) during quiescent and active states and analyzed the relationships between the different spectral indices, blazar classes, and activity states. We found a small scatter in X-ray indices within each class between states, with BL Lac X-ray spectra significantly steeper than in FSRQs, while FSRQs have a highly peaked distribution of X-ray spectral (energy) slopes of -0.60 independent of the state and BL Lacs possess a very broad distribution of X-ray indices during active states.

We use the Suzaku satellite to observe a number of blazars in our sample at 5-20 keV. We plan to explore medium X-ray energies of blazars with the NuSTAR satellite's capabilities.

Previous Observations of Blazars and Radio Galaxies with RXTE

We have carried 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. 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, although delays between the X-ray and optical light curves change with time. We discuss this in detail for the quasar 3C 279 in Chatterjee et al. 2008 and Larionov et al. 2008. The results for the quasar 3C 273 are presented in McHardy et al. 2007.

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. You can view our X-ray, radio, and optical light curves of the RXTE era for 3C 279, PKS 1510-089, and BL Lac as well as an X-ray-only light curve of 3C 273 on our website. 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 both 3C 120 and 3C 111. 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. If the X-ray emitting "corona" is really the base of the jet, the electron density could decrease when the jet flow becomes faster; the lower density would decrease the X-ray flux while the faster flow would form a shock that moves down the jet.

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 multifrequency light curves for 3C 111 up to 2009. More recent results of studying the disk-jet connection in 3C 120 and 3C 111 can be found in Chatterjee et al. 2009, Chatterjee et al. 2011, and Lohfink et al. 2013.

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 many FR II galaxies and quasars. The ratio of X-ray to radio flux for IC-CMB emission 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 (for example, the image of the quasar 0827+243) 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). 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 1E 48 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.

In 2007 we detected the Megaparsec-scale X-ray jet of the BL Lac object OJ 287, which we observed with the Chandra, HST, Spitzer, and VLA (see the image below). We discussed physical conditions in the Megaparsec-scale jet in Marscher & Jorstad 2011.
OJ287 kiloparsec-scale jet
Above: BL Lac object OJ287, at a redshift of 0.306 (distance of 5.15 billion light-years), so that 1" corresponds to 15,000 light-years
False color: X-ray image from the Chandra X-ray Observatory; contours: 1.4 GHz radio image from the Very Large Array

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