GeminiFocus 2019 Year in Review | Page 37

As shown in Figure 9, GR provides an ac- curate description of the star’s positional and velocity data throughout its very large swing in velocity near its closest approach to Sag A*. In contrast, the observations rule out Newton’s law of gravity with a high sta- tistical significance. “The GR model is 43,000 times more likely than the Newtonian mod- el in explaining the observations,” the study concludes. The measurements also provide strong constraints on the black hole’s dis- tance and mass, 8.0 kiloparsecs and 4.0 mil- lion solar masses, respectively. Of course, no one wins forever, and at some point, namely the event horizon of a black hole, GR must also fail. However, although S0-2 plunged precipitously near Sag A*, the minimum distance was roughly 1,000 times larger than the radius of the event horizon. Thus, it may be some time before observa- tional limits encroach on the limits of GR’s validity. Meanwhile, such observations con- tinue to enlighten our understanding of the dynamics and evolution of the center of our Galaxy. The study appears in the journal Science. JULY 2019 Reverberations from an Intermediate-mass Black Hole in a Bulgeless Dwarf For some, the term “reverberation mapping” might suggest the idea of pinpointing the locations of the various garage bands in the neighborhood (all with their amplifiers turned way up) based on the distribution and intensity of the vibrations emanating from one’s walls and window panes. But in actuality, it denotes a powerful technique for determining the masses of the black holes embedded within the active galactic nuclei (AGNs) at the centers of many galax- ies. Interestingly, the two phenomena are January 2020 / 2019 Year in Review not entirely dissimilar. Like the perfect guitar riff, reverberation mapping requires precise timing and can be quite challenging to ex- ecute in practice. In addition, the virtue of both lies in their conceptual simplicity. Reverberation mapping works by applying the familiar virial theorem to the broad line region (BLR) of an AGN. Assuming that the motion of the gas in the BLR is primarily influ- enced by the central black hole, the mass of the black hole M BH will be proportional to σ 2 R, where σ is the velocity dispersion determined from the Doppler width of a broad emission line and R is the characteristic radius of the BLR. The radius is determined from the delay time τ between variations in the intensity of the continuum light from the AGN, which excites the gas within the BLR, and the line emission itself: R = c τ , where c is the speed of light. Because lines of different ionization show different delays, the same line should be used for determining both σ and τ . Typical AGNs powered by supermassive black holes of millions of solar masses (M B ) have delay times measured from Balmer lines ranging from a few days to many months. Figure 9. Top: Zoom in on the radial velocity data from 2018, encompassing the maximum and minimum of the observed radial velocity. Measurements from the three different observatories are indicated; Gemini/ NIFS and Keck/OSIRIS each provided nine measurements during this critical period, over which the observed velocity changed by 6,000 km/s. Bottom: radial velocity residuals with respect to the best-fitting General Relativistic model. Figure from Do et al., Science, 365: 664, 2019. A new study published in Nature Astron- omy has measured the mass of the black hole associated with one of the lowest lu- GeminiFocus 35