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 MatthewsUCLA

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

Bondi radius,

Simulated red noise curves of variousslopes.

RADIO

3-mm light curves from theOwens Valley MillimeterInterferometer(Mauerhan et al. 2004)

Note the excess power at2-3 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 SMA @ 230 GHz (Marrone, Moran et al.): note the power on few-hour time scales …..

Polarization evolution in theQ-U plane.

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

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Hornstein et al. 2006

HK’L’ color composite

1”

K’-L’

Peak 1 Peak 2 Peak 3

K’-L’ (July 2004)

K’-L’

L’ 3.8 K’ 2.2 µmH 1.6 µm

Rapid-switching, broad-band photometry with Keck/NIRC2during July 2005 (2 nights)

L’ 3.8 µmMs 4.7 µm

Hornstein et al. 2006

F =

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.

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.

1 - 3 hour time scales: dynamical time at 20-30 Rs

or recurrence time of an instability, such as the RossbyWave 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 – 23-30 May CXO light curve (2-8 keV) Baganoff et al.

May 28 15:36 UT25x, 4 ks

May 29 18:33 UT13x, 2 ks

May 29 06:03 UT12x, 7 ks

On average, ~ 1 X-ray flare / 105 seconds; compare to near-IR (L’) -- ~10 broad maxima / day

SimultaneousIR/X-ray/submmobservation,2006 July 17

Keck II/ NIRC2Hornstein 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 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.

= -0.54, or within 1- of the2005 value of -0.9.

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.

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.

Summary1. 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.

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.