Key issues for hard X-ray emission from accreting black - hole and neutron-star sources

1. Key issues for hard X-ray emission from accretingblack-hole and neutron-star sources Andrzej A. Zdziarski Centrum Astronomiczne im. M. Kopernika Warszawa, Poland

2. Content: • Radiative processes. • Hard and soft states of black-hole binaries. • Active galactic nuclei, including blazars. • Neutron-star LMXBs.

3. persistent, wind accretion Cygnus X-1: a high-mass X-ray binary (black hole/supergiant)

4. GX 339–4 – a low-mass X-ray binary transient; Roche lobe overflow

5. Main spectral states of accreting black holes unabsorbed spectra Probable detection by Zdziarski & Gierliński 2004

6. The geometry inferred for the hard state: outflow/jet emitting radio/IR/... scattered hard photons variable inner radius direct soft photons reflected photons gravity + Coulomb cold outerdisk hot inner disk blackhole thermal plasma with kTe  50–150 keV in hot accretion solutions

7. The geometry inferred for the soft state: inherently nonthermal scattered hard photons active region soft seed photons reflected photons gravity blackhole cold accretion disk

8. Main relevant radiative processes: • Compton upscattering of low-energy photons by high- • energy electrons; • Compton dowscattering of high-energy photons by low- • energy electrons; • synchrotron emission and absorption; • free-free emission and absorption; • electron-positron pair annihilation. Blackbody and Wien equilibria. Electron distribution can be either thermal (Maxwellian), or hybrid, with a Maxwellian followed by a high-energy tail. Absorption of photons in bound-free transitions; sometimes followed by fluorescence. Modification of photon spectra by relativistic effects close to a compact object.

9. cutoff: E>kTe Comptonized spectrum Lhard Energy gain Thermal Comptonization Seed photons log E F(E) Lsoft log E The photon index, , is a function of kTe and  (the Thomson optical depth). The parameters found in black hole binaries: kTe 50—100 keV,   1.

10. Polarization from Comptonization in a hot disk: Viironen & Poutanen 2004

11. input spectrum: bremsstrahlung + annihilation Wien peak Pair annihilation and Comptonization self-absorption Comptonized spectrum kT = 128 keV T = 8 (pair dominated) Thermal plasma Zdziarski 1984 • pair equilibrium: pair annihilation balanced by pair production:    e e (usually dominant) • e  ee e  p  p e e e e eee e e p  epe e Note that no pair annihilation feature can be seen from thermal plasmas in pair equilibrium. It is masked by Compton scattering into a Wien peak.

12. Nonthermal plasmas (with electron acceleration): pair annihilation may be visible: Lightman & Zdziarski 1987

13. log E F(E) Fluorescent Fe K line Klein-Nishina cutoff reflected photon incident photon Cold matter log E Compton reflection of a power-law incident spectrum from a medium with cosmic composition, e.g., an accretion disk: <bound-free>=T at E 10 keV bound-free absorption 6.4 keV 30 keV 200 keV the integrated albedo: ~20% 7.1 keV The relative reflection strength usually expressed as /2. Note that the albedo is  1. The remaining energy is re-emitted as a (modified) blackbody spectrum with the temperature of the reflecting medium.

14. Cyg X-1 states: Zdziarski & Gierliński 2004 spectra Cyg X-1 power Cyg X-1 hard/low intermediate soft 1996 soft/high 2002

15. Hard (‘low’) states

16. Cyg X-1: typical hard state spectrum Lhot Ldisk 1 thermal Compton soft excess reflection+Fe K disk blackbody E [keV] kTe  100 keV,   1, /2  0.3, L1-2% of LE

17. The universal high-energy cutoff in the hard state of black-hole binaries: Cyg X-1 GX 339–4 thermal Comptonization, kTe~50–100 keV

18. The electron temperature in the presence of Coulomb energy transfer from protons and Compton losses: 2/5 ln me mp kTe mec2  kTe (50–100) keV  (0.1–0.2) (Beloborodov 2001) • Additional thermostatic constraints: • Transition to the relativistic regime of Compton losses. • The onset of electron-positron pair production.

19. A weak high energy photon tail in the hard state: a possible electron tail beyond a Maxwellian NGC 4151 Johnson et al. (1997) McConnell et al. (2002)

20. Consequences of an electron tail for thermal synchrotron emission: Hybrid Compton hybrid synchrotron thermal synchrotron  is the relative energy content in the nonthermal electrons. The presence of nonthermal electrons can greatly amplify the thermal synchrotron emissivity even for a weak corresponding photon tail: Wardziński & Zdziarski 2001 As the hybrid synchrotron photons serve as seeds for thermal Comptonization, a measured photon tail yields an upper limit on B; e.g. the tail measured in Cyg X-1 implies sub-equipartition magnetic field in the hot plasma.

21. The nonthermal synchrotron model The theoretically predicted cutoff from synchrotron emission of power law electrons with an exponential cutoff is notsharp enough compared to the data. Zdziarski et al. 2003 While the hot inner flow may be identical to the base of the jet, the main radiative process in that region is thermal Comptonization, not nonthermal synchrotron.

22. An explanation of the radioX-ray correlation in black-hole binaries: A tight correlation between radio and the bolometric luminosity, L, due to connections between the accretion rate and the accretion luminosity, the accretion rate and the jet power, and the jet power and the radio flux (e.g. Heinz & Sunyaev 2003; Merloni et al. 2003). GX 339–4 Zdziarski et al. 2004

23. Soft (‘high’) states

24. Cyg X-1: a soft-state spectrum Lhot Ldisk 1 disk blackbody L0.05LE thermal Compton nonthermal Compton reflection+Fe K E [keV] A single electron population consisting of a Maxwellian and a high-energy (power-law like) tail.

25. The very high/intermediate state: Lhot/Ldisk~1 hybrid Compton disk blackbody reflection+Fe K Nonthermal nature: no high-energy cutoff up to at least 1 MeV. The very high state has a higher amplitude of the tail,Lhot/Ldisk, indicatingstronger coronal activity. Gierliński & Done 2003

26. XTE J1550-564 Ultrasoft states:What is the origin of the flat (2) power law seen in the disk-dominated states? GRS 1915+105 Cyg X-3 GX 339-4 Zdziarski & Gierliński 2004

27. A high-L case: GRS 1915+105 hybrid Comptonization disk very high state reflection No hard/low state because of its high luminosity Comptonized disk blackbody disk A possible pair annihilation feature soft/high state Zdziarski et al. 2001, 2005

28. OSSE spectra of GRS 1915+105

29. XTE J1550–564 – a transient low-mass X-ray binary Cygnus X-3 Neutron star or a black hole? Cygnus X-3 XTE J1550–564

30. State transitions and flare events

31. GX 339–4 hard-to-soft soft-to-hard Hysteresis in LMXBs 3–12 keV index RXTE/ASM Hard-to-soft transitions can occur at much higher L than soft-to-hard state ones. Lhard up to 30%LEdd. Lsoft down to 1% LEdd. inner disk radius GM/c2 Two accretion solutions in the same range of L. The history determines the actual state. Eddington ratio PCA/HEXTE ASM model Unknown shape of hard X-ray spectra during rise and decline. Zdziarski et al. 2004

32. Accretion solutions LE hot solution A limit cycle in the presence of a variable accretion rate, first increasing and then decreasing. cold solution evaporation the part of the accretion rate that is radiated

33. Flares from Cyg X-1 hard state hard state soft state Gierliński & Zdziarski 2003

34. the spectrum at the peak of the flare with a possible model Flare-related spectra unabsorbed 1996 soft state 16 s before the flare average spectrum from the observation soft state the spectrum at the peak of the flare with a possible model Similarity of the peak spectra, the spectrum before the flare average hard state spectrum unabsorbed L0.3LEdd at the peak hard state

35. Active galactic nuclei

37. NGC 4151 – spectra highest lowest Zdziarski et al. 2002

38. Average spectra from OSSE: Ratio of the high-energy spectra of GX 339–4 and NGC 4151: hard tails? NGC 4151 Sy 1 v. similar shapes Sy 2 Zdziarski et al. (1998, 2001)

39. Consequences of high-energy tails of AGNs for the cosmic X-ray background Zdziarski 1996

40. Narrow-Line Seyfert 1s The best-fit spectrum of 1H 0707-495 blackbody A striking similarity to the soft states of black-hole binaries, noted by Pounds et al. (1995) Comptonization Hard X-ray spectra not measured as yet.

42. Some model spectra: Sikora et al. 2002

43. POLARIMETRY: TheL of theX+ component usually exceeds the L of the IR+O component , up to a factor >10. Both are from the jet. IR+Oare produced by synchrotron mechanism, X+ most likely by Comptonization of external radiation (broad emission lines, IR of hot dust). But X+ can also be duetothe SSC and synchrotron radiation of e+e pair cascades triggered by hadronic processes. Both synchrotron and/or SSC radiation is predicted to be significantly polarized (like synchrotron optical radiation), while polarization of the external Compton (ERC) should be  0.Hence X-ray polarimetry can test the origin of the X+ component.

44. Extremely hard X-ray spectra: Tavecchio et al. 2002

45. Hard X-ray spectra: The ERC model of X+ can be also verified by measurements of the hard X-ray slope. Since only the slow-cooling regime yields the photon index 1.5, and only ERC can produce hard X-ray spectra within the slow cooling regime, any evidence for  < 1.5 would strongly support the ERC model of X+. ASCA and BeppoSAXhave already detected such cases, but additional confirmation is desired noting the possible systematic errors and proximity of the measured  to 1.5.

46. Neutron-star binaries

47. High-energy tails seen in the soft states of neutron-star binaries: GX 17+2 BeppoSAX data Farinelli et al. 2005

48. High-energy tail in Cir X-1: Origin? Iaria et al. 2002

49. The nature of X+emission in neutron-star LMXBs not well understood. 4U 160852 Gierliński & Done 2002

50. Some of the main issues: • Polarization of Comptonized emission. • Pair annihilation features. • High-energy cutoffs and tails. • Jet contribution to hard X-rays. • Nature of hard tails in supersoft states. • New black-hole/neutron-star diagnostics. • Hard X-rays during rise/decline of transients. • AGN tails and the XRB. • SSC vs. external Compton in blazars.