This text was initially revealed at The Conversation. (opens in new tab) The publication contributed the article to House.com’s Expert Voices: Op-Ed & Insights.
A glowing blob referred to as “the cocoon,” which seems to be inside one of many huge gamma-ray emanations from the middle of our galaxy dubbed the “Fermi bubbles,” has puzzled astronomers because it was found in 2012.
In new research (opens in new tab) revealed in Nature Astronomy, we present the cocoon is brought on by gamma rays emitted by fast-spinning excessive stars referred to as “millisecond pulsars” positioned within the Sagittarius dwarf galaxy, which orbits the Milky Way. Whereas our outcomes clear up the thriller of the cocoon, in addition they solid a pall over makes an attempt to seek for dark matter in any gamma-ray glow it could emit.
Seeing with gamma rays
Fortunately for all times on Earth, our environment blocks gamma rays. These are particles of sunshine with energies greater than 1,000,000 instances larger than the photons we detect with our eyes.
As a result of our ground-level view is obscured, scientists had no thought of the richness of the gamma-ray sky till devices had been lofted into space. However, beginning with the serendipitous discoveries made by the Vela satellites (put into orbit within the Sixties to observe the Nuclear Take a look at Ban), increasingly of this richness has been revealed.
The state-of-the-art gamma-ray instrument working right this moment is the Fermi Gamma Ray Space Telescope, a big NASA mission in orbit for greater than a decade. Fermi’s means to resolve positive element and detect faint sources has uncovered a variety of surprises about our Milky Way and the broader cosmos.
Mysterious bubbles
One in every of these surprises emerged in 2010 (opens in new tab), quickly after Fermi’s launch: one thing within the Milky Way’s heart is blowing what appear to be a pair of large, gamma-ray-emitting bubbles. These utterly unanticipated “Fermi bubbles” cowl totally 10% of the sky.
A first-rate suspect for the supply of the bubbles is the galaxy’s resident supermassive black hole. This behemoth, 4 million instances extra large than the sun, lurks within the galactic nucleus, the area from which the bubbles emanate.
Most galaxies host such large black holes of their facilities. In some, these black holes are actively gulping down matter. Thus fed, they concurrently spew out large, outflowing “jets” seen throughout the electromagnetic spectrum.
Thus a query researchers requested after the invention of the bubbles: can we discover a smoking gun tying them to our Galaxy’s supermassive black hole? Quickly, tentative proof did emerge: there was a hint (opens in new tab), inside every bubble, of a skinny gamma-ray jet pointing again in direction of the galactic heart.
With time and additional information, this image turned muddied, nonetheless. Whereas the jet-like function in one of many bubbles was confirmed, the obvious jet within the different appeared to evaporate under scrutiny (opens in new tab).
The bubbles regarded surprisingly lopsided: one contained an elongated vivid spot – the “cocoon” – with no counterpart within the different bubble.
Learn extra: Astronomers have detected one of the biggest black hole jets in the sky (opens in new tab)
The cocoon and the place it comes from
Our recent work (opens in new tab) in Nature Astronomy is a deep examination of the character of the “cocoon.” Remarkably, we discovered this construction has nothing to do with the Fermi bubbles or, certainly, the Galaxy’s supermassive black hole.
Somewhat, we discovered the cocoon is definitely one thing else totally: gamma rays from the Sagittarius dwarf galaxy, which occurs to be behind the southern bubble as seen from the place of Earth.
The Sagittarius dwarf, so referred to as as a result of its sky place is within the constellation of Sagittarius, is a “satellite” galaxy orbiting the Milky Way. It’s the remnant of a a lot bigger galaxy that the Milky Way’s robust gravitational discipline has actually ripped aside. Certainly, stars pulled out of the Sagittarius dwarf might be present in “tails” that wrap across the whole sky.
What’s making the gamma rays?
Within the Milky Way, the principle supply of gamma rays is when high-energy particles, referred to as cosmic rays, collide with the very tenuous gasoline between the celebs.
Nonetheless, this course of can not clarify the gamma rays emitted from the Sagittarius dwarf. It way back misplaced its gasoline to the identical gravitational harassment that pulled away so lots of its stars.
So the place do the gamma rays come from?
We thought of a number of potentialities, together with the thrilling prospect they’re a signature of dark matter, the invisible substance recognized solely by its gravitational results which astronomers imagine makes up a lot of the universe. Sadly, the form of the cocoon intently matches the distribution of seen stars, which guidelines out dark matter because the origin.
A technique or one other, the celebs had been accountable for the gamma rays. And but: the celebs of the Sagittarius dwarf are outdated and quiescent. What sort of supply amongst such a inhabitants produces gamma rays?
Millisecond pulsars
We’re happy there is just one chance: quickly spinning objects referred to as “millisecond pulsars.” These are the remnants of specific stars, considerably extra large than the Solar, which are additionally intently orbiting one other star.
Below simply the best circumstances, such binary programs produce a neutron star – an object about as heavy because the Solar however solely about 20km throughout – that rotates a whole bunch of instances per second.
Due to their fast rotation and powerful magnetic discipline, these neutron stars act as pure particle accelerators: they launch particles at extraordinarily excessive vitality into space.
These particles then emit gamma rays. Millisecond pulsars within the Sagittarius dwarf had been the last word supply of the mysterious cocoon, we discovered.
Learn extra: This newly discovered neutron star might light the way for a whole new class of stellar object (opens in new tab)
The hunt for dark matter
Our findings shed new mild – pun supposed – on millisecond pulsars as sources of gamma rays in different outdated stellar programs.
On the similar time, in addition they solid a pall over efforts to seek out proof for dark matter through observations of different satellite galaxies of the Milky Way; sadly, there’s a stronger “background” of gamma rays from millisecond pulsars in these programs than beforehand realized.
Thus, any sign they produce won’t be unambiguously interpreted as as a result of dark matter.
The hunt for dark matter indicators goes on.
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