No gamma rays from a dwarf galaxy solve a ten-year-old astronomical mystery

PRI ESPL INT .SYDNEY FES9 Gamma Rays No gamma rays from a dwarf galaxy solve an astronomical mystery By Roland Crocker, Australian National University in Sydney, Sep 6 (Conversation) A glowing bubble known as a cocoon, which appears to be inside one of the gamma rays emanated Enormous cocoons from the center of our galaxy called Fermi bubbles have baffled astronomers since their discovery in 2012. In new research published in Nature Astronomy, we show that the cocoon is caused by gamma rays emitted from rapidly rotating extreme stars called millisecond pulsars located in a galaxy. Sagittarius dwarf, which revolves around the Milky Way. While our results clarify the mystery of the cocoon, they overshadow attempts to search for dark matter in any gamma-ray glow it might emit. Seeing Gamma Rays Fortunately for life on Earth, our atmosphere blocks gamma rays. These are particles of light with energies a million times higher than the photons that we detect with our eyes. Because our ground-level view is obscured, scientists had no idea the richness of the gamma-ray sky until the instruments were lifted into space. But, starting with the surprising discoveries of the Vela satellites (which entered orbit in the 1960s to monitor the nuclear test ban), more and more of this richness has been revealed. The most modern gamma-ray instrument in operation today is the Fermi-ray Gamma Space Telescope, a major mission for NASA that has been in orbit for more than a decade. Fermi’s ability to resolve fine details and discover faint sources has revealed a number of surprises about our Milky Way and the wider universe. Mysterious bubbles One such surprise arose in 2010, shortly after the Fermi launch: something in the center of the Milky Way was blowing out what looked like a pair of giant bubbles emitting gamma rays. Totally unexpected Fermi bubbles cover 10% of the sky. The prime suspect in the bubbles’ source is the galaxy’s resident supermassive black hole. This giant, four million times more massive than the Sun, lurks in the galactic core, the region from which bubbles pop. Most galaxies host such giant black holes at their centers. Sometimes these black holes are actively sucking in matter. And so they feed, simultaneously spewing out giant outward-flowing jets visible across the electromagnetic spectrum. And so the researchers asked a question after discovering the bubbles: Can we find a smoke hexagon that connects them to the supermassive black hole of our galaxy? Soon, tentative evidence emerged: inside each bubble there was a hint of a thin jet of gamma rays pointing back toward the galactic center. But with time and more data, this picture has become muddled. While the jet-like feature was confirmed in one of the bubbles, the apparent jet in the other appeared to evaporate under scrutiny. The bubbles seemed oddly unbalanced: one contained a luminous oblong cocoon spot unlike any other. The Cocoon and Where It Came From Our recent work in natural astronomy is a deep examination of the nature of the cocoon. Remarkably, we found that this structure has nothing to do with the Fermi bubbles or, in fact, with the galaxy’s supermassive black hole. Instead, we found that the cocoon is actually something else entirely: gamma rays from the Sagittarius dwarf galaxy, which just so happens to be behind the Southern Bubble as seen from Earth’s location. The Sagittarius dwarf is so named because its location in the sky is in the constellation Sagittarius, which is a satellite galaxy orbiting the Milky Way. It is a remnant of a galaxy so much larger that the Milky Way’s powerful gravitational field has literally been torn apart. In fact, stars pulled from a Sagittarius dwarf can be found in tails that wrap around the entire sky. What makes gamma rays? In the Milky Way, the main source of gamma rays is when high-energy particles, called cosmic rays, collide with the very weak interstellar gas. However, this process cannot explain the gamma rays emitted by the Sagittarius dwarf. She has long since lost her gas to the same oomph annoyances that have drawn so many of her stars. So where do gamma rays come from? We examined several possibilities, including the intriguing possibility that it is a sign of dark matter, the invisible matter known only through gravitational influences that astronomers believe makes up a large part of the universe. Unfortunately, the shape of the cocoon closely matches the distribution of visible stars, ruling out dark matter as an origin. One way or another, the stars were responsible for the gamma rays. However, Sagittarius dwarf stars are old and quiet. What type of source produces gamma rays among this population? Millisecond pulsars We are satisfied that there is only one possibility: fast-spinning objects called millisecond pulsars. These are the remnants of certain stars, much larger than the Sun, which also closely orbit another star. Under just the right conditions, such binary systems produce a neutron star as heavy as the Sun, but only about 20 km wide and which rotates hundreds of times per second. Because of their fast rotation and strong magnetic field, these neutron stars act as natural particle accelerators: they shoot particles with extremely high energy into space. These particles then emit gamma rays. We found that millisecond pulsars in the Sagittarius dwarf were the ultimate source of the mysterious cocoon. Searching for Dark Matter Our findings shed new light on millisecond pulsars as sources of gamma rays in other ancient star systems. At the same time, they cast a shadow over efforts to find evidence of dark matter through observations of other satellite galaxies of the Milky Way. Unfortunately, there is a much stronger gamma-ray background from millisecond pulsars in these systems than previously realized. Thus, any signal they emit may not be unambiguously interpreted as being caused by dark matter. The search for dark matter signals continues. (Conversational) AMS AMS 09060947 NNNN

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