The Character of the Short-Term Variability of Sagittarius A* from the Radio to the Near-Infrared

1. The Character of the Short-Term Variability of Sagittarius A* from the Radio to the Near-Infrared and even to X-rays Mark Morris, Andrea Ghez, Seth Hornstein, Jessica Lu & Keith Matthews UCLA and Fred Baganoff (MIT)

2. Outline • Sgr A* variability: mm, cm, submm • Hourly time scales • Near-IR variability properties • Spectral index invariance • Hourly time scales • “simultaneous” observations of a recent X-ray flare • The question of quasi-periodicities • summary

3. Bondi radius,

4. RADIO 3-mm light curves from theOwens Valley MillimeterInterferometer(Mauerhan et al. 2004) Note the excess power at2-3 hours. Simulated red noise curves of variousslopes.

5. Centimeter-wave variability of SgrA* (Yusef-Zadeh et al. 2006):- note few hour time scale - low-amplitude - phase lag --> expanding plasmon model

6. SMA variability (890 µm) (from Eckart et al. 2006, observations by Marrone, Moran, Zhao)

7. Polarized light curves from the SMA @ 230 GHz (Marrone, Moran et al.):note the power on few-hour time scales ….. Polarization evolution in theQ-U plane.

8. Near-infrared variability: almost an order of magnitude on an hour time scale.

9. HK’L’ color composite 1” Hornstein et al. 2006

10. Peak 1 Peak 2 Peak 3 K’-L’ K’-L’ (July 2004) Rapid-switching, broad-band photometry with Keck/NIRC2during July 2005 (2 nights) L’ 3.8 K’ 2.2 µmH 1.6 µm L’ 3.8 µmMs 4.7 µm K’-L’ F =  Hornstein et al. 2006

11. Contamination by the underlying stellar population Spectral Slopes Spectral indices from uncorrected SgrA* fluxes (obtained from PSF fitting) Spectral indices obtained after subtraction of smallest SgrA* flux, to correct forcontamination by the unresolved, centrally peaked stellar population.

12. These results are consistent withIR = -0.9 ± 0.3 for all pairs of wavelengths at all times. • ==> working hypothesis: the spectral index of the IR emission is constant over intensity, time, and wavelength (1 - 5 µm).This differs from the cm-wave radio result. Conclude: no spectral evolution during the outburst. The “flare” mechanism leaves the energy distribution of the bulk of the IR-emitting electrons unchanged. 1 - 3 hour time scales: dynamical time at 20-30 Rsor recurrence time of an instability, such as the RossbyWave Instability (Tagger & Melia 2006).

13. No X-ray activity during the time of the measured events.It would be extremely important to make this measurement during an X-ray flare to see if the spectral slope changes then.If not --> different electron population needed for X-rays.

14. 2002 – 23-30 May CXO light curve (2-8 keV)Baganoff et al. May 28 15:36 UT 25x, 4 ks May 29 06:03 UT 12x, 7 ks May 29 18:33 UT 13x, 2 ks On average, ~ 1 X-ray flare / 105 seconds; compare to near-IR (L’) -- ~10 broad maxima / day

15. SimultaneousIR/X-ray/submmobservation,2006 July 17 Keck II/ NIRC2Hornstein et al.

16. X-ray flare Sgr A* 230 GHz X-ray Time --> Submillimeter Array (SMA) light curve(Marrone, Moran et al. ) Sgr A* near-IR Keck dataHornstein et al.

17.  = -0.54, or within 1- of the2005 value of -0.9. The near-IR spectral index shows no change during the fall of thelight curve following the X-ray flare, and during the rise of the sub-mm. ==> inconsistent with an expansion process, as might have beensuggested by the time lag between X-rays and sub-mm.

18. 15 - 22 minute periodicities in IR & X? Context: period of innermost stable circular orbit around a 3.6 x 106 M BH is 28 min (a=0) 17 min (a=0.5), where 0 < a  1 is the dimensionless rotation parameter. Belanger et al. 2005: P ~ 22 min (XMM flare) Genzel et al. 2004: P ~ 17 minutes, 2.2 µm Relatively rare -- never seen at Keck. Only structured flare in ~20 now seen

19. Hornstein et al. A good example of a Keck light curve (3 hours at 3.8 µm), simultaneous with Chandra, but with no X-ray activity above the quiescent level.

20. Summary • 1. There have been exciting hints of quasi-periodic variability at periods related to the ISCO, but IMHO, more work remains to be done to really establish: • the frequency of occurrence of periodic signals • the range of periods (presumably not all located at the ISCO) • the temporal coherence length of a periodic signal • whether a “chirp” phenomenon can be demonstrated for a distribution of cases, and whether dP/dt is roughly constant • Polarization studies at both submillimeter and near-IR wavelengths appear to be very promising for elucidating theorbital motion.

21. 2. There is an apparent excess of power on time scales of a few hours at millimeter, submillimeter, and infrared wavelengths. • dynamical time scale? (R ~ 25Rs) or instability recurrence time? • the near-infrared spectrum does not evolve as the intensity undergoes strong changes. • 3. In the near-IR, the variability is essentially continuous, at least at 3.8 µm: • not a succession of “flares” but rather a stochastically varying intensity • At 2 µm, the peaks have often been interpreted as discrete events, perhaps because the background level is more difficult to define there.