3. Discovery of powerful millisecond flares from Cyg X-1 Marek Gierliński University of Durham, England Andrzej Zdziarski N. Copernicus Astronomical Center, Warsaw, Poland

4. Cyg X-1 variability from years to milliseconds

5. Powerful millisecond flares from Cyg X-1 28 October 20034 Cyg X-1: a tame source Persistent source Spectral states: hard and soft Variability nothing like as dramatic as in transients Cyg X-1 is a well-behaved black hole PCA+HEXTE

6. Powerful millisecond flares from Cyg X-1 28 October 20035 Seven years of Cyg X-1 2001/2002 soft state (400 days) ASM BATSE

7. Powerful millisecond flares from Cyg X-1 28 October 20036 Spectral transitions – days to weeks Typical transition between states takes about a week There are faster events with timescales of a day Cui, Feng & Ertmer 2002

8. Powerful millisecond flares from Cyg X-1 28 October 20037 Kilosecond outbursts Stern, Beloborodov & Poutanen 2001 20-300 keV

9. Powerful millisecond flares from Cyg X-1 28 October 20038 Power spectrum of Cyg X-1 Revnivtsev, Gilfanov & Churazov 2000 Not much variability above 100 Hz

10. Powerful millisecond flares from Cyg X-1 28 October 20039 Other black holes too Sunyaev & Revnivtsev 2000

11. Powerful millisecond flares from Cyg X-1 28 October 200310 Cyg X-1 high-frequency PDS Revnivtsev, Gilfanov & Churazov 2000

12. Very short events from Cyg X-1

13. Powerful millisecond flares from Cyg X-1 28 October 200312 X-ray archaeology: 1973 Rothschild et al. 1974

14. Powerful millisecond flares from Cyg X-1 28 October 200313 Shots Flares or ‘shots’ are weak, each of them 2–3 times brighter than the continuum Superposition of hundreds of ‘shots’ Timescales of ~10 and ~100 ms (sum of two exponentials) Feng, Li & Chen 1999

15. So, you don’t expect sudden dramatic events from Cyg X-1…

16. Powerful millisecond flares from Cyg X-1 28 October 200315 …until this happened Enormous flare on 31 July 2002 (during extended soft state) PCA count rate increased by factor 20 in 100 ms  20 PCA 2–60 keV (4 PCUs)

17. Powerful millisecond flares from Cyg X-1 28 October 200316 Is it real? An external event not from Cyg X-1? Perhaps an energetic particle hit the PCA detector? Or is it a solar flare? Or is it a gamma-ray burst? 27 August 1998: SGR 1900+14 flare recorded by PCA looking somewhere else

18. Powerful millisecond flares from Cyg X-1 28 October 200317 Multiple detection The flare was detected by four units of PCA (PCU1 was switched off) and one HEXTE cluster

19. Powerful millisecond flares from Cyg X-1 28 October 200318 Even BATSE can see it! PCA BATSE 16-12-1996

20. Powerful millisecond flares from Cyg X-1 28 October 200319 It really comes from Cyg X-1 HEXTE (15–150 keV) contains two detector clusters switching between the source and background 1.5 deg away When cluster B detected the flare, cluster A looked at background and did not see anything The flare comes from Cyg X-1 source background

21. Flare properties

22. Powerful millisecond flares from Cyg X-1 28 October 200321 More flares We’ve scanned all the PCA data from Cyg X-1 Found 13 strong flares with c peak -  c  > 10  12 flares in the hard state, 1 in the soft state 16-12-1996 14-02-1988 31-07-2002

23. Powerful millisecond flares from Cyg X-1 28 October 200322 Flare profile (soft state)   20 ms   0.7 PCA 20  HEXTE exp(-|t/  |  )

24. Powerful millisecond flares from Cyg X-1 28 October 200323 Very short precursor Increase  10 in 2 ms 2 ms ~ 40GM/c 3

25. Powerful millisecond flares from Cyg X-1 28 October 200324 Flare anatomy (soft state) Flux (3–30 keV)  (2–15 keV)  (6–60 keV) Spectral hardening  ~ 1.8 Spectral hardening  ~ 1.3  30 flux increase L peak ~ 1.4  10 38 erg/s

26. Powerful millisecond flares from Cyg X-1 28 October 200325 Flare anatomy (hard state) Flux (3–30 keV)  (2–15 keV)  (6–60 keV)  10 flux increase L peak ~ 0.9  10 38 erg/s Spectral softening  ~ 0.3

27. Powerful millisecond flares from Cyg X-1 28 October 200326 Flare spectrum (soft state) SAX/OSSE soft state PCA/HEXTE continuum around flare Flare spectrum Model luminosity L bol ~ 0.3L Edd

28. Powerful millisecond flares from Cyg X-1 28 October 200327 Flare spectrum (hard state) SAX/OSSE hard state PCA/HEXTE continuum around flare Flare spectrum Model luminosity L bol ~ 0.25L Edd

29. Powerful millisecond flares from Cyg X-1 28 October 200328 Not only Cyg X-1 XTE J1118+480

30. Powerful millisecond flares from Cyg X-1 28 October 200329 Not only Cyg X-1 GX 339–4

31. Powerful millisecond flares from Cyg X-1 28 October 200330 Rapid flare in Sgr A* Baganoff et al. 2001 Shortest timescale ~30GM/c 3

32. Powerful millisecond flares from Cyg X-1 28 October 200331 Conclusions We have discovered several strong millisecond X-ray flares from Cyg X-1 and other black holes Shortest observed timescales correspond to ~40GM/c 3 or half of the Keplerian period on the marginally stable orbit The flares must be due to a sudden release of accretion energy in the inner region of the disc Magnetic flares? e.g. Beloborodov 1999; Poutanen & Fabian 1999; Machida & Matsumoto 2003 Challenge for theorists!

34. Powerful millisecond flares from Cyg X-1 28 October 200333 Magnetic flares in plunging region Machida & Matsumoto 2003