Superman isn’t the only one with X-ray vision. Many exploding stars are also good at bursting out this type of high-energy light. Now, thanks to a chance discovery, scientists are aware of a new, explosive source of stellar X-ray radiation. The light output of these bursts is unlike any previous cosmic explosion. Meet “millinovas,” a term that will now undoubtedly enter the lexicon of space enthusiasts!
In a new study, astronomers have discovered 28 Minova stars among the stars. Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC)two satellite galaxies Milky Way. They then discovered that the first explosion may have been discovered eight years earlier but had not been identified.
Although scientists don’t quite know how these events occur X-ray examinationthey believe millinovas are caused when the remnants of dead stars are summoned white dwarf Feeding on an expanding companion star.
“We encountered a group of grumpy people variable star Team member Przemek Mróz, a scientist at the University of Warsaw, told Space.com, “It shows a very typical triangular symmetry burst, unlike any previously known variable star. We discovered this by accident. Group new stars.
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The team is scouring 20 years of data Optical Gravitational Lens Experiment (OGLE) For long-term, light bending”Gravitational microlensing event“This may indicate the existence of The black hole left behind after the big bang – The so-called “primordial black hole” – in the halo dark matter Orbiting the Milky Way.
“Over the past few months I have been working on a project aimed at finding a large number of signatures primordial black hole “In the Milky Way’s dark matter halo,” Mroz said.
Often, this can be frustrating for the team. But the result was the discovery of these strange stellar X-ray sources, now known as millinovas (or, more accurately, “millinovae”).
Hotter and brighter than the sun
OGLE data showed that several objects in the LMC and SMC increased their brightness by a factor of 10 to 20 over a period of months. Some even erupt repeatedly every few years, while others erupt only once during the observation period.
One object in particular, called OGLE-mNOVA-11, erupted late last year, allowing the team to study these objects in detail.
“In November 2023, one of the objects went into an outburst, so we decided to do some additional follow-up observations to study it in more detail,” said Mroz. “We obtained one with the Southern Africa Large Telescope (SALT) telescope. group of spectra. We found emission lines from ionized atoms of helium, carbon and nitrogen, indicating extremely high temperatures.”
Mróz added that the researchers also observed the object together with NASA Neil Gales Swift Observatorywhich detects soft X-rays from the source. The team speculates that these X-rays are produced by gas heated to more than 1 million degrees Fahrenheit (600,000 degrees Celsius).
It is about three times hotter than WR 102, the hottest known star in the universe, and 100 times hotter than its surface temperature of the sun. If OGLE-mNOVA-1 occurred in our solar system, it would be 100 times brighter sun From our perspective.
These 28 events resemble a strange, so far seemingly unique cosmic explosion known as ASASSN-16oh, which was discovered in 2016 All-sky supernova automatic survey The team now considers this a million-dollar star.
“We believe that OGLE-mNOVA-11, ASASSN-16oh and 27 other objects form a new class of transient X-ray sources,” Mróz said. “We named them millinovae because their peak brightness is about a thousand times lower than that of classical novae.”
So, what exactly are Minovas, how are they created, and what makes them special?
Another kind of exploding Death Star
Despite the lack of similarities between classical novae and dwarf novae, white dwarf Do Seems to be behind the Mystery of Millinois.
These stellar remnants are created when a star with a similar mass to the Sun runs out of fuel for nuclear fusion, the process of converting hydrogen into helium at its core. As nuclear fusion proceeds in the star’s outer layers, it expands into what’s called a “subgiant” or “subgiant”red giant star”.
Unlike more massive starswhose immense gravity led to the creation of neutron star or black hole When they die, stars like our sun end their lives as smoldering white dwarfs—superdense objects indeed, but not on the same level.
While this is a quiet death for an independent star like the sun, many stars binary partners This would at least temporarily resurrect them. This is because some binary stars are close enough that the white dwarfs can start sucking matter from their companions, reviving them.
In other cases, the star and white dwarf are not close enough to initiate this mass transfer until the companion star expands into a red giant and fills half of its imaginary figure-eight shape, or its “Roche leaves”.
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It is known that white dwarfs acquiring stellar material in this way are responsible for different nova events. The most famous of which is Type Ia supernovain which a white dwarf is obliterated in a runaway thermonuclear explosion after stolen stellar material accumulates on its surface (although there are rare events known as Type Iax supernova, The white dwarf continues to exist as a crashed zombie star).
However, the team found that the optical and X-ray properties of OGLE-mNOVA-11 did not really match those of “classical” novae or Type Ia supernovae, which are caused by white dwarfs dumping stellar material. Produced by a thermonuclear explosion when above it. They also differ from the characteristics of “dwarf novae”, which occur under similar circumstances but are fainter and less destructive and therefore can be repeated.
“We think millinovae are binary systems consisting of a white dwarf and a subgiant, a star that has exhausted its core hydrogen and expanded,” Mroz said. “The two stars orbit each other. Their orbital period is only a few days, allowing material to flow from the subgiant to the white dwarf.”
The University of Warsaw researcher added that while it’s not yet clear how the millinovas’ X-ray emission is generated, he and his team have two preliminary ideas to investigate.
“According to one hypothesis, the X-rays may be produced in the belt around the white dwarf’s equator, where gas from the subgiant strikes the white dwarf’s surface,” explains Mroz. “Alternatively, the X-rays could come from a weak thermonuclear runaway on the white dwarf’s surface, triggered by material falling onto the white dwarf.
“The explosion was weak enough that little or no material was ejected from the white dwarf.”
If so, the white dwarf should continue to grow in mass, which could mean it eventually explodes into a more powerful Type Ia supernova. Therefore, millinovas may be type Ia “ancestors” – an exciting development if true.
Type Ia supernovae are very useful to astronomers because their uniform light output allows them to be used as “standard candle” is used to determine cosmic distances. Having information about when and where Type Ia supernovae are about to explode will help to better understand these events.
Mroz explained the next steps in the Minovas investigation.
“We will monitor the brightness of all 29 objects in real time and wait for the next burst to begin,” he concluded. “We also plan to conduct additional follow-up observations to better understand the physical processes responsible for these bursts.”
The team’s research results were published on December 12 in Astrophysical Journal Letters.