Understanding Blazar Jets Through Their Multifrequency Emission

Being dominated by non-thermal (synchrotron and inverse Compton) emission from a relativistic jet, blazars offer important clues to the structure and radiative processes in extragalactic jets. Crucial information is provided by blazars' spectral energy distributions from radio to gamma-rays (GeV and TeV energies), their trends with bolometric luminosity, and their correlated variability properties. This review is focussed on recent multiwavelength monitorings of confirmed and candidate TeV blazars and the constraints they provide for the radiative properties of the emitting particles. I also present recent observations of the newly discovered class of ``blue quasars'' and the implications for current blazars' unification schemes.Comment: invited opening talk at the GeV-TeV Astronomy International meeting held at the Snowbird resort, Salt Lake City, Utah, August 199

A kpc-scale X-ray jet in the BL Lac source S5 2007+777

X-ray jets in AGN are commonly observed in FRII and FRI radio-galaxies, but rarely in BL Lacs, most probably due to their orientation close to the line of sight and the ensuing foreshortening effects. Only three BL Lacs are known so far to contain a kpc-scale X-ray jet. In this paper, we present the evidence for the existence of a fourth extended X-ray jet in the classical radio-selected source S5 2007+777, which for its hybrid FRI/II radio morphology has been classified as a HYMOR (HYbrid MOrphology Radio source). Our Chandra ACIS-S observations of this source revealed an X-ray counterpart to the 19"-long radio jet. Interestingly, the X-ray properties of the kpc-scale jet in S5 2007+777 are very similar to those observed in FRII jets. First, the X-ray morphology closely mirrors the radio one, with the X-rays being concentrated in the discrete radio knots. Second, the X-ray continuum of the jet/brightest knot is described by a very hard power law, with photon index Gamma_x~1, although the uncertainties are large. Third, the optical upper limit from archival HST data implies a concave radio-to-X-ray SED. If the X-ray emission is attributed to IC/CMB with equipartition, strong beaming (delta=13) is required, implying a very large scale (Mpc) jet. The beaming requirement can be somewhat relaxed assuming a magnetic field lower than equipartition. Alternatively, synchrotron emission from a second population of very high-energy electrons is viable. Comparison to other HYMOR jets detected with Chandra is discussed, as well as general implications for the origin of the FRI/II division.Comment: Accepted for publication in ApJ, 19 pages, 3 figure

A Universal Scaling for the Energetics of Relativistic Jets From Black Hole Systems

Black holes generate collimated, relativistic jets which have been observed in gamma-ray bursts (GRBs), microquasars, and at the center of some galaxies (active galactic nuclei; AGN). How jet physics scales from stellar black holes in GRBs to the supermassive ones in AGNs is still unknown. Here we show that jets produced by AGNs and GRBs exhibit the same correlation between the kinetic power carried by accelerated particles and the gamma-ray luminosity, with AGNs and GRBs lying at the low and high-luminosity ends, respectively, of the correlation. This result implies that the efficiency of energy dissipation in jets produced in black hole systems is similar over 10 orders of magnitude in jet power, establishing a physical analogy between AGN and GRBs.Comment: Published in Science, 338, 1445 (2012), DOI: 10.1126/science.1227416. This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. Corrected typo in equation 4 of the supplementary materia

A Rapid X-ray Flare from Markarian 501

We present X-ray observations of the BL Lacertae (BL Lac) object Markarian 501 (Mrk 501), taken with the Rossi X-ray Timing Explorer in 1998 May as part of a multi-wavelength campaign. The X-ray light curve shows a very rapid flare in which the 2-10 keV flux increased by ~60% in <200 seconds. This rapid rise is followed by a drop-off in the 2-10 keV flux of ~40% in <600 seconds. The 10-15 keV variation in this flare is roughly a factor of two on similar time-scales. During the rise of the flare, the 3-15 keV spectral index hardened from 2.02 +/- 0.03 to 1.87 +/- 0.04, where it remained during the decay of the flare. This is the fastest variation ever seen in X-rays from Mrk 501 and among the fastest seen at any wavelength for this object. The shift in the energy at which the spectral power peaks (from 30 keV during the flare) is also among the most rapid shifts seen from this object. This flare occurs during an emission state (2-10 keV flux approximately 1.2e-10 erg cm^-2 s^-1) that is approximately 25% of the peak flux observed in 1997 April from this object but which is still high compared to its historical average X-ray flux. The variations in the hardness ratio are consistent with the low energy variations leading those at high energies during the development and decay of the flare. This pattern is rare among high frequency peaked BL Lac objects like Mrk 501, but has been seen recently in two other TeV emitting BL Lacs, Mrk 421 and PKS 2155-304. The hard lag is consistent with a flare dominated by the acceleration time-scale for a simple relativistic shock model of flaring.Comment: 11 pages, 3 figures, accepted for publication in Astrophys. J. Letter

A Chandra view of Naked Active Galactic Nuclei

We present the first X-ray observations of three sources belonging to a new AGN class: the naked AGNs. Based on optical spectroscopic studies, these sources appear as classical type 2 (obscured) AGNs, with only narrow emission lines. However, long-term optical monitoring campaigns, carried out over more than two decades, show that the same sources are strongly variable, like type 1 (un-obscured) AGNs. Based on short Chandra observations, the sources appear to be fairly bright in the X-rays, with typical Seyfert 1s values for the photon index (Gamma~1.8) and without significant intrinsic absorption, supporting the conclusion that some bright AGNs may genuinely lack a broad line region. Future, broad-band studies as well as deeper X-ray observations, probing both the spectral and the temporal properties of the naked AGNs, are crucial to shed light on the central engine of these sources, which may be representative of a large class of AGNs.Comment: 7 pages, 3 figures, accepted for publication in Ap

Spitzer IRAC Imaging of the Relativistic Jet from Superluminal Quasar PKS 0637-752

Emission from the relativistic jet located at hundreds of kpc from the core of the superluminal quasar PKS 0637-752 was detected at 3.6 and 5.8 microns with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. The unprecedented sensitivity and arcsecond resolution of IRAC allows us to explore the mid-infrared emission from kiloparsec-scale quasar jets for the first time. The mid-infrared flux from the jet knots, when combined with radio and optical fluxes, confirms a synchrotron origin of the radio-to-optical emission and constrains very well the high energy end of the nonthermal electron distribution. Assuming the X-rays are produced in the relativistically moving knots via inverse Compton scattering of cosmic microwave background (CMB) radiation, the infrared observation puts constraints on the matter content of the quasar extended jet. Specifically, pure electron-positoron pair jet models are unfavorable based on the lack of an infrared bump associated with ``bulk Comptonization'' of CMB photons by an ultrarelativistic jet.Comment: 4 pages, 3 figures; accepted for publication in ApJ Letter