When stars like our sun use up all their fuel, they shrink to form white
dwarfs. Sometimes such dead stars flare back to life in a super-hot
explosion and produce a fireball of X-ray radiation. A research team from
several German institutes including Tübingen University, and led by
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), has now been able
to observe such an explosion of X-ray light for the very first time.
"It was to some extent a fortunate coincidence, really," explains Ole König
from the Astronomical Institute at FAU in the Dr. Karl Remeis observatory in
Bamberg, who has published an article about this observation in the journal
Nature, together with Prof. Dr. Jörn Wilms and a research team from the Max
Planck Institute for Extraterrestrial Physics, the University of Tübingen,
the Universitat Politécnica de Catalunya in Barcelona and the Leibniz
Institute for Astrophysics Potsdam. "These X-ray flashes last only a few
hours and are almost impossible to predict, but the observational instrument
must be pointed directly at the explosion at exactly the right time,"
explains the astrophysicist.
The instrument in this case is the eROSITA X-ray telescope, which is
currently located one and a half million kilometers from Earth and has been
surveying the sky for soft X-rays since 2019. On July 7, 2020, it measured
strong X-ray radiation in an area of the sky that had been completely
inconspicuous four hours earlier. When the X-ray telescope surveyed the same
position in the sky four hours later, the radiation had disappeared. It
follows that the X-ray flash that had previously completely overexposed the
center of the detector must have lasted less than eight hours.
X-ray explosions such as this were predicted by theoretical research more
than 30 years ago but have never been observed directly until now. These
fireballs of X-rays occur on the surface of stars that were originally
comparable in size to the sun before using up most of their fuel made of
hydrogen, and later helium, deep inside their cores. These stellar corpses
shrink until white dwarfs remain, which are similar to Earth in size but
contain a mass that can be similar to that of our sun. "One way to picture
these proportions is to think of the sun being the same size as an apple,
which means Earth would be the same size as a pin head orbiting around the
apple at a distance of 10 meters," explains Jörn Wilms.
"These so-called novae do happen all the time, but detecting them during the
very first moments when most of the X-ray emission is produced is really
hard," adds Dr. Victor Doroshenko from Tübingen University. "Not only the
short duration of a flash is a challenge, but also the fact that the
spectrum of emitted X-rays is very soft. Soft X-rays are not very energetic
and easily absorbed by interstellar medium, so we cannot see very far in
this band, which limits the number of observable objects, be it a nova or
ordinary star. Telescopes are normally designed to be most effective in
harder X-rays where absorption is less important, and that's exactly the
reason why they would miss an event like this," concludes Victor Doroshenko.
Stellar corpses resemble gemstones
On the other hand, if you were to shrink an apple to the size of a pin head,
this tiny particle would retain the comparatively large weight of the apple.
"A teaspoon of matter from the inside of a white dwarf easily has the same
mass as a large truc" Jörn Wilms continues. Since these burnt-out stars are
mainly made up of oxygen and carbon, we can compare them to gigantic
diamonds that are the same size as Earth floating around in space. These
objects in the form of precious gems are so hot they glow white. However,
the radiation is so weak that it is difficult to detect from Earth.
That is true unless the white dwarf is accompanied by a star that is still
burning, and when the enormous gravitational pull of the white dwarf draws
hydrogen from the shell of the accompanying star. "In time, this hydrogen
can collect to form a layer only a few meters thick on the surface of the
white dwarf," explains FAU astrophysicist Jörn Wilms. In this layer, the
huge gravitational pull generates enormous pressure that is so great that it
causes the star to reignite. In a chain reaction, it soon comes to a huge
explosion during which the layer of hydrogen is blown off. The X-ray
radiation of an explosion like this is what hit the detectors of eROSITA on
July 7, 2020, producing an overexposed image.
"The physical origin of X-ray emission coming [from] white dwarf atmospheres
is relatively well understood, and we can model their spectra from first
principles and in exquisite detail. Comparison of models with observations
allows [us] then to learn basic properties of these objects such as weight,
size, or chemical composition," explains Dr. Valery Suleimanov from Tübingen
University.
"The problem in this particular case was, however, that after 30 years with
no photons we suddenly had too many, which distorted the spectral response
of eROSITA, which was designed to detect millions of very faint objects
rather than one but very bright," adds Victor Doroshenko.
"Using the model calculations we originally drew up while supporting the
development of the X-ray instrument, we were able to analyze the overexposed
image in more detail during a complex process to gain a behind the scenes
view of an explosion of a white dwarf, or nova," explains Jörn Wilms.
According to the results, the white dwarf has around the mass of our sun and
is therefore relatively large. The explosion generated a fireball with a
temperature of around 327,000 Kelvin, making it around 60 times hotter than
the sun. "These parameters were obtained by combining models of X-ray
radiation with the models of radiation emitted by very hot white dwarfs
created in Tübingen by Valery Suleimanov and Victor Doroshenko, and very
deep analysis of instrument response in a regime far outside specifications
carried out at FAU and MPE. I think it illustrates very nicely the
importance of collaboration in modern science, and wide range of expertise
within the German eROSITA consortium," adds Prof. Dr. Klaus Werner from
Tübingen University.
Since these novae run out of fuel quite quickly, they cool rapidly and the
X-ray radiation becomes weaker until it eventually becomes visible light,
which reached Earth half a day after the eROSITA detection and was observed
by optical telescopes. "A seemingly bright star then appeared, which was
actually the visible light from the explosion, and so bright that it could
be seen on the night sky by the bare eye," explains Ole König. Seemingly
"new stars" such as this one have been observed in the past and were named
"nova stella," or "new star" on account of their unexpected appearance.
Since these novae are only visible after the X-ray flash, it is very
difficult to predict such outbreaks and it is mainly down to chance when
they hit the X-ray detectors. "We were really lucky," says Ole König.
Reference:
Ole König et al, X-ray detection of a nova in the fireball phase, Nature
(2022).
DOI: 10.1038/s41586-022-04635-y
Tags:
Space & Astrophysics