High Energy Astrophysics

1. High Energy Astrophysics Active Galactic Nuclei

2. Introduction • Active Galactic Nuclei (AGN) are powerful sources of radiation which exist in the centre of 1-10% of all galaxies • Galaxies which host an AGN are known as active galaxies • The span of observed AGN luminosities is huge L~1040 – 1047 erg/s • The more luminous AGN outshine their host galaxies by factors of 1000 or more • AGN are the most luminous long-lived objects in the universe

3. Introduction: Historical discovery • Fath 1908 at Lick took a spectrum of NGC1068 and found emission lines • Slipher 1917 • Hubble 1926 study more systems • Seyfert 1941,1943: galaxies whose nuclei spectra show high-ionization emission lines. Broad emission lines arising from small, bright nucleus and covering a wide range of ionization => Seyfert galaxies

4. Introduction: Historical discovery • After II World War with the development of radioastronomy, radiogalaxies were discovered • Quasars were discovered in 1963 when Maarten Schmidt interpreted the optical emission lines of the known radio source 2C273 as redshifted hydrogen Balmer lines.

5. AGN Zoology • Radio galaxies • Radio Quasars • BL Lac Objects • Optically Violent Variables (OVV’s) • Radio Quiet Quasars (QSOs) • Seyfert I galaxies (SyI) • Seyfert II galaxies (SyII) • Low Ionization Nuclear Emission-Line Regions (LINERS)

6. AGN classification • There is a large diversity in the nature of the observed radiation from different AGN • The classification scheme has varied with time • A fundamental division in the classification scheme is based on the radio properties of AGN: radio-loud or radio-quiet

7. AGN classifiction Radio-loud and radio-quiet AGN • Radio –loud AGN emit collimated jets of plasma which feed energy and high-energy particles into an extended halo or lobe. In extreme cases the halo can be in excess of 1Mpc in extent • Both the jets and radio halo are observed to be sources of continuum radio emission, presumably due to synchrotron emission from relativistic electrons in the jet/halo plasma

8. AGN classifiction Radio-loud and radio-quiet AGN • There are many galaxies where radio imaging reveals well-collimated jets linking the AGN with the halo • Outflowing blobs have been observed within the jets. Sometimes, the apparent proper motion of these blobs exceeds the speed of light (superluminal motion). This probably results from relativistic bulk motion in a direction that is somewhat towards the observer • Some radio-loud AGN present only one observable jet

9. AGN classifiction Radio-loud and radio-quiet AGN • Inferred relativistic jet motion are in the range of Lorentz factors of 3-10 • The physical nature of these jets, along with the mechanisms responsible for their acceleration and collimation are very poorly understood

10. AGN classifiction Radio-loud and radio-quiet AGN • Radio-quiet AGN do not show large scale collimated jets • However, bipolar radio emission, typically with and extent less than 0.5 kpc, is often seen and attributed to an outflow from the AGN

11. AGN classifiction Radio-quiet AGN classification • Within the class of radio-quiet AGN classification is performed primarily on the basis of optical/UV spectral properties Seyfert I galaxies/ radio-quiet quasars • Spectrum is complex and multicomponent • Called broad-line AGN due to the presence of broad (FWHM~2000-20000 km/s) permitted optical/UV emission lines

12. AGN classifiction Seyfert I galaxies/ radio-quiet quasars • Both low-ionization broad lines (MgIIλ2798 and hydrogen lines) and high-ionization broad lines (CIVλ1549, OVIλ1035 and NeVIIIλ774) are common • The region from which these emission line arise is known as broad line region (BLR) • The optical/UV emission lines arise from relatively dense (ne ~ 109 cm-3) photoionized clouds with a small volume filling factor

13. AGN classifiction Seyfert I galaxies/ radio-quiet quasars • The presence of low-ionization lines, coupled with photoionization modeling, suggests that at least some of the material responsible for the lines is optically thick to Lyman continuum radiation • The line widths are due to cloud motions • Collissional de-excitation in this relatively high density plasma dominates over forbidden line emission • The optical/UV spectra also present narrow (~500 km/s) permitted and forbidden emission lines

14. AGN classifiction Seyfert I galaxies/ radio-quiet quasars • Commonly observed narrow lines are the hydrogen lines, [OII]λ3727, [OIII]λ5007, [NII]λ6583, [SII]λ6716/6731. • the material responsible for these lines, which resides in the narrow line region (NLR), is more teneous than that in the BLR and lies at greater distances from the putative black hole • The NLR can be spatially resolved in nearby AGN and is found to have typical sizes of 0.1 kpc

15. AGN classifiction Seyfert I galaxies/ radio-quiet quasars • The distinction between a Seyfert I nucleus and a radio-quiet quasar (RQQ or quasi-stellar object QSO) was traditionally made on the basis of whether the galaxy or AGN was discovered first • SyI nuclei are generally lower-luminosity than radio-quiet quasars: the dividing luminosity is approx Lbol ~ 1045 erg/s LX ~ 1042 erg/s • In practice, SyI nuclei and radio-quiet quasars seem to form a continous sequence in luminosity

16. AGN classifiction Seyfert I galaxies/ radio-quiet quasars • They display strong continuum radiation from IR through gamma-ray wavelengths • The higher-energy emission is often observed to vary rapidly

17. AGN classifiction Seyfert II galaxies • The spectra of Seyfert II galaxies display narrow permitted and forbidden emission lines • They differ from SyI galaxies because no broad lines are observable and the optical/UV radiation is much weaker • The fact that broad optical/UV lines can often be observed at sufficiently long wavelengths suggests that at least some SyII nuclei are simply reddened SyI nuclei and the BLR would have to lie behind the reddening material • X-ray observations reveal photoelectric absorption from cold gas presumably associated with the dust

18. AGN classifiction Narrow line Seyfert I galaxies (NLS1) • They differ from Seyfert I in that their permitted optical/UV lines are significantly narrower than SyI (but still broader that the lines from the NLR) • The ratio of [OIII]λ5007/Hβ < 3, a criterion that seems to differentiate SyI from SyII • They display stronger than typical optical line complexes due to FeII emission • X-ray continuum is softer and more variable than normal SyI

19. AGN classifiction Narrow Emission Line galaxies (NELGs) • They show narrow (FWHM<1000 km/s) emission lines, typically of Hα, Hβ, [OIII]λ5007 • X-ray luminosities Lx ~ 1042 erg/s (two orders of magnitude brighter than normal galaxies) • NELGs are often classified as AGN but part of the emission may be due to star formation • The number of sources and their hard spectra make them good candidates for producing the X-ray background

20. AGN classifiction Radio-Loud AGN classification • Within the class of radio-loud AGN, classification is performed on the basis of both radio morphology and optical/UV spectral properties FR-I (Fanaroff-Riley-I) radio galaxies • Radio images of low luminosity (Lradio < 1042 erg/s) radio-loud AGN usually reveal radio surface brightness profiles that fall continuously as one proceeds from the nucleus to the edge of the radio lobe (edge-darkened morphology) • They display narrow emission lines from a NLR, but normally not broad emission lines

21. AGN classifiction FR-II radio galaxies • Powerful radio-loud AGN (Lradio > 1042 erg/s) display different radio morphology. Apart from the actual nucleus itself, the radio surface brightness can be low in the central regions of the source and dramatically brighten towards the edge of the halo as the jet(s) are seen to terminate at strong shocks (edge-brightened morphology) • The optical properties can be either SyI-like or SyII-like (known as broad line radio galaxies BLRG and narrow line radio galaxies NLRG, respectively)

22. AGN classifiction Radio-loud quasars • Distinction between BLRG and radio-loud quasars (RLQ or quasars) is blurred • A BLRG type object in which the continuum emission of the AGN dominates over the radio emission is known as a RLQ • RLQ often posses one-sided jets which show superluminal motion

23. AGN classifiction Blazars • Radio-loud objects displaying very strong and variable continuum at all wavelengths • the emission lines are either very weak or absent from the optical/UV spectrum • The presence or absence of lines leads to the subclassification: optically-violent variable (OVV) or BL Lac object

24. The supermassive black hole model • The current paradigm is that the fundamental power source of all AGN is accretion onto a supermassive black hole • Some theoretical models predict that there is a massive black hole at the center of most galaxies due to processes probably related to galaxy formation • At some level the interstellar medium (ISM) of the host galaxy must accrete onto a central black hole • Non-axisymmmetric potentials resulting from either galactic bar formation or a nearby companion galaxy can further increase the mass flow rate into the central regions of the galaxy

25. The supermassive black hole model • Together with any stellar mass loss associated with a nuclear star cluster, significant quantities of gas can be deposited in the central parts of a galaxy and enter the region where the black hole mass dominates the gravitational potential • Gas inevitably posses a non-zero angular momentum when it enters the region dominated by the potential of the black hole. As a result, the inflowing gas will form a flattened, rotationally supported structure, an accretion disk

26. The supermassive black hole model • Accretion disks are the engine via which some fraction of the gravitational potential energy of the infalling material is transformed into observable radiation and kinetic energy of jets/outflows • Accretion disks posses some form of “viscosity” in order to transport angular momentum outwards and allow matter to flow inwards, thereby liberating energy • It is probably a self-sustaining magnetic dynamo which probably provides the required angular momentum transport

27. The supermassive black hole model • In the standard model, the accretion disk is geometrically thin and radiatively efficient

28. The supermassive black hole model • Data supporting the existence of supermassive black holes • The width of the broad optical/UV emission lines is interpreted as being due to material motions under the influence of only gravity, then a deep gravitational potential is needed • The velocity dispersions of stars in the central regions of many galaxies appear to increase as one proceeds inwards to the core of the galaxy. These observations imply a mass concentration at the galactic nucleus of the sort expected if a black hole were present

29. The supermassive black hole model • Data supporting the existence of supermassive black holes • The large amplitude variability of X-ray emission implies that, at any one time, the emission is dominated by a very small number of sources • The efficiency in which matter is converted into energy exceeds that possible from any thermonuclear process. The only process (apart from particle anihiliation) this efficient is accretion of matter onto some compact object with R < 100 Rsch

30. The supermassive black hole model • Data supporting the existence of supermassive black holes • In order that the accretion flow does not halt due to the pressure of the outgoing radiation, the accreting object must posses a minimum mass of M ~ 106 Msolar • The only object known with this mass 106 Msolar in that radius 100 Rsch is a black hole

31. The supermassive black hole model • Data supporting the existence of supermassive black holes • Imaging and spectroscopy from HST reveal rotating disks indicating large masses in small radii therefore arguing in favor of black holes

32. The supermassive black hole model • Data supporting the existence of supermassive black holes • Many AGN produce H2O and OH masers. The intrinsically narrow width of the maser emission line allows accurate velocities to be determined from a measurement of the line frequency. High spatial VLBI observations map the velocity field in very small radius • The M101 maser finds almost perfect Keplerian motion demonstrating that, at these scales, 0.5 pc from the source, the potential is dominated by a point-like mass with M~107 Msolar

33. The supermassive black hole model • Data supporting the existence of supermassive black holes • The X-ray fluorescent Kα emisison line is thought to originate from the innermost regions of the accretion disk and thus probes a regime where orbital velocities are mildly-relativistic and gravitational redshifts are becoming large • The profile of the iron line is in agreement with that expected from material in a thin accretion disk near a black hole

34. AGN unification • The central feature of the unification schemes is that the observed properties, and thus the classification, of a given AGN depend upon its orientation • The main ingredients of this scheme are: • A supermassive black hole 106-10 Msolar • An accretion disk and corona, heated by magnetic and/or viscous processes so that it radiates at optical through soft X-ray energies • high velocity gas, often referred as the BLR • lower velocity gas in the NLR

35. AGN unification • The main ingredients of this scheme are: • an obscuring torus (or other geometrical form) of gas and dust, hiding the BLR from some directions • a relativistic jet, formed within ~ 100 Rsch of the black hole, and extending outwards for tens of kpc, and in some cases as much as a Mpc

36. AGN unification Radio-Quiet AGN unification • The strongest case for unification can be made for SyI and SyII galaxies • Some SyII appear to be SyI apart from the fact that the BLR and continuum radiation are heavily reddened • The prototypical SyII galaxy NGC1068 shows broad optical emission lines when observed in polarized light. Interpreting polarization due to scattering, this shows that there are directions from which NGC1068 displays a SyI spectrum

37. AGN unification Radio-Quiet AGN unification • Antonucci & Miller 1985 proposed that all Seyfert galaxies posses a dusty torus of gas at distances intermediate between the BLR and NLR. And observer whose line of sight to the black hole intercepts this torus would see a heavily reddened (or completely extinguished) BLR and central continuum radiation but an unreddened NLR. This would be identified as a SyII galaxy. If the line-of-sight does not intercept the torus, the central regions can be observed leading to a SyI classification

38. AGN unification Radio-Quiet AGN unification • Powerful radio-quiet quasars may be more luminous versions of the same basic central engine, probably due to a higher accretion rate • It is unclear whether most radio-quiet quasars have dusty tori as not many type-II quasars have been found • NLS1 have unusual properties hard to accommodate in this scheme

39. AGN unification Radio-Quiet AGN unification • NELGs appear to be simply low-luminosity obscured Seyfert nuclei • However, there may be intrinsically narrow-line object (without BLR) amongst this population. These objects may have an advection-dominated accretion disks rather than the standard thin disks. Within this model, the low luminosity of these objects results from the small radiative efficiency of the accretion disk (most of the energy is advection through the event horizon). The resulting lack of a strong photoionizing continuum could explain the lack of BLR

41. AGN unification Radio-Loud AGN unification • Blazars, radio-loud quasars and FR-II galaxies have unified under a similar scheme to the Seyfert one. The only difference is the inclusion of relativistic jets of synchrotron emitting plasma. The relativistic beaming and Doppler shifts associated with this jet are a source of further anisotropy in addition to the obscuration of the torus

42. AGN unification Radio-Loud AGN unification • Blazars are objects in which our line-of-sight lies within the jet cone of the source. The strong relativistic aberration and Doppler shifting of the jet emission produces the highly variable and continuum dominated emission • RLQ are objects in which the line-of-sight is close to, but not within, the jet cone. The continuum is strongly beamed but not as strong as to swamp the emission from the BLR

43. AGN unification Radio-Loud AGN unification • In objects with larger orientation angles, the central continuum flux falls resulting in an FR-II BLRG • For yet larger angles, the dusty torus obscures the central continuum source and BLR, resulting in an FR-II NLRG • It is unclear how FR-I radio galaxies fit into this scheme. Clearly some mechanism produces different radio morphologies for FR-I and FR-II and this mechanism is related to source power

44. AGN unification Radio-Loud AGN unification • An FR-I unification model as above is not straight forward. Some blazars can be beamed FR-I, but there is a lack of BL FR-I. This suggests either a torus with a small opening angle or the absence of the BLR in FR-I sources

46. broad lines blazars, Type 1 Sy/QSO The AGN Unified Model Urry & Padovani, 1995

47. Type 1 AGN SED X-rays mm far-IR near-IR Optical-UV Manners, 2002

48. radio galaxies, Type 2 Sy/QSO narrow lines The AGN Unified Model Urry & Padovani, 1995

49. Type 2 AGN SED X-rays Radio far-IR optical-UV Norman et al, 2002

50. Type 1 AGN SED X-rays mm far-IR near-IR Optical-UV Manners, 2002