Measuring a Black Hole’s Mass with Robotic Telescopes

Who needs humans? Robotic observations made by telescopes in the Las Cumbres Observatory Global Telescope network (LCOGT) have tracked variability in the active galaxy Arp 151 over 200 days. These observations have proven to be enough information to estimate the mass of the black hole at the galaxy’s center.

Mapping Echoes

Measuring the masses of supermassive black holes is notoriously difficult. Except in the few cases where we’re able to resolve actual objects orbiting around the supermassive black hole (for instance, in the case of the black hole at the center of the Milky Way), our estimates of black-hole mass must come from indirect measurements.

One clever approach is called “reverberation mapping.” In an active galactic nucleus (AGN), continuum emission from the black hole’s accretion disk photoionizes gas clouds in the nearby broad-line region, causing the clouds to emit light. In reverberation mapping, we track the time lag between variability in the disk’s continuum emission and the clouds’ broad-line emission, obtaining a distance scale. Combining this information with a velocity (provided by the broad-line width) allows us to infer the enclosed mass — in this case, that of the black hole.

So what’s the catch? Getting this information requires a lot of man-hours and telescope-hours, because AGN need to be observed over long periods of time to see the variability and the lags needed to make these inferences. This is where LCOGT comes in.

Robotic Network

Arp 151 light curves

Arp 151 light curves. The top panel shows the continuum emission from the disk; the remaining panels show various emission lines from the broad-line-region clouds. The variability of the line emission lags slightly behind that of the continuum emission. [Valenti et al. 2015]

LCOGT is a completely robotic telescope network. Everything from the scheduling to the telescope alignment is done without human involvement. Because of this feature, the LCOGT is an ideal facility for conducting time-intensive observations of AGN.

A team of scientists led by Stefano Valenti (LCOGT, UC Santa Barbara) has published the first results from the AGN Key Project, a project which uses the LCOGT network to conduct several studies of AGN — including reverberation mapping of both local and high-redshift objects. In these first results, the team reports the outcome of a 200-day observing campaign of the galaxy Arp 151, which has a highly variable active nucleus.

The LCOGT observations successfully show a measurable lag between the continuum emission and the broad emission lines for Arp 151. They are also enough-resolved that a velocity can be measured from the broad emission lines, allowing the team to calculate the mass of the black hole enclosed. Valenti and collaborators announce a mass of 6.2 million solar masses — consistent with previously measured masses for this system.

The success of this test demonstrates the viability of this approach, as well as the powerful capabilities of robotic telescope networks for long-term AGN time domain campaigns.


S. Valenti et al 2015 ApJ 813 L36. doi:10.1088/2041-8205/813/2/L36

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