Latest News
Black Hole Feasts on
Star, Bite by Bite

The active
galaxy ESO 253-G003, pictured here, is the first one shown to host
regularly recurring flares coming from its central black hole. Credit: Michael Tucker (University of
Hawai'i) and the AMUSING survey
Like a cosmic blinker, a distant supermassive
black hole is flaring every 114 days. Astronomers believe the
eerily regular signal betrays the piecemeal devouring of a
star.
On November 14, 2014, the All-Sky Automated
Survey for Supernovae (ASAS-SN), a global network of two-dozen
14-centimeter robot telescopes, detected a flare in the core of ESO
253-G003, a strangely shaped galaxy 570 million light-years away in
the southern constellation Pictor. Catalogued as ASAS-SN 14ko, the
flare was thought to be a supernova, or maybe a temporary
brightening of an active galactic nucleus (AGN) powered by a
supermassive black hole.
However, when Anna Payne (University of
Hawai'i, Mānoa) studied six years' worth of data on the galaxy, she
discovered that similar flare-ups occur every 114 days. AGNs often
show quite dramatic brightness changes, but never in a fashion so
regular, it's reminiscent of the famous Old Faithful geyser in
Yellowstone National Park.
"This behavior provided me with a unique
opportunity to predict the next flare," Payne told a press
conference at the virtual 237th meeting of the American
Astronomical Society. Three of the most recent flares (on May 17,
September 7, and December 20, 2020) have now been studied by
various ground-based facilities (including four large amateur
telescopes in Australia, South Africa and Brazil), and by several
X-ray space observatories: NASA's Swift and NuSTAR satellites, and
ESA's XMM-Newton. The team has posted preliminary results on the
arXiv preprint
server.
Each individual flare consists of a rapid,
five-day rise in luminosity, followed by a much slower decline. As
luck would have it, one earlier event, on November 7, 2018, was
observed by the Transiting Exoplanet Survey Satellite (TESS),
resulting in an extremely detailedlight curvethat traces how the
brightness changes over time. According to Payne, the December 20,
2020, flare also occurred in TESS's field of view, but she and her
team have not analyzed those data yet.
Observations with the Multi Unit Spectroscopic
Explorer on the European Southern Observatory's Very Large
Telescope reveal that ESO 253-G003 actually contains two actively
feeding supermassive black holes, separated by some 4,500
light-years. In a second paper, submitted to Monthly Notices of the
Royal Astronomical Society, a team led by Michael Tucker
(also at the University of Hawai'i, Mānoa) suggests that the galaxy
is in the late stage of a major merger, which explains its
irregular shape. The periodic flaring is seen in the brighter of
the two nuclei.
So what causes the regular flares of ASAS-SN
14ko? They can't be due to an interaction between the two black
holes; they are too far apart and orbit each other much too slowly.
Instead, Payne and her colleagues envision a star in an elongated
orbit around the larger of the two black holes, which has an
estimated mass of 80 million Suns or so. Every 114 days, the star
passes dangerously close to the black hole, which rips away about
three Jupiter masses' worth of stellar material. This gas falls
into the black hole, causing the flare. However, the star survives,
only to lose another chunk at the next pass.
Close Encounters of the
Supermassive Kind
Such periodic partial tidal disruption events
(TDEs) were first studied theoretically in 1986 by Jean-Pierre
Luminet (Laboratory of Astrophysics, Marseille, France). In fact,
comments Luminet, "I'm surprised that this kind of event has not
been detected earlier, because partial TDEs must be more frequent
than complete TDEs, for which we already have a catalog of dozens
of cases."
Luminet's calculations showed that a
supermassive black hole's tidal forces would rip apart a more or
less homogenous Sun-like star if it ventured too close, or leave it
unharmed if it remained at a safe enough distance. In contrast, at
a range of distances, the black hole could tidally strip the
low-density outer layers of a red giant, while the star's dense
core would remain intact. "Hence, a red giant can lose a few
Jupiter masses at each passage along its periodic orbit," he
says.
Astronomers don't know what kind of star is
involved in ASAS-SN 14ko, or how long the AGN has been producing
periodic flares. Therefore, it's hard to tell how long the
phenomenon will last, says Payne. The team plans to extensively
study future flares, expected in April and August 2021.
There's much to learn. Luminet recalls that
when a low-density gas cloud, called G2, passed close to the
supermassive black hole in the galactic center in 2014, most
astronomers expected it to be destroyed by tidal forces, but
nothing happened. "A lot of work remains to be done to better
understand tidal disruption events," he says.
Source:
Sky and Telescope
|



|