259 Acta Polytechnica CTU Proceedings 1(1): 259–264, 2014 259 doi: 10.14311/APP.2014.01.0259 A Review of Astrophysical Jets James H. Beall1,2 1St. John’s College, Annapolis, MD 2Space Sciences Division, Naval Research Laboratory, Washington, DC Corresponding author: beall@sjc.edu Abstract Astrophysical jets are ubiquitous: this simple statement has become a commonplace over the last three decades and more as a result of observing campaigns using detectors sensitive from radio to gamma-ray energies. During this epoch, theoretical models of these sources have become more complex, moving from assumptions of isotropy that made analytic calculations possible, to fully anisotropic models of emission from the jets and their interactions with the interstellar and intra-cluster medium. Such calculations are only possible because we have extensive computational resources. In addition, the degree of international cooperation required for observing campaigns of these sorts is remarkable, since the instruments include among others the Very Large Array (VLA), the Very Long Baseline Array (VLBA), and entire constellations of satellite instruments, often working in concert. In this paper, I discuss some relevant observations from these efforts and the theoretical interpretations they have occasioned. Keywords: astrophysical jets - active galactic nuclei - UHE cosmic rays - quasars - microquasars. 1 Introduction Extended linear structures, usually interpreted as as- trophysical jets, are found in star-forming regions, in compact binaries, and of course in active galactic nuclei (AGNs), and galaxy clusters. Gamma-ray bursts can be associated with astrophysical jets associated with stel- lar collapse where the jets is directed toward Earth. We have become aware of considerable details of these results through decades of sustained observing cam- paigns. The results of these campaigns show similarities and significant differences in the data from some epochs of galactic microquasars, including GRS 1915+105, the concurrent radio and x-ray data (Beall et al., 1978) on Centaurus A (NGC 5128), 3C120 (Marscher (2006), and 3C454.3 as reported by Bonning et al. (2009), to name a few. The Bonning et al. analysis of the 3C454.3 data showed the first results from the Fermi Space Tele- scope for the concurrent variability at optical, UV, IR, and γ-ray variability of that source. In combination with observations of microquasars and quasars from the MOJAVE Collaboration (see, e.g., Lister et al., 2009), these observing campaigns provide an understanding of the time-dependent evolution of these sources at mil- liarcsecond resolutions (i.e., parsec for AGNs, and As- tronomical Unit scales for microquasars). In blazar sources (see, e.g., Ulrich et al. 1997, and Marscher, 2006) there seems to be a confirmed connec- tion of jets with accretion disks. In sources without large-scale linear structures (i.e., jets), as Ulrich et al. (1997) note, the source variability could result from the complex interactions of the accretion disk with an x-ray emitting corona. But to the extent that ”small” jets are present in these sources, the disk-jet interaction must still be of paramount importance, since it provides a mechanism for carrying away energy from the disk. Current theories (see e.g., Hawley 2003 and Bisnovatyi-Kogan 2013 for a discussion of disk struc- ture and jet-launch issues, respectively) suppose that the jet is formed and accelerated in the accretion disk. But even if this is true in all sources, it is still unclear whether or not astrophysical jets with shorter propaga- tion lengths are essentially different in constitution from those that have much longer ranges, or whether the ma- terial through with the jet propagates determines the extent of the observational structures we call jets. At all events, the complexities of the jet-ambient medium interaction must have a great deal to do with the ulti- mate size of emitting region. This sort of argument has applications to both quasars and microquasars, especially if essentially sim- ilar physical processes occurs in all these objects (see, e.g., Beall, 2003, and Marscher, 2006). To some, it has become plausible that essentially the same physics is working over a broad range of temporal, spatial, and lu- minosity scales. Hannikainen (2008) and Chaty (2007) have discussed some of the emission characteristics of 259 http://dx.doi.org/10.14311/APP.2014.01.0259 James H. Beall microquasars, and Paredes (2007) has considered the role of microquasars and AGNs as sources of high en- ergy γ-ray emission. In fact, the early reports of the concurrent radio and x-ray variability of Centaurus A can be plausibly interpreted as the launch of a jet from Cen A’s central source into the complex structures in its core. Additionally, these observations are remarkably similar to the observations of galactic microquasars and AGNs, including the observations from the Fermi Space Telescope of concurrent γ-ray, IR, Optical, and UV vari- ability of 3C454.3 (Bonning et al., 2009), and observa- tions by Madejski et al. (1997) for BL Lac, among others. 2 Concurrent Variability of AGN Jet Sources as an Indication of Jet Launch or Jet-Cloud Interactions I now turn to two such observations in this paper, the concurrent radio and x-ray variability of Centau- rus A (Beall et al., 1978), and the gamma-ray, UV, and IR concurrent variability discussed by Bonning et al., (2009) using the Fermi LAT and Swift spacecraft. 2.1 Radio and X-ray variability measurements of Centaurus 1 (NGC 5128) The first detection of concurrent, multifrequency vari- ability of an AGN came from observations of Centau- rus A (Beall et al., 1978, see Figure 1 of that paper). Beall et al. conducted the observing campaign of Cen A at three different radio frequencies in conjunction with observations from two different x-ray instruments on the OSO-8 spacecraft in the 2-6 keV and 100 keV x- ray ranges. These data were obtained over a period of a few weeks, with the Stanford Interferometer at 10.7 GHz obtaining the most data. Beall et al. also used data from other epochs to construct a decade-long ra- dio and x-ray light curve of the source. Figure 1a of Beall (2011) shows the radio data during the interval of the OSO-8 x-ray observations, as well as the much longer timescale flaring behavior evident in the three different radio frequencies and at both low-energy (2-6 keV, see Figure 1b of Beall, 2011) and in high-energy (∼ 100 keV, see Figure 1c of Beall, 2011) x-rays. As noted by Beall (2011), a perusal of Figure 1a in that paper shows the that the radio data (represented as a “+” in that figure) generally rise during 1973 to reach a peak in mid-1974, then decline to a relative minimum in mid-1975, only to go through a second peak toward the end of 1975, and a subsequent decline toward the end of 1976. This pattern of behavior is also shown in the ∼ 30 GHz data and the ∼ 90 GHz data albeit with less coverage at the higher two radio frequencies. Several points are worthy of note. First, as Beall et al. (1978) show, the radio and x-ray light curves track one another. This result demonstrated the first report of concurrent radio and x-ray variability of an active galaxy. Mushotzky et al. (1978), using the weekly 10.7 GHz data obtained by Beall et al., (1978) demonstrate that the 10.7 GHz radio data track the 2-6 keV x-ray data on weekly time scales, also. The concurrent vari- ability at radio and x-ray frequencies argues that the emitting region is the same for both the radio and x- ray light. This, as was noted by Beall and Rose (1980), can be used to set interesting limits on the parameters of the emitting region. In addition, the observations at the three radio frequencies (10.7 GHz, ∼ 30 GHz, and ∼ 90 GHz) clearly track one another throughout the interval whenever concurrent data are available. Figure 1: Fermi LAT data (see, e.g., Lott et al. 2013) showing the time history of the flaring from 3C454.3 (Figure 1b), along with the Bonning et al. (2009) data showing concurrent gamma-ray, IR, optical, and UV data (Figure 1b). The time of the jet launch from the MOJAVE ”movie” is illustrated by the arrow. The re- sult, first reported here, shows radio flaring consistent with the Bonning et al. data. A plausible hypothesis for the observations we have witnessed is that they arise from physical processes as- sociated with the ”launch” of an astrophysical jet into the complex structures in the core of Centaurus A. The timescale of the evolution from early 1973 through 1977 appears to be associated with the evolution of a larger structure over a more extended region. The observa- tions are consistent with the interaction of the astro- physical jet with an interstellar cloud in the core of Cen A. 260 A Review of Astrophysical Jets It is clear from this discussion that a distinction needs to be made about which observational signatures are associated with the jet launch, the jet itself, and the ambient medium’s reaction to the jet. In considering such a scenario applied to microquasars, the observa- tions of Sco X-1 by Fomalont, Geldzahler, and Brad- shaw (2001), as discussed by Beall et al. (2013) are extremely informative. 2.2 A jet launch coincident with the 3C454.3 multi-frequency flaring Bonning et al. (2009) performed an analysis of the multi-wavelength data from the blazar, 3C454.3, using IR and optical observations from the SMARTS tele- scopes, optical, UV and X-ray data from the Swift satellite, and public-release γ-ray data from the Fermi- LAT experiment. In that work, she demonstrated an excellent correlation between the IR, optical, UV and gamma-ray light curves, with a time lag of less than one day. Urry (2011) noted that 3C454.3 can be a labora- tory for multifrequency variability in Blazars. While a more precise analysis of the data will be required to de- termine the characteristics of the emitting regions for the observed concurrent flaring at the different frequen- cies, the pattern of a correlation between low-energy and higher-energy variability is consistent with that ob- served for Cen A, albeit with the proviso that the en- ergetics of the radiating particles in 3C454.3 is consid- erably greater. A perusal of the data for 3C454.3 at milliarcsecond scales taken from the MOJAVE VLBA campaign dur- ing the Bonning et al (2009) flare show that the time- dependent flare occurs during launch of a new compo- nent of the jet that originates from the core. Figure 2: Figure 2: 3C454.3 shown at milliarcsecond scales for data taken from the MOJAVE VLBA cam- paign during the Bonning et al (2009) flare. A perusal of the time-dependent flare clearly shows that the jet launch from the core of the AGN is coincident with the double peaked flaring shown in the Bonning et al. re- sults. The pattern of variability reported by Bonning et al., is consistent with the injection of relativistic parti- cles into a region with relatively high particle and radi- ation densities (i.e., an interstellar cloud). The picture that emerges, therefore, is consistent with the observa- tions of spatially and temporally resolved galactic mi- croquasars and AGN jets. It should not escape our notice that other epochs in the 3C454.3 data from MOJAVE and FERMI also show similar periods of injection of radio blobs that are asso- ciated with a double-peaked structure in the gamma-ray light. For the Cen A data, and for data from 3C454.3, it is the concurrent variability that suggests that the ra- dio to x-ray (in Cen A’s case) and the IR, Optical, and UV to γ-ray fluxes (in 3C454.3’s case) are created in the same region. This leads to the possibility of esti- mates of the source parameters that are obtained from models of these sources. VLBI observations of cores vs. jets (see, e.g., the study of BL Lac by Bach et al. 2006) show the structures of the core vs. jet as they change in frequency and time. It has thus become possible to sep- arate and study the time variability of the jet and the core of AGN at remarkably fine temporal and spatial scales. Van der Laan (1976) discussed the theoretical in- terpretation of cosmic radio data by assuming a source which contained uniform magnetic field, suffused with an isotropic distribution of relativistic electrons. The source, as it expanded, caused an evolution of the ra- dio light curve at different frequencies. Each of the curves in Van der Laan’s paper represents a factor of 2 difference in frequency, the vertical axis representing intensity of the radio flux and the horizontal axis repre- senting an expansion timescale for the emitting region. Van der Laan’s calculations show a marked difference between the peaks at various frequencies. The data from Cen A (as discussed more fully in Beall (2008, 2010) are, therefore, not consistent with van der Laan expansion (Van der Laan, 1976), since for van der Laan expansion, we would expect the different frequencies to achieve their maxima at different times. Also, the peak intensities should decline with increas- ing frequencies at least in the power-law portion of the spectrum. The most likely explanation for the changes in the spectrum of the Cen A data at 100 keV (Beall, et al., 1978) is that the emitting region suffered an injection of energetic electrons. That is, a jet-ambient-medium interaction dumped energetic particles into a putative ”blob,” or, equivalently, that there was a re-acceleration of the emitting electrons on a timescale short compared to the expansion time of the source. An analysis of the 3C120 results compared with the data from the galactic microquasar, Sco X-1, under- 261 James H. Beall taken by Beall (2006) shows a similar radio evolution, with rapidly moving “bullets” interacting with slower moving, expanding blobs. It is highly likely that the elements of these sources that are consistent with van der Laan expansion are the slower-moving, expanding blobs. I believe that the relativistically moving bullets, when they interact with these slower-moving blobs, are the genesis of the flaring that we see that seems like a re-acceleration of the emitting, relativistic particles. I note that a similar scenario could be operating in Cen A and 3C454.3. This is not to say that the ”slower-moving blobs” are not themselves moving relativistically, since the bi- polar lobes have significant enhancements in brightness due to relativistic Doppler boosting for the blobs mov- ing toward us. The true test of this hypothesis will require concur- rent, multifrequency observations with resolutions suffi- cient to distinguish jet components from core emissions in galactic microquasars as well as for AGN jets. One of the most remarkable sagas regarding the dis- covery of quasar-like activity in galactic sources comes from the decades long-investigation of Sco X-1 by Ed Fomalont, Barry Geldzahler, and Charlie Bradshaw (Fomalont, Geldzahler, and Bradshaw, et al. 2001). During their observations, an extended source changed relative position with respect to the primary object, dis- appeared, and then reappeared many times. We now know that they were observing a highly variable jet from a binary, neutron star system. The determinant ob- servation was conducted using the Very Large Array (VLA) in Socorro, New Mexico and the VLBA inter- ferometer (see, e.g., Beall, 2008) for a more complete discussion). The observations of the concurrent IR, Optical, UV, and γ-ray variability of 3C454.3, and its associated jet launch in the MOJAVE data, argue for a reinvestigation of these data sets in the near future. It is worthy of note that the milliarcsecond observa- tions show a complex evolution of structure at parsec scales, including an apparently sharp change of direc- tions associated with changes in the polarization of the radio light at that point in the jet’s evolution (see Figure 2). Furthermore, the multi-frequency flares reported by Bonning et al. (2009) are consistent with the launch of another component of the astrophysical jet in the core region. Regarding the acceleration region and the possible mechanisms for the collimation of the jets, a number of models have been proposed (see, e.g., Kundt and Gopal-Krishna (2004), Bisnovaty-Kogan et al. (2002), Romanova and Lovelace (2009), and Bisnovaty-Kogan (2012) that might help explain the complexity present in these data. 3 Concluding Remarks The data discussed therein suggest a model for the jet structures in which beams or blobs of energetic plasmas propagate outward from the central engine to interact with the ambient medium in the source region. This ambient medium in many cases comes from prior ejecta from the central source, but can also come from clouds in the Broad Line Region. The jet can apparently also excavate large regions, as is suggested by the complex structures in, for example, 3C120. The physical pro- cesses which can accelerate and entrain the ambient medium through which the jet propagates, have been discussed in detail in several venues (see, e.g., Rose et al., 1984, 1987, Beall, 1990, Beall et al., 2003, and Beall, 2010). In this paper, we have focused on the patterns of concurrent radio and x-ray variability for Cen A, and on concurrent radio, gamma-ray, optical, IR, and UV variability for 3C454.3. The data can be interpreted as being associated with a jet-launch scenario for these sources, and this paper represents the first report of the association between a jet launch in the MOJAVE data for 3C454.3 and its gamma-ray flare from FERMI. In addition, jets from microquasars show similar patterns of variability to those of AGNs. These data require us to abandon our assumptions both of spherical sym- metry and of single-zone productions in our models of these sources. 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If you recall the data I showed for 3C454.3, the region where the get seems to turn shows marked variations in brightness that seem to be associated both with the blobs from the jet traversing that region, and with the jet luminosity at the ”elbow.” In fact, it seems likely that the acceleration and collimation region of the jet in 3C454.3 extends many parsecs from the launch re- gion. CARLOTTA PITTORI Can you say something about the southern jet of the Crab nebula? As I actually showed at a previous vulcano workshop and promptly noticed by W. Kundt, the jet orientation for x-ray im- ages from Chandra changed in the ten years from 2000- 2010. How can this be explained? By kink instabilities? Where does the field close in this system? JIM BEALL This is only a conjecture, but if we look at the large distance where the jet in 3C454.3 changes direction, it appears that magnetic field structures can extend out to several parsecs and maintain their coher- ence, at least in AGN sources. If the same is true in the Crab, then the magnetic fields might be responsible for the changes in the jet orientation. WOLFGANG KUNDT Can you tell me when and where Geoff Burbidge argued in favor of hadronic jets on the grounds of their stability? In my model with Gopal, a pair-plasma jet is thought to be stable on the grounds of their ExB drift, for example in New Astron- omy Reviews, 2008, 52, pp 364-369. JIM BEALL Geoff Burbidge’s article was published in Astrophysical Journal, 1956, 124, 416. That paper was in the main an argument that the energy supplied to the giant radio lobes must be of order 1060 ergs, and could only be delivered by hadronic jets. 264 Introduction Concurrent Variability of AGN Jet Sources as an Indication of Jet Launch or Jet-Cloud Interactions Radio and X-ray variability measurements of Centaurus 1 (NGC 5128) A jet launch coincident with the 3C454.3 multi-frequency flaring Concluding Remarks