A new measurement could be the farthest back in time astronomers have ever reached when measuring a black hole’s spin.
Earlier this year I wrote about astronomers measuring a distant black hole’s spin thanks to the magnifying, lens-like effect of a closer elliptical galaxy. The radiation from the black hole’s accretion disk had traveled 6 billion years to reach us, meaning the active galactic nucleus (AGN) has a redshift of 0.658. At the time, it was the most distant black hole spin yet measured.
That same team has now upped the ante, using the technique to measure the spin of the AGN Q2237+0305. This quasar has a redshift of 1.695, meaning its radiation has traveled nearly 10 billion years before reaching us. That’s brushing up against the peak of galaxy and star formation in the cosmos, which happened at roughly a redshift of 2 (10.4 billion years ago).
Q2237+0305 lies behind a barred spiral galaxy from our perspective. The spiral galaxy’s gravity lenses Q2237+0305’s emission into four images, creating what’s known as the Einstein Cross.
To determine the black hole’s spin, Mark Reynolds (University of Michigan, Ann Arbor) and colleagues used 26 observations taken over 13 years by NASA’s Chandra X-ray Observatory of the lensed quasar. Because the object is so distant and faint, they stacked the observations together into one image to boost the signal.
The team analyzed the combined X-ray emission, looking for evidence of reflection off the black hole’s accretion disk. The innermost accretion disk is deep in the black hole’s gravitational well, where the spinning black hole drags spacetime around with it. When X-rays from the hot corona around the accretion disk reflect off that innermost disk, they’re imprinted with spectral markers that reveal how the black hole has warped spacetime — and therefore how fast the black hole is spinning.
The team analyzed Q2237+0305 a couple of different ways, determining that they do see signs of reflection. The researchers also found that the numbers cranked out a bit better when they assumed a high level of iron (ionized iron is the most prominent feature in reflection spectra, and the amount of iron can affect the measurement).
The spin comes out to about 74% of the maximum possible value for a black hole of this mass (roughly a billion Suns). That’s high for a black hole (anything above 50% is high), suggesting that the beast grew by slurping down a steady stream of gas and not by merging with other black holes.
The caveat here is that the result depends on stacked data. Stacking data together can smooth out variability, and variability affects the disk’s spectral signal, potentially messing with the signature from reflection and, therefore, from the spin. But Reynolds says his team didn’t find signs of significant variation that would muck up the analysis, and he doesn’t think it’s a problem for this source, even on the years-long timescale they’re working at.
Sadly, this is the last high-redshift system for which the team has enough data to confidently estimate the spin. Astronomers know of a couple dozen lensed quasars, but the observations are generally less detailed than the archival data the team used for Q2237+0305. The team hopes to use X-rays to clock the spins of another five to ten black holes in the early universe, but Reynolds cautions that it’ll take them several years.
Reference: M. T. Reynolds et al. “A Rapidly Spinning Black Hole Powers the Einstein Cross.” Astrophysical Journal Letters. September 1, 2014.
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