In search of the galaxy’s magnetic Magnetars unleash mammoth bursts of energy, but how and why? Astronomers 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 magNetar’S gIaNt flareS, magnetic field 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 neutron Six years later, in 2004, colleagues magnetic neutron stars — dense remnants because of a confluence of theory and obser- powerful. Don Dixon for Astronomy stars (later dubbed “magnetars”), they finally recognized Duncan and Thompson of massive stars that expired in fiery vational data from some of nature’s most had a hard time convincing themselves (now at the University of Texas and the supernova blasts. impressive high-energy displays. For astron- magnetic field of about a million billion powerful flare from outside our solar sys- I that the notion made sense. Five years University of Toronto, respectively) for Armed with this knowledge, researchers omers, says Thompson, the turning point (1015) gauss. (Earth’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 second, 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 Sun gives off in ence, they were given just 3 minutes to National Space Science 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 Aquila. 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 astrophysics. these questions can tell astronomers how tron star’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 rotation, 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 Milky Way | 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 pulsars (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 Magellanic Clouds 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 Bryan Gaensler, 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 pulsar (AXP) (AXP) Small SmallMagellanic Magellanic Cloud Cloud SGR 0526–66SGR 0526–66 Carina. “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 Australia’s Parkes radio inferred that SGR 1806–20’s progenitor had magnetar galactic plane, 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 star cluster; this implied a mass 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 Suns. 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 Neutron star 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 Westerlund 1 with NASA’s Chandra clusters looks strong. Moreover, both magne- massive star’s core formed in a super- least 40 times the Sun’s mass. X-ray Observatory, 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 nova. 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 luminosity and spectrum, 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. masses, 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 European Space Agency’s XMM-Newton and cover four objects. With any success, they’ll archival search through XMM-Newton and ond) neutron star whose radiation dentally, contained that same magnetar. The about 40 solar masses on the magnetar’s pre- NASA’s Chandra X-ray observatories 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.