The Lowest Frequency Detection of the Black Hole at the Center of Our Galaxy

Introduction: Early radio observations revealed that the central region of our Galaxy is radio bright and located in the constellation of Sagittarius. Subsequent observations showed that this region, known as Sagittarius A, is a complex of radio sources including a supernova remnant, several ionized hydrogen regions, and a radio point source known as Sagittarius A*. Studies of stellar and gas motions in the vicinity of Sagittarius A* strongly suggest the presence of roughly three million solar masses of dark mass in a region less than the size of the solar system. This dark mass is now considered to be the black hole at the center of our Milky Way Galaxy.

Mass accreting onto the black hole is heated to great temperatures and is the source of the radiation coming from Sagittarius A*. This radiation has been detected at many frequencies in the radio regime (1 to 230 GHz) and recently in the near infrared and X-ray bands. However, one of the mysteries surrounding this object is its overall lack of emission, which suggests that either the black hole is not accreting at the rate expected, the accretion process creates radiation inefficiently, or both. Detecting Sagittarius A* at the widest range of frequencies will help elucidate the nature of its radiation processes.

Until recently, radio observations at frequencies below 1 GHz were severely limited by the Earth's ionosphere. However, recent work at NRL in conjunction with work at the National Radio Astronomy Observatory1 has allowed for data-adaptive ionospheric compensation, and allowed this area of the radio spectrum to be observed with high spatial resolution.

Methodology: The Galactic Center region was imaged at the radio frequency of 330 MHz with the National Radio Astronomy Observatory's Very Large Array radio interferometer. Previous observations at similar radio frequencies had not detected Sagittarius A*. These nondetections were attributed to an ionized hydrogen region, known as Sagittarius A West, blocking emission from Sagittarius A* at frequencies below ~1 GHz. However, when imaging of the region was completed, a region of emission was observed at the location of Sagittarius A* (Fig. 7).

FIGURE 7
Image of the region surrounding Sagittarius A^*. White areas indicate high brightness due to the background supernova remnant Sagittarius A East, dark regions indicate low brightness due to absorption by the ionized hydrogen region, Sagittarius A West. The blue oval in the center is the location and size of Sagittarius A^* as extrapolated from higher frequency measurements. Note that a local maximum in 330 MHz intensity occurs at this location. The oval in the lower left represents the resolution element of the interferometer (33 by 53 µrad). The image is roughly 0.5 mrad on a side.

The region around the black hole thought to emit radio signals is less than 10 light-hours (the distance light travels in 10 hours, roughly 1010 km) in diameter (~0.5 nrad at a distance of 25,000 light years). However, density fluctuations in the interstellar plasma along the line of sight produce scattering that causes the observed size of the radio-emitting region to be much larger, with the size rising as the inverse square of the observation wavelength. When the 330 MHz size of Sagittarius A* is extrapolated from measurements at higher frequencies in this way, the expected 330 MHz size and shape agree well with the size and shape of the emission region in Fig. 7. Figure 8 demonstrates that the integrated brightness of the source also agrees well with measurements at higher frequencies. We therefore conclude that we have detected Sagittarius A* at 330 MHz; this is the lowest frequency at which it has ever been detected.

Interpretation: Little theoretical work has gone into modeling the emission of Sagittarius A* at frequencies below 1 GHz; the source was considered unobservable in this frequency regime. However, this detection does suggest several interesting properties of the region surrounding the black hole. As shown in Fig. 8, the integrated 330 MHz emission is what is expected from extrapolations from higher frequencies. Significant absorption from the Sagittarius A West ionized hydrogen region is seen along lines of sight near the source. Therefore, it is surprising that Sagittarius A* is detected at all at 330 MHz, much less at the level expected from extrapolations from higher frequencies. Although it is possible that the intrinsic brightness of Sagittarius A* is rising to offset partial absorption, we feel that the observation is best explained simply by the lack of any significant absorption along the line of sight.

FIGURE 8
Radio spectrum of Sagittarius A^*. The red point is the detection; the yellow point is a recent 610 MHz detection; green represents the radio regime; and blue is the sub-millimeter regime.² For reference, the solid line indicates a power law spectrum with a spectral index of 0.3. The integrated emission is on the abscissa and is in units of Janskys (Jy), with one Jansky being equal to 10^-26 W M^-2 Hz^-1.

The lack of absorption could be modeled in at least two ways. Either the absorbing medium is inherently clumped, with the line of sight to the black hole having low absorption by chance, or the black hole itself is responsible for evacuating the plasma in its immediate environs. Unfortunately, our data cannot differentiate between these two possibilities, but both possibilities are of interest to understanding the environs of Sagittarius A*.

This detection also contains information about high-energy particles in the vicinity of the black hole. The radio emission of Sagittarius A* is synchrotron in origin; high-energy electrons spiraling around magnetic fields radiate according to their energy and the strength of the magnetic fields. This 330 MHz emission comes from electrons with lower energy than the higher frequency detections, probing the lower end of the high-energy particle spectrum.

Conclusions: We have used the Very Large Array radio interferometer to image the radio source associated with the black hole at the center of our Galaxy at the lowest radio frequency at which it has ever been detected. This detection was unexpected. Emission from the source was expected to be absorbed by intervening ionized gas at frequencies below ~1 GHz. The detection suggests that the line of sight to the black hole is relatively free of obscuring plasma, and it probes the high-energy particle spectrum of the region surrounding the black hole.

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