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. 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)

4. AGN classifiction Radio-quiet AGN classification Seyfert I galaxies/ radio-quiet quasars Seyfert II galaxies Narrow line Seyfert I galaxies (NLS1) Narrow Emission Line galaxies (NELGs, LINERS)

5. AGN classifiction Radio-Loud AGN classification FR-I (Fanaroff-Riley-I) radio galaxies FR-II radio galaxies Radio-loud quasars Blazars

6. 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 • In the standard model, the accretion disk is geometrically thin and radiatively efficient • In some instances, there may be advection dominated disks that radiatively inefficient

7. The supermassive black hole model • Data supporting the existence of supermassive black holes • The width of the broad optical/UV emission lines • The stellar kinematics in the central regions of many galaxies • The large amplitude variability of X-ray emission • The efficiency in which matter is converted into energy • Balance between accretion flow and radiation pressure • Masers • The X-ray fluorescent Kα emission line

8. 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

9. 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

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

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

14. History of X-ray Observations of AGN • The first successful X-ray observations of AGN were performed in the 1960s with rocket and balloon experiments. They detected emission from the bright QSO 3C273 • The third Uhuru catalog (Giacconi et al 1974) identified X-ray emission from the 3 brightest AGN in the sky: 3C273, NGC 4151, Cen-A • The Ariel-V sky survey established the ubiquity of X-ray emission from Seyfert I galaxies • The first X-ray spectral result from OSO-7 and Uhuru were published in the mid 1970s

15. History of X-ray Observations of AGN • The first measurements of X-ray variability came from OSO-7 and Copernicus in the mid 1970s • Ariel-V showed that the X-ray variability on timescales greater than one day was a common property of AGN • Improved X-ray spectra of NGC 4151 and Cen-A from OSO-8 and Ariel-V established the power-law shape of the X-ray continuum, measured a significant low energy roll-over due to photoelectric absorption (NH>1022 atoms/cm2) and detected the 6.4 keV Fe line in emission in both objects

16. History of X-ray Observations of AGN • The first large spectral samples of AGN, obtained with the HEAO-1 satellite (early 1980’s), were well modeled by a power law in the 2-20 keV band, with little intrinsic absorption, and the observed range of photon spectral indexes were narrow and centered on Γ~1.7

17. History of X-ray Observations of AGN • The HEAO-1 mission also performed the first extended and sensitive search for variability, finding very few objects that exhibited rapid (shorter than a few hours) large amplitude (a factor of 2) changes

18. History of X-ray Observations of AGN • The Einstein observatory (HEAO-2) increased the known number of X-ray AGN by more than an order of magnitude • The imaging proportional counter showed that AGN of all types were powerful X-ray sources detecting ~1000 • The solid state spectrometer gave the first higher quality spectra of AGN and confirmed the narrow distribution of spectral slopes around Γ~1.7 in the 0.7-4 keV band • However, some of the lower luminosity Seyferts were not well described by a single power law with absorption by cold material

19. History of X-ray Observations of AGN • Einstein low resolution spectra with its imager (IPC) showed that at lower energies 0.2-4 keV the spectral slope presented higher variations and was not so uniform as at higher enegies 2-20 keV Wilkes & Elvis 1987

20. History of X-ray Observations of AGN • EXOSAT, launched in 1983, provided the first detailed temporal studies of large samples of Seyfert galaxies. The data showed that variations were stochastic, with no characteristic time scale • There was a large amplitude variability in Seyfert galaxies (in contrast with the HEAO-1 results) • The broad energy coverage resulted in the discovery that a sharp rise at low energy, the so-called soft excess, occurs in the spectra of ~50% of Seyfert galaxies

21. Arnaud et al 1985

22. History of X-ray Observations of AGN • EXOSAT, launched in 1983, provided the first detailed temporal studies of large samples of Seyfert galaxies. The data showed that variations were stochastic, with no characteristic time scale • There was a large amplitude variability in Seyfert galaxies (in contrast with the HEAO-1 results) • The broad energy coverage resulted in the discovery that a sharp rise at low energy, the so-called soft excess, occurs in the spectra of ~50% of Seyfert galaxies • The power-law index distribution in the energy range 2-20 keV is found uniform as previously found

23. Turner & Pounds 1989

24. History of X-ray Observations of AGN • Ginga, launched in 1987, data allowed detailed spectral decomposition to be performed for the first time, showing that a significant fraction of all Seyfert galaxies exhibit Fe 6.4 keV lines and at a level that is much more intense than can be explained by the general small columns of absorbing material in the line of sight Pounds et al 89

25. History of X-ray Observations of AGN • Ginga data also showed a spectral flattening at E>8 keV which could be most readily attributed to the reprocessing of a (considerable) fraction of the incident X-ray flux by cold material near the central engine, either totally or partially out of the line of sight Lightman & White 88

26. History of X-ray Observations of AGN • Ginga obtained the first crude spectra of a significant sample of quasars in the 2-20 keV band (Williams et al 1992)

27. History of X-ray Observations of AGN • ROSAT, launched in 1990 and with good soft energy sensitivity carried out the ROSAT All Sky Survey in which tens of thousands of AGN were detected. At low fluxes the fraction of NELG increses compared to type I AGN • ROSAT provide detailed studies of the soft X-ray excess and made a clear connection of this excess to the width of the permitted lines (NLS1)

28. History of X-ray Observations of AGN • ASCA with its superior spectral resolution resolved the Fe K line into components. In particular it showed a broad component in SyI’s which was a dynamical signature of the innermost, relativistic accretion disk around the supermassive black hole • ASCA measured narrower and stronger Fe K line from Sy II’s as expected from the unified model predictions

29. Nandra et al 1996

30. History of X-ray Observations of AGN • ASCA also provided spectral evidence for intrinsic “warm” absorption Otani et al 96

31. History of X-ray Observations of AGN • RXTE, launched in 1995, has provided the best temporal resolution of both the line and continuum variability • The Fe K line variability is not universal and does not correlate with the continuum • BeppoSAX, launched in 1996, have the best hard energy response and have been fundamental in the study of blazars and the hard X-ray-soft gamma ray connection • Chandra & XMM

32. History of X-ray Observations of AGN • Chandra & XMM have brought the best spatial and spectral resolution and collecting area • They have acquired the deepest X-ray images with which the X-ray background is resolved • They are also providing detailed data to constrain the AGN structure

33. X-ray Spectra of AGN Overview • To first order the 2-50 keV spectrum of the great majority of Seyfert I galaxies can be characterized by a simple power law of the form F(E) = A E-Γ ph/cm2/s/keV • The X-ray spectra are continuum dominated • The strongest broad spectral feature in the range 0.1-100 keV is low energy absorption due to photoelectric absorption of cold or partly ionized material in the line of sight to the nucleus

34. X-ray Spectra of AGN Overview • Fe is the most abundant heavy element and its large absorption cross section at high energies, combine with a high fluorescent yield, make that the strongest spectral features at E>6 keV are due to Fe. These include the Fe fluorescent emission at 6.4 keV (the only strong spectral line from near-neutral (or cold) material), He-like and H-like lines at 6.7 and 6.97 keV from ionized material

36. X-ray Spectra of AGN Overview • In the 0.3-3 keV band, the spectral features are due to K shell transitions of O, Mg, Si, and S, and L shell transition of Fe • However, since these elements have low fluorescent yields, the strongest spectral features at low energy from cold material are due to absorption. Highly ionized material have both emission and absorption features across the entire X-ray spectral band. The strongest features in both emission and absorption at E<2 keV are due to O K and Fe L shell transitions

37. X-ray Spectra of AGN Seyfert I Continuum • Simple fits to power-law spectra over the 2-20 keV band give photon number indices that are narrowly distributed around Γ~1.7 with a 1 σ width of 0.13 which is represent a true dispersion in spectral slopes around the mean. This spectral slope appears to be an average of a intrinsic steeper spectrum Γ~2.0 and an additional flatter component which is produced as a result of reprocessing in material close to the central source

38. X-ray Spectra of AGN Seyfert I Continuum • The spectrum of the X-ray background provides an integral constraint on the total flux from AGN. Since AGN make most of the XRB, the spectrum of the average AGN must steepen at energies above 60 keV in the observer’s frame in order not to exceed the XRB

39. X-ray Spectra of AGN Seyfert I Fe K emission lines • The 6.4 keV Fe emission line due to fluorescence of cold material is present on most Seyfert I galaxies • The distribution of EW is quite broad 50<EW<350 eV • There is no apparent correlation between the line strength and the line-of-sight column density • The line at 6.4 keV comes from fluorescence from cold material with low velocities • There is no sign of a 6.7 keV thermal Fe line

40. X-ray Spectra of AGN Seyfert I Fe K emission lines • BLRG show also Fe K lines

41. X-ray Spectra of AGN Seyfert I Features at high energies • Apart from the 6.4 Fe line there is a decrement between 7-8 keV and a flattening of the spectrum at higher energies • The mean flattening between 2-10 keV and 10-18 keV is ΔΓ~0.5

42. X-ray Spectra of AGN Seyfert I Interpretation • The Fe K line and “hard tail” features are most probably due to the reprocessing (or reflection) of primary X-rays in optically thick material subtending a substantial solid angle to the X-ray source, possibly the accretion disk • The reflection albedo is energy dependent, at E<2 keV, there are many elements (C,N,O) that will photoelectrically absorb any incident flux, so few photons are reflected

43. X-ray Spectra of AGN Seyfert I Interpretation • However, the albedo increases at higher enegies due to the decreasing abundance of the higher Z elements required for photoelectric absorption • Fe is the last abundant element that can significantly affect the probability of reflection, leading to a marked “pseudo-absorption feature” in the reflected flux from 7.1-9 keV, together with the associated Fe Kα fluorencent line with a predicted equivalent width of ~150 keV

44. X-ray Spectra of AGN Seyfert I Interpretation • At higher energies, Compton down-scattering and the reduction of the scattering cross-section deplete the number of photons reflected, resulting in a broad band spectral bump, peaking at 20-30 keV

45. X-ray Spectra of AGN Seyfert I Interpretation • This Compton reflection model introduces four new free parameters into a spectral fit: the fraction of all photons that are reflected by the scatterer (essentially the solid angle covered by the scatterer), the inclination of the scatterer to the line of sight, the element abundance, and the ionization state of the material

46. X-ray Spectra of AGN Seyfert I Interpretation • Observationally, the covering fractions of the material are large and there is an intrinsic slope of Γ~1.9 which translates into an observed Γ~1.7 when the Compton reflection component is taken into account. The distribution is narrow implying a strong coupling between the spectral index and the fraction of solid angle occupied by the reflector. This could be a selection effect where most of SyI are observed with small inclination angles, consistent with the unified models

47. X-ray Spectra of AGN Seyfert I Interpretation • The Fe K line can provide information about these selection effects. If the material is in the form of an accretion disk, Doppler and gravitational broadening will combine to produce a characteristic skewed line profile, where the detailed line shape depends on the inclination and the emissivity law of the disk • Typically such a line will be broad, with a Gaussian width of at least several hundred eV

48. Fabian et al 1989

49. X-ray Spectra of AGN Seyfert I Interpretation • In principle measurements of the Fe K line profile can determine the distance of the line emitting region from the central object, in units of the Schwartzschild radius, and thus “prove” the existence of a compact massive object • Similarly, an absorption edge will be distorted by these effects

50. X-ray Spectra of AGN Seyfert I Interpretation • An earlier competing explanation to a reflection disk invoked partial covering of the X-ray source by cold, dense material. Only a fraction of the photons seen by the observer are absorbed by quite thick material while the rest is little affected by the absorption.