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

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

ABSTRACT Observations of Sgr A* over the past 4 years with the Keck Telescope in the near-infrared, coupled with millimeter and submillimeter observations, show that the 3.7 x 10^6 Msun Galactic Black Hole, Sagittarius A*, displays continuous variability at all these wavelengths, with the variability power concentrated on characteristic time scales of ~2 hours, and with a variability fraction that increases with wavelength. Interpreted as a dynamical time, the few-hour variability time scale corresponds to a radial distance of 2 AU, or 25 Schwarzchild radii. Searches for quasi-periodicities in the near-infrared data from the Keck Telescope have so far been negative. One implication of the character of these variations is that they most likely result from a recurring disk instability, rather than from variations in the mass accretion rate flowing through the outer boundary of the emission region. Other implications for activity in and near the presumed accretion disk will be discussed.

Bondi radius,

Simulated red noise curves of various slopes. RADIO 3-mm light curves from the Owens Valley Millimeter Interferometer (Mauerhan et al. 2004) Note the excess power at 2-3 hours.

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

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

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

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

Hornstein et al HK’L’ color composite 1”

 K’-L’ Peak 1Peak 2Peak 3  K’-L’ (July 2004)  K’-L’ L’ 3.8 K’ 2.2 µm H 1.6 µm Rapid-switching, broad-band photometry with Keck/NIRC2 during July 2005 (2 nights) L’ 3.8 µm M s 4.7 µm Hornstein et al F = 

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

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 hour time scales: dynamical time at R s or recurrence time of an instability, such as the Rossby Wave Instability (Tagger & Melia 2006).

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.

2002 – May CXO light curve (2-8 keV) Baganoff et al. May 28 15:36 UT 25x, 4 ks May 29 18:33 UT 13x, 2 ks May 29 06:03 UT 12x, 7 ks On average, ~ 1 X-ray flare / 10 5 seconds; compare to near-IR (L’) -- ~10 broad maxima / day

Simultaneous IR/X-ray/submm observation, 2006 July 17 Keck II/ NIRC2 Hornstein et al.

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

The near-IR spectral index shows no change during the fall of the light curve following the X-ray flare, and during the rise of the sub-mm. ==> inconsistent with an expansion process, as might have been suggested by the time lag between X-rays and sub-mm.  = -0.54, or within 1-  of the 2005 value of -0.9.

minute periodicities in IR & X? Context: period of innermost stable circular orbit around a 3.6 x 10 6 M  BH is 28 min (a=0) 17 min (a=0.5), where 0 < a  1 is the dimensionless rotation parameter. Genzel et al. 2004: P ~ 17 minutes, 2.2 µm Belanger et al. 2005: P ~ 22 min (XMM flare) Relatively rare -- never seen at Keck. Only structured flare in ~20 now seen

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. Hornstein et al.

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 the orbital motion.

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 ~ 25R s ) 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.