Blazing BL Lacertaes. Jacob Trier Frederiksen. November 9, 2006

Transcription

1 Blazing BL Lacertaes Jacob Trier Frederiksen November 9, 2006

2 BL Lacs, Introduction Blazars comprise the most variable class of active galaxies and are usually classified as a subgroup of quasars. The name blazar arises from merging BL Lac and quasar the first blazar was observed in the constellation Lacerta. Blazars (BLs) are extremely luminous jets emanating from active galactic nuclei (AGN), their inferred luminosity isotropical equivalent can reach of order L BL L. The variability is one of their main characteristics, and in some cases, the jet has been seen to change its luminosity by a factor of 2-3 within a week. Furthermore, the spectral extent over which radiation intensity from these extreme objects is approximately constant in almost all cases a striking 15 orders of magnitude(!) see also fig. A.1. As sketched in figure 1, there are two classes of highly variable quasars or blazars separated in classification by their spectral characteristics. One class comprise the OVV Quasars or Optically Violent Variable Quasars have distinct (strong) line features, both broad line features (highly Figure 1: A classification of galaxies in terms of (from top-left to rightbottom); band spectral features, radio loudness, temporal variability, line features. NB: radio loudness is not unique to radio galaxies; but variability is not a characteristic of radio galaxies. Doppler shifted lines this may indicate both very hot thin gas and/or large turbulent gas velocities) and narrow line features (indicating - what?). The other class of highly variable quasars comprises the BL Lacertae dubbed so due to the first discovery of such an object being in the constellation Lacerta. The main feature distinguishing BL Lacs from other blazars (OVVs) is the lack of spectral lines; this can be seen for the near-u and V bands in figure 2 1. Further, BL Lacs possess high degrees of polarization (up to 40%). Figure 2: Typical spectra of the OVV and BL Lac AGN in UV & V bands BL Lac, subclassification From a study of 233 BL Lacertae objects [2] found that these samples can be categorized into two classes; LBL, consisting of low-frequency peaked BL Lacs, and a class, HBL, consisting of high-frequency peaked BL Lacs. In figure 4A can be seen a sketch of the spectral range of BL Lac objects the skecth is based on observations [2] but drawn in the figure as curves for clarity. It is likely, but unclear whether the transition from the LBL to the HBL BL Lac objects is continuous or if these are separate objects with different emission mechanisms. I have marked the spectral contributions in that figure with blue (synchrotron component) and red (Compton upscattering) - respectively [8]. 1 The remaining spectra for other classes of galaxies can be found in figure A.1. 1

3 BL Lacs, Model In what follows, we shall concentrate on the more extreme objects commonly classified as BL Lacertae objects, characterized by strong, polarized, featureless (no line emission/absorption) and highly variable radiation signature. The (unified) BL Jet Model In the archetypical model, Blazars are relativistic jets from AGN that are launched by accretion of matter onto super massive central black hole (BH) 2. When we observe these relativistic jets down the funnel at low declination angles we see a Blazar rather than the host AGN since the former is at least as bright as the latter on our line of sight. At the same time, the model also provides a prudent attempt at a unifying theory of AGN see cartoon, fig. 3), in which the blazars become special cases of the same observational phenomenon from AGN. If viewed down the jet inside its opening angle, the AGN is a Blazar (BL Lac with no lines OVV if it has lines). If viewed slightly from the side, the spectra, luminosity and temporal behavior changes characteristics and we have different object. Adding to the confusion, however, it has become clear (with reasonable substantiation, [11] and refs therein), that Gamma-Ray Bursts (GRBs), which seem to originate from the same basic physical process (BH accretion + relativistic jet launch, Synchrotron & Comptonization) are quite different in spectral behaviour. BLs are redder when brighter, while GRBs are bluer when brighter. Both BLs and GRBs are potentially valuable tools in constraining cosmological parameters (next chapter). Figure 3: A cartoon of a supposed unified model of AGN (upper/lower half with/without jet). Arrows indicate observational line of sight. Whether or not this picture holds true for all AGN classes remains to be determined however, for the case of blazars, this downthe-funnel model is generally accepted scenario. From observations, jets are inferred to travel from their host galactic center at relativistic velocities, meaning that the average bulk speed of the jet approaches the speed of light; typical bulk Lorentz factors approach Γ Or maybe from multiple BHs... 2

4 Although blazars are becoming intensively observed with increasing accuracy and sensitivity, three important questions about blazars are still largely unanswered to date: Redshift of BL Lacs Could the high-z BL Lacs cores even form in time for us to see them at z 5 6? Jet launching: How are blazar jets launched in the first place? This ties strongly with the problem of removal of angular momentum from the (alleged) accretion disc around the central massive BH; in order to go from the disc to the jet ejecta, a rapid dispersion of angular momentum is needed the process by which the AGN achieves this is unanswered. γ-ray production: How are the (TeV) γ-rays produced? In many cases the gamma-rays are dominating the luminosity (see fig. 4B), this is particularly true when the BL Lac flares. γ- rays are thus a fundamental component in our understanding of how the bulk of the energy is produced in blazar jets. This might be closely related to the question of particle acceleration and UHECRs. Spectral subclasses: The physical origin of the difference in spectral characteristics of BL Lacertae subclasses LBL and HBL is unknown. The sparse (but growing) observational evidence hints that two distinct classes exist, but it cannot be inferred from the data whether there is a continuous family of BL Lacs ranging between LBL and HBL (compare fig. 4A and fig. 4B). The data show that, generally, the HBLs are less luminous but peaked at higher energies whereas LBLs are more luminous but at lower photon energies. Further, as can be seen from fig. 4A, during flaring, a shift occurs to higher flux and higher photon energies over the entire spectrum. NB: The different viewing-angle (figure 3) scenario seems unable to explain this difference in the SED peaks. Magnetic fields: In order to account for the synchrotron self-compton (SSC) model of broadband emission in Blazar jets, we need to understand the interplay of the magnetic field and jet plasma. Recent polarization measurements by [6] indicate that the LBL and HBL (BL Lac) subclasses (see sect. ) have different B-field topologies. This in itself is a scientifically interesting topic, since it seems that there is an apparent difference between how the central engine (the AGN environment) works for the LBL and HBL cases [6]. The main problems in answering these questions arise from an inadequacy of theory to produce a consistent treatment. In particular, determining the radiation processes that produce the spectra from these jets is paramount to our understanding of the formation and prop- Figure 4: Left: General form of BL Lac spectra for LBL and HBL subclasses. Right: Nearest known BL Lac object, Markerian 421, in all energy bands (top insets) and broadband spectral fit. agation of these jets. Hence, we need also determine both the micro- and macro-physics if we are to add these powerful lighthouses to our toolbox for cosmological surveys, cosmology in general and maybe even as a tool in the search for dark matter. 3

5 Concluding remarks, unsolved issues Redshift of BL Lacs Concluding that a jet actually exists in BL Lacs (and other blazars) will be explained in part here, in part below. The biggest problem then becomes of course how that jet is launched? This is not as trivial a question as it may seem; even if we do find a good way to launch a jet from the Blazar, we can still not be sure that this way is physically allowed in this universe. The jet central engine might be too old compared with the time is has been able to grow measured from the Big Bang. Following [12] it can be estimated for sufficiently high redshift BL Lacs z 5, when the Universe was at most years old, that to achieve a luminosity of L ergs/s, a Black hole of M BH M is required to extract enough gravitational energy. For such a BH to grow from a BH of initial mass M BH 10M, with growth rate estimated at τ BH η 0.1 (L E /L)yr, about 16-efoldings are needed 3. Figure 5: From [11]. Left: Hubble diagram with SN Ia from the Legacy Survey (red points) and GRBs analyzed by Firmani et al. (blue points). Right: Constraints on Ω M and Ω Λ set by SN Ia (green contours), and SN + GRBs (red area). BL Lacs have been observed over the entire redshift range of both SN Ia and GRBs, and could potentially help narrowing constraints. Black cross gives Concordance Model demand on Ω M and Ω Λ This means that the AGN (central black hole) cannot be younger than τ SMBH 16τ BH 3 Here, η 0.1 is the accretion energy efficiency, and L E is Eddington luminosity we can take them to be η 0.1 1, (L E /L) 1 4

6 η 0.1 (L E /L) yrs. Thus, it is unlikely that a super-massive BH has had time to form. This leaves us with a few possible outcomes: 1. The highest-z observed BL Lacs, i.e. at z 5.4 or so, are not jets from AGN super-massive BHs. At least not as massive as estimates tell us. 2. Super-massive BHs can form faster than expected in the early universe and the highest-z BL Lacs are in fact from SMBHs in AGN. 3. The redshifts on the highest-z BL Lacs might have to be adjusted i.e. z true z inferred The number fit exactly such that we see the very first SMBH AGN in the Universe when observing these highest-z BL Lacs. Take your pick. Item number 3 could possibly be answered if the radiation processes in the relativistic jets was modified. We shall take a closer look at this below. Item number 1 is an interesting although not as likely. Item number 2 is interesting from a GR perspective as well as from a Cosmological one. Item number 4 is very interesting from the perspective that high-z BL Lac objects could potentially tell us about the very first galaxies. The advantage over fx GRBs is that BL Lacs can be observed continuously and over many years span, thus we do not fact the transient issue in GRBs. Both phenomena can become very important constrainers and telltales on our Cosmology, see for example figure 5 here, taken from [11]. Jet launching This subject is mainly connected with the angular momentum that needs to be dissipated from the accretion disc as matter falls toward the central BH. We shall not go into further detail, but estimating the initial angular momentum, J i in a Keplerian orbit of some M at distance R K 10kpc, and final J f for M BH M and R f 0.01pc 4, we get that the angular momentum must be reduced by a factor of J i /J f So somehow, almost all angular momentum must be dissipated by some (unknown) mechanism. More problems arise as the jet collimation and acceleration are questioned we shall not go into detail about this either. Magnetic fields This could be one of the most efficient methods of dissipating the angular momentum through a (non-local) viscous drag on the accretion disc. However, it is not known to what extent the magnetic fields play a role a most modelers have so far made it a hallmark to exclude magnetic fields from both calculations and simulations. This, even though we saw [6] that magnetic fields do play a role in the AGN environment. In fact, magnetic fields will also play a role in the radiation processes inasmuch as the spectra can be interpreted as having an embedded synchrotron signature. However, how the fields are generated in these jet scenaria is not known. There are basically two ways to facilitate moderateto-strong B-fields (and E-fields) in the jet: 4 Inferred from Eddington estimates + observed luminosities 5

7 Poynting dominated outflow explains the presence of a magnetic field as carried out from source along with the jet, this field can then be (curled up and) expelled along with the gas. This is mechanism is often dubbed the Blandford-Znajek model. Local field generation explains the presence of the electromagnetic fields by simple plasma physical mechanisms that must be present in the relativistic jet. The field is here generated naturally by the jet as an inevitable consequence of the phenomenon. Those two scenaria are to my knowledge not known mutually exclusive, it is remains to be theorized (or simulated), whether the relativistic jet can possess both. A very important feature that can help determine the jet structure and internal dynamics is the high degree of polarization the BL Lacs seem to have in common. As mentioned in model description, the difference in LBL and HBL classes spectra cannot be explained by a variation in viewing angle on the jet 5. In figure is shown the polarization statistics for a number of Blazars, only a few of those are BL Lacs. It is interesting to note that there is a difference between the HBLs and LBLs in the transverse and parallel polarization degree [6]. The HBLs seem to lean toward a more jet parallel field whereas the LBLs have a more jet perpendicular field. This could turn out to be a strong indicator of how the magnetic field is produced and therefore an important probe of the physics in the BL Lacs. Spectral subclasses The absence often complete of emission (or absorption) lines from BL Lacs is not understood at present. There are - again - several options: 1. The unified Blazar (and Seyfert) scenario in fig. 3 attempts to explain this by a steep angle (low column density) with the halo and dick surrounding the AGN. 2. Another way to interpret this is of course that the there is no radiation from anything except from a strongly pair loaded (γ + γ e + + e, see next section) plasma outflow and that we see only the radiation from the jet. 3. An explanation in close resemblance with the previous, is that there is simply no metals in or around the outflow. This would indicate very young systems, but this does not go well with the experience that shows BL Lacs to reside mainly in ellipticals (there is no way to win is there :o). 5 There are presently theory in development that might turn out to remove this inadequacy but that is far beyond the scope of this essay. However, I am be willing to this with whomever might be interested. 6

8 The most straightforward explanation seem here to be tied to point number 3. Since some BL Lacs have been observed to have very weak (but detectable) lines, it seems that a correlation between redshift and line strength should be able to probe this point. γ-ray production The final problem is strongly coupled to most of the previous problems. In particular, the presence of γ-rays should be ascribed to relativistic outflows in order for the very high energy photons (TeV range) to be observed. This argument is strengthened by the apparent super-luminous motion of jets from blazars. This superluminous motion can only be achieved if the ejecta is moving close to the speed of light creating a relativistic temporal contraction of events along the LOS see fx [13]. If pair production is important in the jet and launch volume, then seed photons in the early jet phase could be produced with energies in the X-ray band, since E(e + + e ) 511keV E X,typ {10 1, 10 4 }kev, in the plasma bulk rest frame. These photons can then Compton upscattered a few orders by electrons and reach a final energy approaching the TeV range. Still, the problem remains how to produce the observed spectra. It is conjectured that the radiating electrons must be powerlaw distributed, and it follows that the radiating particles must be accelerated to very high energies, far from the central engine. So we re left with much the same questions again from inadequacies in theory, before we can reliably interpret observations correctly and understand Blazars, in particular BL Lacs; to explain field generation, particle acceleration and radiative processes in high compactness, optically thin, hot plasmas. Once the above goal is achieved, can we hope to get a better picture of what goes on in these extreme BL Lac objects, and even maybe get a better idea about BH formation and cosmology constraints. 7

9 Bibliography [1] Author et al., journal [2] Padovani, P., & Giommi, P., MNRAS, 277, 1477 (1995) [3] Krawczynski et al., ApJ, 559, 187 (2001) [4] Jones et al., W. A., ApJ, 188, 353 (1974) [5] Aharonian et al., astro-ph/ , Accepted to A & A [6] Kharb P., astro-ph/ [7] Dondi, L., and G. Ghisellini,MNRAS, 273, 583 (1995) [8] Ghisellini, G., and Madau, P., MNRAS, 280, 67 (1996) [9] Cheung et al., ApJ, 650, Issue 2, (2006) [10] Author et al., astro-ph/ [11] Ghisellini, astro-ph/ [12] T. Padmanabhan, Theoretical Astrophysics, Vol.III; Galaxies and Cosmology, Cambridge Univ. Press (2002) [13] L. S. Sparke & J. S. Gallagher, Galaxies in the universe, Cambridge Univ. Press (2000) 8

10 Appendix A Appendix Figure A.1: The typical spectra, in the waveband λ {3000Å,6000Å}, for all major classes of galaxies. The Blazars are in the upper left two spectra. The OVVs are the main contributor to the mean blazar spectrum below the BL Lac spectrum. The BL Lac is completely featureless (flat spectrum no(!) lines). 9