A glowing blob known as "the cocoon," which appears to be inside one of the enormous gamma-ray emanations from the center of our galaxy dubbed the "Fermi bubbles," has puzzled astronomers since it was discovered in 2012.
In new research published in Nature Astronomy, we show the cocoon is caused by gamma rays emitted by fast-spinning extreme stars called "millisecond pulsars" located in the Sagittarius dwarf galaxy, which orbits the Milky Way. While our results clear up the mystery of the cocoon, they also cast a pall over attempts to search for dark matter in any gamma-ray glow it may emit.
Seeing with gamma rays
Thankfully for life on Earth, our atmosphere blocks gamma rays. These are particles of light with energies more than a million times higher than the photons we detect with our eyes.
Because our ground-level view is obscured, scientists had no idea of the richness of the gamma-ray sky until instruments were lofted into space. But, starting with the serendipitous discoveries made by the Vela satellites (put into orbit in the 1960s to monitor the Nuclear Test Ban), more and more of this richness has been revealed.
The state-of-the-art gamma-ray instrument operating today is the Fermi Gamma Ray Space Telescope, a large NASA mission in orbit for more than a decade. Fermi’s ability to resolve fine detail and detect faint sources has uncovered a number of surprises about our Milky Way and the wider cosmos.
One of these surprises emerged in 2010, soon after Fermi's launch: something in the Milky Way's center is blowing what look like a pair of giant, gamma-ray-emitting bubbles. These completely unanticipated "Fermi bubbles" cover fully 10% of the sky.
A prime suspect for the source of the bubbles is the galaxy's resident supermassive black hole. This behemoth, 4 million times more massive than the sun, lurks in the galactic nucleus, the region from which the bubbles emanate.
Most galaxies host such giant black holes in their centers. In some, these black holes are actively gulping down matter. Thus fed, they simultaneously spew out giant, outflowing “jets” visible across the electromagnetic spectrum.
Thus a question researchers asked after the discovery of the bubbles: can we find a smoking gun tying them to our Galaxy’s supermassive black hole? Soon, tentative evidence did emerge: there was a hint, inside each bubble, of a thin gamma-ray jet pointing back towards the galactic center.
With time and further data, this picture became muddied, however. While the jet-like feature in one of the bubbles was confirmed, the apparent jet in the other seemed to evaporate under scrutiny.
The bubbles looked strangely lopsided: one contained an elongated bright spot – the "cocoon" – with no counterpart in the other bubble.
The cocoon and where it comes from
Our recent work in Nature Astronomy is a deep examination of the nature of the "cocoon." Remarkably, we found this structure has nothing to do with the Fermi bubbles or, indeed, the Galaxy’s supermassive black hole.
Rather, we found the cocoon is actually something else entirely: gamma rays from the Sagittarius dwarf galaxy, which happens to be behind the southern bubble as seen from the position of Earth.
The Sagittarius dwarf, so called because its sky position is in the constellation of Sagittarius, is a "satellite" galaxy orbiting the Milky Way. It is the remnant of a much larger galaxy that the Milky Way's strong gravitational field has literally ripped apart. Indeed, stars pulled out of the Sagittarius dwarf can be found in "tails" that wrap around the entire sky.
What’s making the gamma rays?
In the Milky Way, the main source of gamma rays is when high-energy particles, called cosmic rays, collide with the very tenuous gas between the stars.
However, this process cannot explain the gamma rays emitted from the Sagittarius dwarf. It long ago lost its gas to the same gravitational harassment that pulled away so many of its stars.
So where do the gamma rays come from?
We considered several possibilities, including the exciting prospect they are a signature of dark matter, the invisible substance known only by its gravitational effects which astronomers believe makes up much of the universe. Unfortunately, the shape of the cocoon closely matches the distribution of visible stars, which rules out dark matter as the origin.
One way or another, the stars were responsible for the gamma rays. And yet: the stars of the Sagittarius dwarf are old and quiescent. What type of source amongst such a population produces gamma rays?
We are satisfied there is only one possibility: rapidly spinning objects called "millisecond pulsars." These are the remnants of particular stars, significantly more massive than the Sun, that are also closely orbiting another star.
Under just the right circumstances, such binary systems produce a neutron star – an object about as heavy as the Sun but only about 20km across – that rotates hundreds of times per second.
Because of their rapid rotation and strong magnetic field, these neutron stars act as natural particle accelerators: they launch particles at extremely high energy into space.
These particles then emit gamma rays. Millisecond pulsars in the Sagittarius dwarf were the ultimate source of the mysterious cocoon, we found.
The hunt for dark matter
Our findings shed new light – pun intended – on millisecond pulsars as sources of gamma rays in other old stellar systems.
At the same time, they also cast a pall over efforts to find evidence for dark matter via observations of other satellite galaxies of the Milky Way; unfortunately, there is a stronger "background" of gamma rays from millisecond pulsars in these systems than previously realized.
Thus, any signal they produce might not be unambiguously interpreted as due to dark matter.
The hunt for dark matter signals goes on.
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Areas of expertise
- High Energy Astrophysics; Cosmic Rays
- Galactic Astronomy
- Galactic Centre
- Fermi Bubbles
- Starburst galaxies
"Kikundi hicho cha ""cocoon"" kinajulikana kama ""Fermi bubbles"" na kinaonekana kuwa ndani ya moja ya miale ya gamma inayotokana na nyota ya Milky Way, na imekuwa ikisumbua wataalamu wa nyota tangu ilipogunduliwa mwaka 2012." "Utafiti mpya uliochapishwa katika jarida la Nature Astronomy unaonyesha kwamba ""cocoon"" hiyo hutokana na miale ya gamma inayotokezwa na nyota zinazozunguka kwa kasi, zinazoitwa ""millisecond pulsars,"" ziko katika galaksi ndogo ya Sagittarius, ambayo inazunguka njia ya Maziwa." Wakati matokeo yetu kuondoa fumbo la cocoon, wao pia kutupa shaba juu ya majaribio ya kutafuta nyenzo giza katika glow yoyote gamma-mwanga inaweza kutoa. Kwa bahati nzuri kwa ajili ya maisha duniani, angahewa letu huzuia miale ya gamma. Ni chembe za mwanga zenye nishati kubwa mara milioni moja kuliko ile ya fotoni. Kwa kuwa macho yetu ya juu ya ardhi hayakuonekana, wanasayansi hawakuwa na wazo lolote kuhusu utajiri wa anga la miale ya gamma hadi vyombo vilipopelekwa angani. Lakini kuanzia na uvumbuzi wa bahati mbaya uliofanywa na satelaiti za Vela (kuwekwa katika mzunguko katika miaka ya 1960 ili kufuatilia marufuku ya majaribio ya nyuklia), utajiri huu umefunuliwa zaidi na zaidi. "Kama mfano, ""Fermi Gamma-Ray Space Telescope"" ni chombo cha NASA ambacho kimekuwa kikifanya kazi kwa zaidi ya miaka kumi." Uwezo wa Fermi wa kutatua maelezo madogo na kugundua vyanzo vidogo umefunua mambo mengi ya kushangaza kuhusu Mfereji wetu wa Maziwa na ulimwengu mzima. "Mmoja wa mshangao huu ulijitokeza mwaka 2010, muda mfupi baada ya uzinduzi wa Fermi: kitu fulani katikati ya Mfereji wa Maziwa kinapiga kile kinachoonekana kama jozi kubwa ya Bubbles za ""Gamma-Ray-emitting.""" "Fermi Bubbles"" ni sehemu ya anga ya dunia yenye ukubwa wa asilimia 10." Mtuhumiwa mkuu wa chanzo cha Bubbles ni galaxy ya mkazi supermassive black hole. Mnyama huyo mkubwa, mwenye uzito mara milioni nne kuliko jua, yuko katika kiini cha galaksi, eneo ambalo Bubbles zinatoka. Makundi mengi ya nyota yana mashimo meusi makubwa kama hayo katikati yake. Katika baadhi yao, mashimo hayo meusi yanameza vitu. Kwa hiyo, wanapolishwa, kwa wakati mmoja hutokeza "mlipuko" mkubwa unaoonekana katika wigo wote wa umeme. "Kwa hivyo, swali ambalo watafiti waliuliza baada ya ugunduzi wa Bubbles: ""Je, tunaweza kupata bunduki ya kuvuta moshi inayowaunganisha na shimo nyeusi kubwa la Galaxy yetu?""" Baada ya muda mfupi, ushahidi wa awali ulionekana: ndani ya kila Bubble kulikuwa na ishara ya mtiririko mwembamba wa miale ya gamma uliokuwa ukielekeza nyuma kuelekea kituo cha galaksi. Hata hivyo, kadiri wakati ulivyopita na habari zaidi zilipopatikana, picha hiyo ikawa mbaya zaidi. Wakati kipengele cha jet-kama katika moja ya Bubbles ilikuwa kuthibitishwa, jet dhahiri katika nyingine ilionekana evaporate chini ya uchunguzi. Bubbles inaonekana strangely lopsided: moja ilikuwa na elongated bright spot <unk> "cocoon" <unk> na hakuna mwenzake katika Bubble nyingine. "Kukosa kwa ""cocoon"" ni jambo la kawaida katika ulimwengu wa kisayansi, lakini kwa sababu ya utafiti wetu wa hivi karibuni katika Nature Astronomy, tunaona kwamba kuna uwezekano mkubwa kwamba kuna uwezekano mkubwa wa kuwa na ""cocoon"" katika ulimwengu wa kisasa." "Kwa kushangaza, tuligundua muundo huu hauna uhusiano wowote na Bubbles za Fermi au, kwa kweli, shimo jeusi kubwa la Galaxy. """ "Kama ilivyoelezwa hapo juu, ""cocoon"" ni kitu kingine kabisa: miale ya gamma kutoka Sagittarius dwarf galaxy, ambayo hutokea kuwa nyuma ya Bubble ya Kusini kama inavyoonekana kutoka kwa nafasi ya Dunia." Nyota ndogo ya Sagittarius, inayoitwa hivyo kwa sababu iko katika kundi la nyota la Sagittarius, ni galaksi ya "satellite" inayozunguka Mstari wa Maziwa. Ni mabaki ya galaksi kubwa zaidi ambayo uwanja mkubwa wa mvuto wa Milky Way umevunja kabisa. Kwa kweli, nyota zilizochukuliwa kutoka kwa nyota ndogo ya Sagittarius zaweza kupatikana katika "mikia" inayozunguka anga lote. Ni nini kinachotokeza miale ya gamma? Katika Mwangaza wa Maziwa, chanzo kikuu cha miale ya gamma ni wakati chembe zenye nishati nyingi, zinazoitwa miale ya angani, zinapogongana na gesi nyembamba sana kati ya nyota. Hata hivyo, utaratibu huo hauwezi kueleza miale ya gamma inayotokezwa na nyota ndogo ya Sagittarius. Muda mrefu uliopita ilipoteza gesi yake kwa sababu ya mkazo uleule wa uvutano uliovuta mbali nyota zake nyingi. Kwa hiyo, miale ya gamma hutoka wapi? "Tulifikiria uwezekano kadhaa, ikiwa ni pamoja na uwezekano wa kusisimua kwamba ni saini ya ""matu ya giza,"" dutu isiyoonekana inayojulikana tu kwa athari zake za mvuto, ambayo wanaastronomia wanaamini inafanya sehemu kubwa ya ulimwengu." Kwa bahati mbaya, umbo la kifuniko hicho linalingana sana na usambazaji wa nyota zinazoonekana, jambo ambalo linakataza kwamba ni vitu visivyoonekana ambavyo vimetokana na nyota hizo. Kwa njia moja au nyingine, nyota ndizo zilizosababisha miale ya gamma. Na bado: nyota za nyota ndogo ya Sagittarius ni za zamani na hazijabadilika. Ni aina gani ya chanzo miongoni mwa idadi hiyo ya watu kinachotokeza miale ya gamma? Tuna hakika kwamba kuna uwezekano mmoja tu: vitu vinavyozunguka haraka vinavyoitwa "pulsar za milisekundi". Hizi ni mabaki ya nyota fulani, zenye uzito mkubwa kuliko Jua, ambazo pia zinazunguka nyota nyingine kwa ukaribu. Katika hali sahihi, mfumo huo wa binary hutokeza nyota ya neutroni - kitu chenye uzito kama wa jua lakini kipenyo cha kilomita 20 tu - ambacho huzunguka mamia ya mara kwa sekunde. Kwa sababu ya mzunguko wao wa haraka na uwanja wao wa nguvu wa sumaku, nyota hizi za neutroni hufanya kazi kama kiharakishi cha chembe za asili: zinatupa chembe za nishati ya juu sana angani. Kisha chembe hizo hutoa miale ya gamma. Pulsars ya milisekundi katika dwarf ya Sagittarius ilikuwa chanzo cha mwisho cha cocoon ya ajabu, tuligundua. Utafiti wetu unatoa mwangaza mpya juu ya pulsar za milisekunde kama vyanzo vya miale ya gamma katika mifumo mingine ya nyota za zamani. "Wakati huo huo, pia waliangusha juhudi za kupata ushahidi wa ""dutu nyeusi"" kupitia uchunguzi wa galaxies nyingine za satellite za Milky Way; kwa bahati mbaya, kuna ""msingi"" mkubwa wa miale ya gamma kutoka kwa pulsar za milisekundi katika mifumo hii kuliko ilivyogunduliwa hapo awali." Kwa hiyo, ishara yoyote wanayozalisha huenda isifafanuliwe waziwazi kuwa ni ya vitu vilivyo giza. Utafiti wa ishara za vitu visivyo na rangi unaendelea. Fuata masuala yote ya Expert Voices na mijadala <unk> na kuwa sehemu ya majadiliano <unk> kwenye Facebook na Twitter. Maoni yaliyoonyeshwa ni ya mwandishi na si lazima kutafakari maoni ya mchapishaji. Kupata Space.com Newsletter Breaking nafasi ya habari, updates karibuni juu ya roketi launches, skywatching matukio na zaidi! Maeneo ya utaalam - High Energy Astrophysics; Cosmic Rays - Galactic Astronomy - Galactic Center - Fermi Bubbles - Starburst galaxies