Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre

Abstract

Recent measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* (refs 4, 5) at the Galactic Centre is a 3.6-million-solar-mass black hole. Sgr A* is remarkably faint in all wavebands other than the radio region, however, which challenges current theories of matter accretion and radiation surrounding black holes. The black hole's rotation rate is not known, and therefore neither is the structure of space-time around it. Here we report high-resolution infrared observations of Sgr A* that reveal 'quiescent' emission and several flares. The infrared emission originates from within a few milliarcseconds of the black hole, and traces very energetic electrons or moderately hot gas within the innermost accretion region. Two flares exhibit a 17-minute quasi-periodic variability. If the periodicity arises from relativistic modulation of orbiting gas, the emission must come from just outside the event horizon, and the black hole must be rotating at about half of the maximum possible rate.

Flux monitoring of Sagittarius A* at mm-wavelengths

“…Considering these results, it seems that radio IDV in Sgr A * are not rare, especially at shorter wavelengths. The intraday flare at mm-wavelengths has a similar increase timescale as those known in the X-ray and infrared flares, but has a smaller amplitude ( [13,14,15,16]). The shortest increase timescale, 1.5 hr, suggests that the physical size of the emitting region in the accretion disk is compact on a scale at or below about 12 AU (∼ 150 R S ; the Schwarzschild radius R S = 2GM/c 2 , assuming a black hole mass of 4 × 10 6 M ) [10].…”

“…Moreover, the intraday variations (IDVs) in the Sgr A * flux density at mm-wavelengths were detected [10]. In recent years, X-ray and infrared flares have also been detected, indicating very short timescales and violent intensity increases (X-rays: [13,14]; IR: [15,16]). …”

High-frequency QPOs as a problem in physics: non-linear resonance

“…For ∼ 1M ⊙ neutron stars, ν QPO ≈ 1 kHz, for a ∼ 10M ⊙ black hole ν QPO ≈ 0.1 kHz, and for the 4 · 10 6 M ⊙ black hole at the Galactic center the recently reported period (of an infrared flare) is 17 minutes [11], i.e., ν QPO ≈ 1 mHz. It seems quite safe to assume that the HF QPOs are closely related to orbital frequencies in Einstein's gravity.…”

Testing for periodicities in near-IR light curves of Sgr A*

“…Advances in adaptive optics (AO) technology have offered improved sensitivity to infrared emission from Sgr A* against the stellar background, such that observations in its faint states are now possible [11]. At both NIR and X-ray wavelengths, a possible quasi-periodic oscillation (QPO) signal with a ∼ 20 min period has been reported in light curves of Sgr A* [12][13][14]. Models that aim to produce QPO signals include both a class of models involving the Keplerian orbits of 'hot spots' of plasma at the last stable orbit [15,16] as well as the result of rotational modulations of instabilities in the accretion flow [17].…”