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Magnetars Unleash Mammoth Bursts of Energy, but How and Why? Astronomers Are Working to Understand These Bizarre Stellar Objects

Magnetars Unleash Mammoth Bursts of Energy, but How and Why? Astronomers Are Working to Understand These Bizarre Stellar Objects

In search of the ’s magnetic unleash mammoth bursts of , but how and why? are working to understand these bizarre stellar objects. By Steve Nadis monsters

n 1987, when Robert Duncan and Chris- the 5-day event, seemingly lumped in the objects constitute a distinct class of pul- Blasts from beyond In a ’s giant flares, lines break and reconnect, releasing a burst of topher Thompson first contemplated the fringe category. sars. They are rapidly spinning, intensely Scientists’ now believe magnetars exist energy. This process resembles solar-flare formation, except magnetars’ flares are much more existence of ultramagnetized Six years later, in 2004, colleagues magnetic neutron — dense remnants because of a confluence of theory and obser- powerful. Don Dixon for stars (later dubbed “magnetars”), they finally recognized Duncan and Thompson of massive stars that expired in fiery vational data from some of ’s most had a hard convincing themselves (now at the University of Texas and the blasts. impressive high-energy displays. For - magnetic field of about a million billion powerful flare from outside our solar sys- Ithat the notion made sense. Five years , respectively) for Armed with this knowledge, researchers omers, says Thompson, the turning point (1015) . (’s magnetic field reaches tem astronomers had ever recorded. In later, when they got their first opportunity their theoretical work on magnetars. Join- then turned their attention to a broad range came in 1998. just 0.6 gauss.) just 0.2 , the magnetar released to present their ideas at a scientific confer- ing them was Chryssa Kouveliotou of the of questions, such as: Where do these curi- In May of that year, a team led by Then, in August 1998, a powerful blast more energy than the gives off in ence, they were given just 3 minutes to National and Technology ous objects, with the most powerful mag- Kouveliotou showed that the soft gamma of gamma rays and X rays zapped Earth’s 250,000 years. make their case. Later, in 1998, at a meet- Center (NSSTC) in Huntsville, Alabama, netic fields known to exist, come from? Or, repeater (SGR) 1806–20, a pulsing X-ray outer atmosphere. The burst came from An international group of astronomers ing of the American Astronomical Society, for observations that confirmed the sce- put in other terms, why do some stars source about 50,000 light-years from SGR 1900+14, some 20,000 light-years analyzing the December 27 event supported Duncan was the last scheduled speaker at nario. The three received the Bruno Rossi become magnetars rather than black holes Earth, was likely a magnetar. Kouveliotou’s away in the direction of . Kouve- Duncan and Thompson’s hypothesis that Prize for outstanding contributions to or other kinds of neutron stars? Answering team measured the rate at which the neu- liotou and her colleagues showed, based on magnetar flares arise from twisting mag- Steve Nadis is a frequent Astronomy contributor. high-energy . these questions can tell astronomers how tron ’s spin was slowing down. A mag- the object’s spin-down rate, that it, too, netic fields. These fields warp and strain the He is a co-author of the forthcoming book The Thus, after almost two decades of abundant magnetically powered stars are, netic field could supply the drag to slow must be a magnetar. star’s crust, creating shearing stress across a Shape of Inner Space, tentatively scheduled to be doubts, astrophysicists at last acknowl- thereby providing clues to their astronomi- the star’s , but it had to be incred- On December 27, 2004, SGR 1806–20 region several kilometers long. This shear published in 2010 by W. W. Norton. edged magnetars are real. These unusual cal importance. ibly powerful. The scientists estimated a let loose again. The eruption was the most zone in some ways resembles a geological

© 2010 Kalmbach Publishing Co. This material may not be reproduced in any form 64 The | without 2009 permission from the publisher. www.Astronomy.com www.Astronomy.com 65 SGR 1900+14SGR 1900+14 fault that gives rise to an earthquake. Eventu- AXP 1EAXP 1841–045 1E 1841–045 AXP 1EAXP 1048.1–5937 1E 1048.1–5937 ally, the star’s crust cracks open. AXP 4UAXP 0142+61 4U 0142+61 SGR 1806–20SGR 1806–20 AXP CXOAXP J164710.2–455216 CXO J164710.2–455216 This work answered some questions about Two types of bursters SGR 0501+4516SGR 0501+4516 the magnetar flares. But where do these over- Magnetars break into two subclasses: soft AXP 1EAXP 1547.0–5408 1E 1547.0–5408 magnetized neutron stars come from? “You gamma repeaters (SGRs) and anomalous can’t see anything directly related to the pro- X-ray (AXPs). Scientists know of six genitor [star] by looking at the flare,” Duncan probable SGRs (four of which have been explains. Nevertheless, he adds, you can get confirmed) and 10 likely AXPs. Fourteen hints simply by seeing where astronomers of these 16 magnetars are in our galaxy; find magnetars. the Large and Small hold one each. SGR 1801–23SGR 1801–23 AXP 1EAXP 2259+586 1E 2259+586 SGR 1627–41SGR 1627–41 Digging deeper for answers As their name implies, SGRs emit “soft,” AXP XTEAXP J1810–197 XTE J1810–197 or low-energy, gamma rays. Soft gamma AXP AXAXP J1845–0258 AX J1845–0258 AXP 1RXSAXP J170849–4009101RXS J170849–400910 That’s the approach , then of rays have slightly higher energy than Harvard Smithsonian Center for Astrophys- “hard” X rays. The giant flares from AXPs LargeLarge Magellanic Magellanic Cloud Cloud ics, took when he searched the vicinity of a aren’t as intense as those from SGRs, magnetar called AXP 1E 1048.1–5937, although they’re still more intense than Soft gammaSoft gamma repeater repeater (SGR) (SGR) located roughly 9,000 light-years away in those from run-of-the-mill pulsars. AnomalousAnomalous X-ray X-raypulsar (AXP) (AXP) Small SmallMagellanic Magellanic Cloud Cloud SGR 0526–66SGR 0526–66 . “It occurred to me that the environ- — Liz Kruesi ments around magnetars might tell us some- Known and suspected magnetars thing about them,” Gaensler recalls. AXP CXOUAXP CXOU J010043.1–721134 J010043.1–721134 Gaensler’s team studied the region’s hydro- Most MAGNETAR CANDIDATES lie in the Milky Way’s disk along the gen emissions using ’s Parkes radio inferred that SGR 1806–20’s progenitor had magnetar , where the most massive stars now reside. Two other telescope and the Australia Telescope Com- to be bigger than the biggest stars still stand- came from even known magnetars are extragalactic, with one in the Large Magellanic pact Array. The astronomers found a cavity ing in the ; this implied a of though that star disap- Cloud (LMC) and the other in the Small Magellanic Cloud (SMC). carved out, they think, by stellar outflows at least 50 . peared long before X-ray Astronomy: Roen Kelly; background: 2MASS/J. Carpenter, M. Skrutskie, R. Hurt from the magnetar’s original star. Knowing Michael Muno, then of the University of astronomy began.” ASTROspeak that the star’s mass is proportional to the cav- California at Los Angeles, used a similar Both Muno and Gaensler believe the ity’s size and expansion speed, the team de- technique. While surveying the massive star association of magnetars with massive star The dense remnant of a once- duced that the progenitor star contained at cluster with NASA’s Chandra clusters looks strong. Moreover, both magne- massive star’s core formed in a super- least 40 the Sun’s mass. X-ray , he found an X-ray pulsar tars and high-mass clusters are rare, which Once Figer spots the clusters, others in Gaensler, on the other hand, argues that if . Such stars may contain more Donald Figer, then of the Space Telescope lurking in the cluster’s center. Based on the makes chance alignments less likely. the team will perform follow-up X-ray magnetars are actually spawned by stars larger than twice the Sun’s mass crammed Science Institute in Baltimore, probed the object’s and , Muno Muno’s next step was to confirm that his observations with Chandra. They’ll look for than 40 times the mass of the Sun, scientists into a sphere roughly 12 miles (20 connections between magnetars, their deemed it a probable magnetar. mystery source is, indeed, a magnetar. He pulsing X-ray sources and monitor their have already found most of them. In 2007, he, kilometers) across. , and star clusters. Prior to SGR 1806– His survey showed that Westerlund 1 is observed AXP CXO J164710.2–455216 inter- spin-down rates. So far, the team has been Muno, and Andrei Nechita, then a Harvard 20’s massive blast, Figer explored a cluster teeming with high-mass stars, all roughly the mittently over the course of a year using the granted 11 hours on Chandra — enough to University undergraduate, completed a broad Pulsar A rapidly rotating (many times a sec- filled with massive young stars that, coinci- same age. This survey placed a lower limit of ’s XMM-Newton and cover four objects. With any success, they’ll archival search through XMM-Newton and ond) neutron star whose dentally, contained that same magnetar. The about 40 solar masses on the magnetar’s pre- NASA’s Chandra X-ray to see then seek approval to plow through the Chandra data to look for periodic pulsing X- beam passes Earth on each rotation, most massive stars die first as supernovae, decessor. It’s intriguing, says Muno, “that we how its rotation changes. This enabled entire list of about 100 clusters. ray sources that might be magnetars. They creating a pulse. and less-massive stars expire later. Figer have a good idea of the mass of the star this him to estimate the object’s magnetic-field “With the discovery of a magnetar in searched through some 1,000 observations. strength. His team calculated a magnetic field Westerlund 1, we have good reason to believe “From this huge search, we didn’t find any Magnetar of 1014 gauss, which means the object is in this strategy will work,” Muno says. “Finding magnetars except for the ones we already A neutron star with a magnetic field 1,000 times stronger than normal. Westerlund 1 in visible light Westerlund 1 in X rays fact a magnetar. a second case would show we weren’t just knew about,” Gaensler says. “That tells us lucky the first time and that we can find a there aren’t many magnetars out there, or they Searching star clusters significant number of objects.” are much fainter than we thought, or they are Figer, now at the Rochester Institute of Tech- By looking at many clusters, Figer’s team often sleeping. Now we need to figure out nology, is winding up an ambitious, 5-year hopes to determine the masses of magnetar which possibility is right.” Ibrahim shares Figer’s view that the galaxy effort aimed at finding all Milky Way star progenitor stars to within about 5 solar may host 100 “live” magnetars. He notes this clusters containing at least 1,000 stars. To this masses. Such precision is crucial for estimat- 90 percent to go tally doesn’t include magnetars that have end, he’s utilizing instruments at the ing the fraction of massive stars that develop Recent findings point to a new class of tran- spun down and lost their heightened powers. Keck Observatory in Hawaii and aboard into magnetars. sient magnetars whose X-ray brightnesses He believes the elusive transient objects NASA’s Hubble and Spitzer space telescopes How many magnetars might our galaxy can vary wildly. This makes them detectable might resolve the discrepancy between the to probe our galaxy’s dusty disk. Figer pre- host? Figer believes astronomers have seen at some times and invisible at others. A team observed and expected magnetar popula- CXO J164710.2-455216 dicts the final tally will be in the vicinity of only about 10 percent of the Milky Way’s led by Alaa Ibrahim, then of George Wash- tions. Figer’s team is surveying for exactly this hundreds of clusters. massive clusters — shrouds the rest from ington University, discovered, with the aid of type of now-you-see-it, now-you-don’t object. massive STAR Cluster Westerlund 1 is home to the anomalous X-ray pulsar (AXP) CXO “Our approach,” explains Figer, “is to iden- view. By extension, this means we’ve discov- NASA’s Rossi X-ray Timing Explorer, just It’s “trying to see whether there is a popula- J164710.2-455216, a magnetar candidate. Astronomers discovered the object in 2005, and they tify the breeding grounds of magnetars — ered only about 10 percent of the galaxy’s such a magnetar in 2003, when the object’s tion of faint magnetars that hasn’t turned up hope to uncover additional magnetars as they explore the Milky Way’s most massive star clusters. these massive clusters — and then use magnetars. Because scientists have identified luminosity suddenly increased by a factor of yet,” says Muno. Visible: ESO; X ray: NASA/CXC/M. Muno et al. (UCLA) pointed observations to see how many mag- 14 magnetars, the number anticipated in the 100. A different group found another strong Peter Woods, an X-ray at netars are there.” Milky Way is around 100. candidate at about the same time. NSSTC, agrees that pointing telescopes

66 The Milky Way | 2009 www.Astronomy.com 67 1,000,000 Gamma-ray spike oversaturated detectors Magnetar candidates Inside a magnetar Atmosphere Name Location Rotation Year A MAGNETIC field 1,000 times stronger than normal elevates () discovered an ordinary pulsar to a magnetar. Drag from such an ultrastrong Outer crust field slows a star‘s spin, which makes a magnetar rotate slower SGR 0526–66 8.0 1979 Inner crust than a pulsar. Right: Astronomers think SGR 1806–20’s 2004 flare SGR 1900+14 Aquila 5.16 1979 Beam 120,000 The blast began when the magnetar’s surface cracked. Based on study of Outer core SGR 1806–20 Sagittarius 7.56 1979 the flare, scientists estimate the star’s crust is only about a mile heard ‘round SGR 1801–23 Sagittarius — 1997 (1.6 km) thick. Astronomy: Roen Kelly Inner core the world SGR 1627–41 6.4 1998 SGR 0501+4516 5.8 2008 The 2004 flare from SGR AXP 1E 2259+586 Cassiopeia 7.0 1981 1806–20 was the largest Magnetic field AXP 1E 1048.1–5937 Carina 6.4 1985 flare ever recorded from AXP 4U 0142+61 Cassiopeia 8.7 1993 115,000 outside our . First, a massive flood of AXP 1RXS J170849–400910 11.0 1997 gamma rays hit Earth’s AXP 1E 1841–045 11.8 1997 Cracked surface atmosphere; then came AXP AX J1845–0258 Aquila 7.0 1998 Axis of rotation oscillating X rays. These AXP CXOU J010043.1–721134 Small Magellanic Cloud 8.0 2002 X rays came in flashes AXP XTE J1810–197 Sagittarius 5.5 2003 spaced by 7.56 seconds AXP CXO J164710.2–455216 Ara 10.6 2005 Neutron star — the magnetar’s AXP 1E 1547.0–5408 2.1 2007 110,000 . Hot spots Astronomy: Roen Kelly, after Magnetic axis

J. Borkowski et al. toward massive stars will increase the odds of ing suggests other extragalactic magnetars massive star.” And although astronomers can finding transient magnetars. But identifying exist. A flare within 100 million light-years measure how fast a star’s exterior is spinning, magnetars in their “quiescent” state won’t be of Earth could be detected with current they still can’t correlate rotation in the outer easy, even with extensive telescope time on X-ray and gamma-ray instruments, provided layer with what’s going on inside. Chandra or XMM-Newton. “We don’t know the flare is as bright as SGR 1806–20’s 2004 Given the uncertainties at the theoretical

Photons per second 105,000 enough about transient magnetars,” he says. event, says University of California at Berke- end, perhaps our best recourse is to see where “For example, we don’t know their duty cycle ley astronomer Kevin Hurley. “Since there the observations are taking us, Woods notes. — how long they’re bright versus how long are many within this range, we Duncan agrees: “Many details of how magne- they’re dim. X-ray astronomy is only about 30 should see these events frequently,” he notes. tars behave are poorly understood. To a great years old, and we don’t know whether these NASA’s Swift , which launched in extent, theorists are now being led by the objects stay dim for 30 years or 100 years.” November 2004 to find gamma-ray bursts, observations.” That’s ironic, because when he Figer doesn’t know either, but he isn’t can potentially “open up a new field of astron- many but may still spot an appreciable num- (about 1,000 times per second) at the end of and Thompson first dreamed up magnetars, 100,000 deterred. “More magnetars are still being dis- omy — the study of extragalactic magnetars,” ber over its lifetime.” its . In Duncan and Thompson’s picture, there was little evidence the objects existed. covered, so it would seem we’re not done dis- says Duncan. For example, on September 6, The goal, simply put, is to build up statis- the core acquires its through a Thompson, for his part, finds it exhilarat- covering them,” he says. Rather than relying 2005, Swift spotted a short-duration burst that tics, says Hurley. “We’d like to know the mag- dynamo effect by converting rotational ing to track the magnetar data now coming on estimates of how many magnetars there might prove to be a magnetar flare emanating netar birth rate, which may be difficult to energy into magnetic energy. in. While the findings have supported some should be, Figer intends to look and “see from a distant galaxy, although not all scien- calculate if we’re limited to the magnetars in The best current models indicate massive of the early views he and Duncan advanced, what turns up.” tists accept this interpretation. our own galaxy.” stars rotate more rapidly as they die than they also have served to underscore the many Swift and other instruments detected a In addition to computing the fraction of when they’re young. To up as a magne- puzzles scientists must still resolve. X-ray oscillations Where are these stars hiding? November 3, 2005, burst from M81, a galaxy stars that turn into magnetars, researchers tar, however, the star has to shed much of its Fortunately, growing numbers of astrono- 95,000 The search for magnetars is not confined to about 13 million light-years away. This, says would like to ascertain these stars’ properties. mass before it goes supernova, perhaps by mers are now interested in taking on these 7.56 seconds our galaxy. In fact, the first known magnetar Duncan, was “the first identified extragalac- Why does one supernova become a expelling it in strong stellar-wind outflows. challenges. Indeed, many high-energy astro- burst, which was detected in , tic magnetar flare outside the .” magnetar, while another produces a pulsar? Stars with high content— elements physicists regard the December 27, 2004, came from the Large Magellanic Cloud, a The flare’s energy release appears comparable The question extends beyond an interest in heavier than — have stronger winds, magnetar burst as a watershed event. It’s com- to the Milky Way. This find- to SGR 1806–20’s big blast in 2004. magnetars alone. “We’re talking about the Gaensler notes. So, in addition to looking at parable in significance to supernova 1987A, For astronomers hunting magnetars out- endpoint of stellar ,” Gaensler the masses of cluster stars, scientists should the first naked-eye supernova in centuries side of our galaxy, NASA’s Swift is easily the explains. “If you want to learn about the star look at metal content, too. “You wouldn’t and the only one from which were best instrument around. Still, the satellite cycle and understand what happens when expect to find magnetars in a low- detected. Recognition of this has brought 90,000 isn’t optimized for this task. Swift’s detectors stars die, you need to understand magnetars.” cluster,” he says. more attention to magnetars. are tuned to lower-energy (spectrally Mass, the property that most determines a To Thompson, the question of the source “It’s certainly becoming more of a main- “softer”) events expected from neutron-star star’s destiny, is definitely an important part of magnetars boils down to this: “What kinds stream field, and we’re attracting talented, mergers. “With the right instruments flying,” of the equation, but it’s not the whole story. of stars end up with rapidly rotating cores?” new people all the time,” Woods explains. Duncan explains, “we could detect a magne- To make a magnetar, the core of the pre- That’s difficult to say, he notes, “since no one “That has led to some nice new results, and 0 60 120 180 240 300 tar flare each week. Swift can’t find that supernova progenitor star must rotate rapidly knows how rotation evolves in the center of a it’s only going to get better.” Time (seconds) 68 The Milky Way | 2009 www.Astronomy.com 69

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