Once every 10,000 years or so, a galaxy’s center lights up as its supermassive black hole shreds a passing star. The “tidal disruption event” occurs in a literal flash, as the central black hole pulls in stellar material and releases massive amounts of radiation in the process.
Astronomers know of about 100 tidal disturbance events (TDE) in distant galaxies, based on the rush of light reaching telescopes on Earth and in space. Most of this light comes from x-rays and optical radiation.
Tuning in conventional X-ray and ultraviolet/photonic bands, MIT astronomers have discovered a new tidal disturbance event, shining brightly in the infrared. It is one of the first times scientists have directly determined TDE at infrared wavelengths.
Moreover, the new outburst is the closest tidal disturbance event observed to date: the flare was found in NGC 7392, a galaxy about 137 million light-years from Earth, which corresponds to a region in its own cosmic backyard. coffee beans. Quarter the size of the next nearest TDE.
This new flare, called WTP14adbjsh, did not stand out in the standard X-ray and optical data. Scientists believe that these conventional surveys missed near TDE, not because they did not emit X-rays and ultraviolet light, but because that light was blocked by a massive amount of dust that absorbs radiation and gives off heat in the form of infrared. energy.
The researchers determined that WTP14adbjsh occurred in a young star-forming galaxy, unlike the majority of TDEs that have been found in quieter galaxies. Scientists predicted that star-forming galaxies should host TDEs, since the stars they produce would provide plenty of fuel for the galaxy’s central black hole to devour. But observations of TDEs in star-forming galaxies have been rare until now.
The new study suggests that conventional optical and X-ray surveys may have missed TDE in star-forming galaxies because these galaxies naturally produce more dust that could block out any light coming from their cores. Searching in the infrared range can reveal many previously hidden TDEs in active star-forming galaxies.
“Finding such a close TDE means, statistically, there must be a large number of these events for which traditional methods were not constrained,” says Christos Panagio, a postdoctoral researcher at MIT’s Kavli Institute for Astrophysics and Space Research. “. “So, we should try to find them in infrared if we want a complete picture of black holes and their host galaxies.”
A paper detailing the team’s discovery appears today in Astrophysical Journal Letters. MIT co-authors at Panagiotou are Kechalay D, Megan Masterson, Irene Cara, Michael Calzadilla, Anna-Christina Ehlers, Daniel Frostige, Nathan Lowery, and Rob Simko, along with Viraj Karambilkar, Mansi Kasliwal, Robert Stein, and Jeffrey Zolkwer of Caltech, and Aaron Meissner at the National Science Foundation’s National Optical and Infrared Astronomy Research Laboratory.
A flash of possibility
Panagiotto did not intend to research tidal disturbance events. He and his colleagues were looking for signals of transient general sources in the observational data, using a search tool developed by De. The team used the De method to search for possible transient events in archival data captured by NASA’s NEOWISE mission, a space telescope that has conducted regular surveys of the entire sky since 2010, at infrared wavelengths.
The team detected a bright flash that appeared in the sky towards the end of 2014.
“We could see that there was nothing in the beginning,” Panagiotto recalls. “Then suddenly, in late 2014, the source got brighter and by 2015 had reached a high luminosity, and then it started to return to its former stillness.”
They tracked the flash to a 42-megapixel galaxy from Earth. The question then was, what triggered this? To answer this, the team looked at the brightness and timing of the flash, and compared actual observations with models of various astrophysical processes that could produce a similar flash.
“For example, supernovae are sources that suddenly explode and light up, then come back down, on time scales similar to tidal disturbance events,” Banagio notes. “But the supernovae are not as luminous and energetic as what we have observed.”
Working through the various possibilities of what the nudge could be, the scientists were finally able to rule out all but one: the flash was most likely a TDE, the closest one ever observed.
“It’s a very clean light curve and really follows what we expect from the time evolution of TDE,” Banagio says.
red or green
From there, the researchers took a closer look at the galaxy where TDE originated. They gathered data from multiple ground-based and space-based telescopes that happened to observe the part of the sky where the galaxy is located, across various wavelengths, including the infrared, optical, and X-ray ranges. With this accumulated data, the team estimated that the supermassive black hole at the center of the galaxy was about 30 million times more massive than the sun.
“That’s about 10 times more massive than the black hole we have at the center of our galaxy, so it’s pretty massive, even though black holes can be as large as 10 billion solar masses,” says Banagio.
The team also found that the galaxy itself is actively producing new stars. Star-forming galaxies are a class of “blue” galaxies, in contrast to the quieter “red” galaxies that have stopped producing new stars. Blue star-forming galaxies are the most common type of galaxy in the universe.
The “green” galaxies are somewhere between red and blue, and each time they produce a few stars. Green is the least common type of galaxy, but it is intriguing that most of the galaxies discovered so far have been traced back to these rare galaxies. Scientists have struggled to explain these discoveries, as theory predicts that galaxies composed of blue stars should display TDEs, as they would present more stars for black holes to disrupt.
But star-forming galaxies also produce a lot of dust from interactions between and between stars near the galactic core. This dust can be detected at infrared wavelengths, but it can block any X-rays or ultraviolet rays that optical telescopes can pick up. This may explain why astronomers have not detected TDE in star-forming galaxies using conventional optical methods.
says Suvi Gezari, an astronomer and co-chair of the science staff at the Space Telescope Science Institute in Maryland, who was not involved in the study. “Using infrared surveys to capture the dust echoes of the obscured TDEs… has already shown us that there is a host of TDEs in the dusty star-forming galaxies that we’ve been missing.”
Christos Panagiotou et al., a candidate for a luminous dust-obscured tidal disturbance event in a 42-megapixel star-forming galaxy, Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/ac02f
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