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In ways similar to experiments in nuclear andparticle physics, high-energy astrophysics usesgamma rays and energetic charged particles toprobe processes that involve large energy trans-fers. Since its launch in 2008, the international

Fermi Gamma-Ray Space Telescope has been exploringnatural particle accelerators and the interactions of high-energy particles in the universe. Withsources ranging from thunderstorms on Earth togalaxies and exploding stars in distant parts of thecosmos, the telescope’s subjects of study are al-most as diverse as were those of the scientistwhose name it bears.

Although the universe is largely transparent togamma rays with MeV and GeV energies, Earth’s at-mosphere presents a forbidding absorption barrier. Sodirect detection of cosmic gamma rays is inherently aspace-based activity. The ways in which the gammarays interact with detector materials—by Comptonscattering, electron–positron pair production, andthe photoelectric effect—do not allow reflection or refraction. A gamma “telescope” is, therefore, animaging gamma detector that uses techniqueslargely drawn from accelerator experiments, withadaptations to operation in space.

The principal stimulus for the Fermi missionwas the wealth of scientific information gleaned inthe 1990s from NASA’s pioneering Compton Gamma-Ray Observatory, whose instruments demonstratedthe broad scope of gamma-ray astrophysics and thedynamic nature of the gamma-ray sky. (See the arti-cle by Neil Gehrels and Jacques Paul in PHYSICSTODAY, February 1998, page 26.)

Fermi, shown in figure 1, carries two instru-ments that are direct successors of the EGRET andBATSE instruments aboard Compton: ‣ The Large Area Telescope (LAT), Fermi’s pri-mary instrument, measures the arrival direction,energy, and time of individual gamma rays withenergies from about 20 MeV to over 300 GeV,which produce e+e− pairs in the LAT.1 Using silicon-strip charged-particle trackers instead of theEGRET-era spark chambers to image the trajecto-ries of the pair, the LAT achieves an effective

www.physicstoday.org November 2012 Physics Today 39

David J. Thompson, Seth W. Digel,

and Judith L. Racusin

David Thompson at NASA’s Goddard Space Flight Center in Greenbelt,Maryland, is a deputy project scientist for Fermi. Seth Digel at SLAC inMenlo Park, California, and Judith Racusin at Goddard are members ofthe Fermi Large Area Telescope collaboration.

Exploring theextreme universe with the

The Fermi orbiter, mapping the entire gamma-ray sky every three hours, monitors the cosmos for high-energy phenomena bothfleeting and enduring.

NA

SA


detector area of about a square meter for gammarays with energies above 1 GeV. And its angularresolution for a single gamma ray is finer than 1°,which allows localization of most gamma-raysources to within 10 arcminutes on the sky. For thebrightest sources, the localization is better than 1 arcmin. The LAT’s field of view exceeds 2.4 stera-dians. Overall, its sensitivity exceeds EGRET’s bymore than an order of magnitude.‣ Fermi’s Gamma-Ray Burst Monitor (GBM) is anarray of sodium iodide and bismuth germanatecrystal scintillators that views everything in the skynot occulted by Earth.2 It’s sensitive to x rays andgamma rays with energies from 8 keV to 40 MeV.The GBM uses counting rates in the different detec-tors to measure the energy spectra and celestial lo-cations of bright gamma sources, particularly brieftransients such as gamma-ray bursts. Although theGBM is physically smaller than BATSE was, itsbroader energy range and optimized triggering giveit almost the same sensitivity as its predecessor.

Unlike most telescopes, which have fields ofview that are arcminutes across and point at indi-

vidual targets, Fermi takes advantage of the instru-ments’ huge fields of view to survey the full skyevery three hours—that is, two orbits aroundEarth. That “scanning mode” of operation makespossible two simultaneous approaches to gamma-ray astrophysics:‣ Constantly deepening exposure of the gamma-ray sky. The instruments accumulate ever morephotons with each orbit, giving deeper and clearerviews of persistent sources and spatially extendedfeatures as time progresses. ‣ Time-domain gamma-ray astronomy. By contin-ually monitoring the cosmos, the instruments aresensitive to changes in the gamma-ray sky on timescales ranging from microseconds to years.

We now review examples of discoveries madewith those approaches.

The persistent gamma-ray sky

The instruments aboard Fermi improve the gamma-ray sky map with every scan, probing deeper intothe cosmos to reveal ever more distant and fainterobjects. The increased exposure is particularly im-portant at the highest energies because thosegamma rays, which carry information about themost energetic astrophysical interactions, are fewestin number.

Figure 2 shows false-color maps of the sky ingalactic coordinates at gamma energies above 1 GeVand above 10 GeV. Much of the bright equatorialband across the sky is diffuse emission from theMilky Way, produced by interactions of high-energycharged cosmic-ray particles with interstellar matterand photon fields. The relevant gamma-producingprocesses are neutral-pion decay, bremsstrahlung,and inverse Compton scattering. This diffuse galac-tic emission is a valuable source of informationabout the distribution and interactions of galacticcosmic rays, interstellar radiation, gas, and diffusivemagnetic fields.

In fact, the maps show that the gamma-ray skyis not really dark in any direction. The isotropiccomponent is the extragalactic gamma-ray back-ground. It’s attributed, in part, to unresolved dis-crete sources at great distances. But it may includea truly diffuse component.

A major surprise from the LAT survey wasthe detection of previously unknown giant struc-tures, the so-called Fermi bubbles, above andbelow the direction of the galactic center. They aremost visible at the highest LAT energies.3 And re-cently they’ve been seen to correspond to featuresof the microwave sky recorded by the orbitingPlanck telescope.4

Aspects of the LAT data indicate a unique ori-gin for the Fermi bubbles. Their energy spectrum isflatter (less steeply decreasing with increasing en-ergy) than most diffuse emission in the Milky Way.So the Fermi bubbles stand out at energies above 10 GeV, as one can see by comparing figures 2a and2b. That implies an origin different from the usualcosmic-ray interactions that dominate the MilkyWay emission.

Furthermore, the bubble edges are relativelysharp, transitioning over less than 10° on the sky,

40 November 2012 Physics Today www.physicstoday.org

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Figure 1. The Fermi Gamma-Ray Space Telescope and its two scientific instruments.11 With its silicon-strip tracker and electron–positron calorimeter, the Large Area Telescope measures arrival directions and energies of photons with energies from 20 MeV tomore than 300 GeV. The smaller Gamma-Ray Burst Monitor, with itsarray of 14 crystal gamma-ray detectors, is primarily designed todetect low-energy transient gamma-ray outbursts at photon energies from 8 keV to 40 MeV.


a

b

which suggests an origin in a particular event ratherthan a long-term diffusion process. One possibilityis that the Milky Way may once have had activity atits nucleus like that of an active galaxy, includingparticle-accelerating jets powered by accretion ofmatter onto the black hole at the center of the galaxy.So the bubble features may well have been createdwithin the past few million years.

Long-term observation of sources

The most recently published LAT catalog includes1873 sources.5 (That’s seven times as many as wereincluded in the final EGRET catalog.) The twolargest source classes are active galactic nuclei(AGNs) and pulsars within our own galaxy. Of par-ticular interest is the fact that nearly one-third of thecataloged gamma-ray sources have no obviouscounterparts at other wavelengths. Are they all sim-ply sources from which the longer-wavelengthemission has not been recognized? Or is there somereally new type of gamma-ray source? That’s an on-going study involving both multiwavelength obser-vations and theoretical modeling.

Thus far, the identified classes ofgamma-ray sources in our galaxy are asso-ciated in some way with endpoints of stellar evolution: supernova remnants,white dwarf stars, neutron stars, andblack holes. The energetic shock frontsand strong magnetic fields around suchcollapsed objects have the extreme condi-tions needed for particle acceleration andgamma-ray production.

A century after Victor Hess’s discovery of cos-mic rays (see the article by Per Carlson in PHYSICSTODAY, February 2012, page 30), supernova rem-nants (SNRs)—the expanding shells of gas, dust,and shocked plasma left behind by supernovae—remain the prime suspects for their accelerationsites. But the case is not yet closed.

Gamma rays with energies above a TeV(1012 electron volts) are largely invisible toFermi. But observations by ground-basedtelescopes such as HESS, MAGIC, and VERITAS, which record Cherenkov radia-tion from TeV gamma rays in the atmosphere,leave little doubt that SNRs can accelerate electrons to very high energies. The Fermi LATresults provide strong indications that the cosmic-ray protons also come from SNRs.

The brightest SNRs in GeV gamma rays are notthe youngest ones such as the 300-year-old Cas-siopeia A, but rather much older SNRs that producegamma rays in profusion by feeding cosmic rays todense nearby clouds of interstellar matter. Radio, x-ray, and TeV-gamma observations of SNRs, consid-ered in the context of the LAT results, suggest thatmost intragalactic cosmic-ray protons are producedby diffusive shock acceleration in multiple crossingsof moving SNR shock fronts (see PHYSICS TODAY,January 2010, page 13).

Pulsars

Appearing as highly regular pulsed point sources,pulsars are spinning, city-sized neutron stars left

behind by supernovae. They are ultrastrong mag-netic dipoles with surface fields ranging from 108

to more than 1013 gauss, rotating about an axis not aligned with the dipole. Pulsar periods, givenby the neutron star’s spin rate, range from milli -seconds to seconds.

To a distant observer, the misalignment lookslike a blinking lighthouse. And locally, it producesdynamo action. In certain regions of pulsar magne-tospheres, huge induced electric fields directly ac-celerate electrons and other charged particles tovery high energies, and those particles radiateacross much of the electromagnetic spectrum. TheFermi LAT results for more than 100 gamma-ray pul-sars show that the particle acceleration leading togamma radiation does not take place near the neu-tron star’s surface, where the fields are strongest.6

Rather, the acceleration occurs in the outer magne-tosphere, close to the limiting distance at which the


Figure 2. Brightness of the gamma-ray sky mapped over four years bythe Fermi orbiter’s Large Area Telescope for all photon energies (a) above1 GeV and (b) above 10 GeV. The maps are in galactic coordinates. Thebright equatorial strip is the Milky Way’s disk seen edge on, centered onthe galactic center. The pointlike sources at high galactic latitudes aremostly active galaxies beyond ours, while bright points nearer the galacticequator are mostly associated with pulsars and supernova remnants in theMilky Way. The diffuse “Fermi bubbles” more prominent in panel b createthe pale dumbbell-shaped feature that extends about 50° north and southfrom the vicinity of the galactic center. They may result from a relativelyrecent episode of unusual activity near the Milky Way’s central black hole.(Figure courtesy of the Fermi LAT collaboration.)



magnetic field co-rotates with the star at the speedof light.

More than 30 gamma-ray pulsars have beendiscovered by periodicity searches of the accumu-lated LAT data. They provide an entirely new win-dow on neutron stars. The absence of radio counter-parts for most of them implies that the gamma-raybeams are very broad. That suggests that they rep-resent a relatively unbiased sample of rotating neu-tron stars within 3000 light-years of Earth.

Radio astronomers searching for counterpartsof otherwise unidentified LAT sources have dis-covered more than 40 new millisecond pulsars notin globular clusters of stars. That harvest com-pares favorably with the 70 previously known.Though it seems counterintuitive, the pulsars withthe highest spin rates are the oldest. They werespun up over long times by binary partners; that’swhy they’re called recycled pulsars. The surfacemagnetic fields of those millisecond oldsters areseveral orders of magnitude smaller than those ofyoung pulsars like the thousand-year-old one inthe Crab SNR. But they all produce gamma raysin a similar fashion.

Beyond the intrinsic interest in how milli -second pulsars work, there’s also a practical interestin aid of other disciplines. Radio astronomers havebeen eager to find more fast pulsars because of theiraccurate timing properties. An array of well-timedmillisecond pulsars can in principle be used to de-tect the predicted stochastic background of gravita-tional waves in the galaxy.

Time-domain gamma-ray astronomy

Fermi’s scanning mode, coupled with the instru-ments’ ability to measure gamma-ray arrival times,

makes possible variability studies of gamma-raysources over time scales from microseconds to mul-tiple years. Blazars and other AGNs are prime ex-amples of sources with variability measured downto Fermi’s orbital time scale.

Although the Milky Way dominates thegamma-ray sky, it does so primarily because it’s ourneighborhood. The brightest extragalactic gamma-ray sources are AGNs, galaxies with cores that aremuch more luminous than our own over a broadrange of photon energies. AGNs are thought to bepowered by active accretion of matter onto theircentral supermassive black holes. Many AGNs pro-duce powerful jets of photons and particles with ul-trarelativistic bulk velocities that serve as strong,highly variable sources of collimated gamma rays.The largest single class of sources seen by the LATis blazars, AGNs for which the relativistic jets happen to be beamed within a few degrees of ourdirection.

Because the jet emission is seen across the elec-tromagnetic spectrum, study of AGNs is most pro-ductively a multiwavelength endeavor. Fermi’scontinuous monitoring of the entire sky allows thevariability studies to be correlated with observa-tions at other wavelengths, often involving a mul-titude of telescopes. The results, including rapidflaring seen on time scales of hours as well as evolution extending over years, show that blazarsexhibit significant diversity.

The detection of “orphan” blazar flares in the op-tical, x-ray, GeV, or TeV bands—without counterpartsin other wavelength bands—demonstrates that thosejets are not homogeneous flows. So does the frequentabsence of broadband correlated variability in non-orphan blazar flares. In some cases, multiwave-length observations suggest that the jet must con-tain an ordered magnetic field that maintains itsstructure over long distances. The observations alsosuggest that the particle acceleration involves mul-tiple processes, including simple shock and turbu-lent-plasma acceleration, and possibly magneticfield-line reconnections that convert stored mag-netic-field energy into strong electric fields andhence kinetic energy of charged particles.

Local surprises

In March 2010 a previously unknown brightgamma-ray source appeared in the constellationCygnus and then faded away about two weeks later.A comparison with optical and x-ray observationsshowed that the new source was something unex-pected: a gamma-ray nova. Labeled V407 Cygni, itmanifests an unusual stellar system called a symbi-otic binary, with a red giant star and a white dwarfbound in close orbit.

When enough material shed by the red giantfalls onto the white dwarf, it triggers a thermo -nuclear conflagration that produces a bright opti-cal outburst—a nova. Such novae were not ex-pected to be capable of accelerating particles to theenergies required for producing gamma rays. Butin V407 Cygni, the blast apparently generated ashock wave that accelerated material to such ener-gies. Because novae are more numerous than the

Fermi telescope

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Figure 3. A terrestrial gamma-ray flash recorded by Fermi’s Gamma-Ray Burst Monitor on 13 August 2009. The spectrum exhibits the 511-keV annihilation line resulting from positrons interacting with thematerial of the satellite. The blue curve is a model fit that takes accountof the burst monitor’s instrumental resolution. (Adapted from ref. 7.)



far more powerful supernovae, the discovery thatat least one nova is a particle accelerator suggestsanother class of cosmic-ray sources in the galaxy.

As an energetic young pulsar spins down, mostof its lost energy is carried away in a magnetizedparticle wind. That wind expands into the sur-rounding medium, decelerating as it sweeps upejecta from the supernova that gave it birth andforming a so-called termination shock front. Pulsar-wind nebulae like the Crab SNR contain particlesaccelerated by their pulsars as well as particles ac-celerated in their termination shocks. In the lattercase, the acceleration has generally been thought tobe second-order Fermi acceleration—particles accu-mulating kinetic energy in a turbulent plasma byscattering from colliding regions of magnetizedplasma.

Fermi’s GBM and LAT have both seen surpris-ing behavior from the Crab nebula, which had longbeen considered a steady calibration source becausethe Crab pulsar appeared to pump energy into thenebula at an essentially constant rate. But the GBMand other x-ray telescopes found a steady decline inthe Crab’s hard-x-ray flux on a time scale of years.And recently the LAT (along with the small Italiangamma-ray telescope AGILE) has detected intensegamma-ray flares exhibiting variability on timescales of hours (see PHYSICS TODAY, March 2011,page 12). Those flares are strongly peaked at photonenergies just below 1 GeV. Their rapid variability ar-gues against Fermi acceleration. Instead, it suggeststhe alternative mechanism of magnetic reconnec-tion, with some of the charged particles being accel-erated to PeV (1015 eV) energies.

Nearer to home, the Sun, when it flares, canbriefly become by far the brightest gamma-raysource in the sky. The particles responsible for asolar flare are probably accelerated by magnetic re-connection. In addition to producing distinctgamma-ray spectral lines by creating nuclear exci-tations, accelerated particles also interact with theambient solar medium by bremsstrahlung and pionproduction to generate gamma rays over a broadrange of energies. A solar flare on 7 March of thisyear produced gamma rays with energies up to 4 GeV. It was detectable with the LAT for more than20 hours. Fermi was able, for the first time, to localizethe gamma-ray emission to a specific active regionon the Sun’s disk where, presumably, the instigatingparticle acceleration was occurring.

Gamma-ray bursts

Visible over large intergalactic distances, gamma-ray bursts (GRBs) are thought to result from cata-clysmic stellar events such as the collapse of amassive star or the merger of two compact ob-jects—neutron stars or black holes. The outbursts,often described as the most powerful explosionssince the Big Bang, last from a fraction of a secondto minutes. But their afterglows at lower photonenergies last much longer. Sensitive high-energyobservations with the LAT and GBM have madepossible detailed studies of the temporal and spectral behavior of GRBs over seven decades ofphoton energy.

Such studies provide insight into GRB emissionmechanisms. A remarkable finding by Fermi is thediscovery of systematic behaviors that provide vitalclues to the underlying physics. The emission ofphotons with energies above 100 MeV starts afterthe onset of the keV-to-MeV burst, and the higher-energy burst lasts longer, decaying with a scale-freepower-law dependence on time. This observationimplies that the gamma radiation has at least twodistinct components.

In some bursts, additional thermal and nonther-mal components are also seen in the prompt emis-sion spectra, revealing more complexity in thoseGRBs. Fermi observations of unusually energeticGRBs and follow-up observations at longer wave-lengths reveal ultrarelativistic bulk-matter ejections.The total energy release implied by such observa-tions is too high for conventional GRB models. Per-haps there’s a separate class of hyperenergetic GRBscreated by different mechanisms.

One particularly interesting GRB has yieldedthe most stringent constraint to date on quantumtheories of gravity that predict violations ofLorentz invariance. Using sharp temporal featuresin GRB emission, one might be able to detect tinyenergy-dependent variations in the speeds of photons traveling over cosmological distances.Observing a high-redshift, short-burst GRB on 10 May 2009, Fermi recorded that the gamma-rayphotons, with energies from 8 keV to 31 GeV, arrived within one second of each other after the7-billion-year journey implied by the GRB’s red-shift (z = 0.903). For the quantum-gravity theories,that null result translates into a lower limit of1.4 × 1019 GeV (1.2 Planck masses) on the energyscale of photon velocity variation.

Cosmological GRBs have modest mimics in ourown atmosphere. Though Fermi’s principal goals arestudies of gamma rays from far away, one intriguingfinding involves terrestrial gamma-ray flashes

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Figure 4. Fraction of the cosmic-ray flux of electrons and positronsattributable to the much rarer positrons, as measured by Fermi andearlier orbiters. The Fermi data confirm and extend the surprising riseof the positron fraction with increasing energy first reported by thePAMELA team. (Adapted from ref. 8.)


(TGFs)—radiation associated with thunderstorms.First seen by the Compton orbiter and more recentlystudied by AGILE and NASA’s RHESSI satellite,TGFs are attributed to runaway electromagneticcascades produced by particles accelerated in thestrong electric fields of thunderstorms (see PHYSICSTODAY, January 2011, page 16). In addition to mil-lisecond bursts of gamma rays with energies up to100 MeV, the atmospheric cascades generate elec-trons and positrons. Trapped on geomagnetic fieldlines, the positrons can annihilate with Fermi’s material to produce characteristic 511-keV annihi-lation gamma rays. The annihilation peak is seen infigure 3, which shows the spectrum recorded7 bythe GBM during a TGF on 13 August 2009.

Fermi as a particle detector

Though designed primarily to detect gamma-rayphotons, the LAT is inherently a charged-particledetector. Judicious event selection has produced im-portant results about cosmic-ray electrons andpositrons. Measurements of electrons with energiesfrom GeV to TeV by the LAT—with far greatercounting statistics than obtained by previous instru-ments—have established that the spectrum is flatterthan expected by conventional models of cosmic-ray diffusion in the Milky Way. And those data haveset stringent limits on the anisotropy of galactic cos-mic-ray electrons. The flatter spectrum suggests asource in our local corner of the Milky Way.

Although Fermi doesn’t carry a magnet that can

distinguish a rare cosmic-ray positron from an elec-tron by the sign of its curvature, one can exploit thegeomagnetic field to that end. Thus Fermi has beenable to measure the small positron fraction of thecosmic-ray flux above the atmosphere at energiesbeyond those accessible to earlier missions.8

The results, plotted in figure 4, confirm and ex-tend results from the PAMELA satellite that showeda surprising rise in the ratio of positrons to electronswith increasing energy above about 10 GeV. Thatrise contradicts expectations from models that takeall local positrons to be secondary products of cos-mic-ray interactions in interstellar space. It suggeststhe presence of one or more nearby pulsars gener-ating high-energy electrons and positrons.

The search for dark matter

For more than half a century, astronomers haveknown from the clustering and rotation of galaxiesthat a significant fraction of the gravitating massin the universe is invisible. The evidence for adominant nonbaryonic component (without pro-tons or neutrons) of dark matter on cosmic scalesis compelling.

It’s not known what the nonbaryonic dark-matter particles are, but a great variety of cosmo-logical and astrophysical observations constrainmany of their properties. Favored candidatesnowadays are weakly interacting massive particles(WIMPs), perhaps 100 times as heavy as the proton,predicted by extensions of the standard model ofparticle physics. WIMPs are presumed to be theirown antiparticles and therefore capable of mutualannihilation.

But no such particles have as yet been discov-ered in satellite searches for WIMP-annihilationgamma rays, direct searches for WIMPs interactingin ultrasensitive underground detectors, or acceler-ator experiments that might actually produceWIMPs in collisions between high-energy beam par-ticles. The three-pronged WIMP search is deemed tobe essential for discovery and elucidation.

In the satellite search for dark matter, what theLAT does not see is important. The most stringentupper limits thus far on the interaction cross sectionand flux of WIMPs come from the observation ofdwarf spheroidal galaxies, satellites of the MilkyWay that are known to have little ongoing star for-mation and much dark matter.

Surveillance of a number of such dwarf galaxiesby the LAT has thus far revealed no clear signal ofWIMP annihilation. With plausible models and as-sumptions, the LAT null results yield upper limitson the product of the annihilation cross section andsome characteristic collision velocity.9 Those upperlimits are shown in figure 5 as a function of putativeWIMP mass. (The cross sections in the figure con-sider only readily detectable annihilation channelsinvolving the creation of b-quark pairs.) For refer-ence, the horizontal line at 3 × 10–26 cm3/s roughly in-dicates the minimum value required if WIMPs aloneare to account for the generally accepted nonbary-onic mass density of the cosmos. That requirementalready seems to exclude a WIMP mass lower thanabout 20 GeV.


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Figure 5. Upper limits (at 95% confidence) on the product ofannihilation cross section and mean collision velocity for WIMPs, asyet undiscovered weakly interacting massive particles thought tobe responsible for most of the dark matter in the cosmos. The jointlimit as a function of WIMP mass comes from null results of Fermigamma-ray observations of 10 nearby dwarf galaxies. Coloredcurves show individual upper limits from five of them. The red lineat 3 × 10−26 cm3/s represents the minimum required if WIMPs areto account for the presumed mean density of nonbaryonic darkmatter in the cosmos. (Adapted from ref. 9.)


Fermi is less than halfway through its nominal10-year mission. Its orbit is stable, the detectors haveno consumables that restrict their useful lives, andnone of the instrumentation on board has yet shownany significant degradation. Because the gamma-ray sky is dynamic on so many time scales, new re-sults and surprises are virtually assured.

Because the LAT’s integrated sensitivity above10 GeV is limited by the accumulating number of ar-riving high-energy photons rather than by the dif-fuse background, it grows almost linearly with timeat the upper end of its energy range. That growingsensitivity will be particularly important in ongoingsearches for signs of dark matter.

The top of its energy range is also where theLAT has its best angular resolution. That excellentresolution promises more detailed mapping ofsources such as SNRs in the future. The GBM isshifting to a new data-taking mode that will time-tag each individual gamma-ray photon recorded bythe burst monitor. The new mode should yield im-proved studies of transient phenomena like the ter-restrial gamma-ray flashes.

All of Fermi’s gamma-ray data are released tothe scientific community immediately, along withanalysis software and documentation, all coordi-nated with the instrument teams by the Fermi Sci-ence Support Center10 at NASA’s Goddard SpaceFlight Center. A guest-investigator program pro-vides both funding and telescope time at multiwave-length resources such as the Arecibo radio telescope,National Radio Astronomy Observatory, the Na-tional Optical Astronomy Observatory, the Suzakux-ray telescope, and the VERITAS TeV telescope.

A key lesson from the past decade has been thevalue of coordinated observations. With new facili-ties coming on line for the observation of celestialneutrinos and gravitational waves as well as pho-tons, Fermi will continue to play an essential role inthe exploration of the high-energy universe.

This article was written in cooperation with Julie McEnery,the Fermi project scientist. We thank the project staff, theinstrument teams, and the broader user community forcontinuing efforts to produce scientific results such as thosedescribed here.

References1. W. B. Atwood et al., Astrophys. J. 697, 1071 (2009).2. C. Meegan et al., Astrophys. J. 702, 791 (2009). 3. M. Su, T. R. Slatyer, D. P. Finkbeiner, Astrophys. J. 724,

1044 (2010).4. ”Planck all-sky images show cold gas and strange

haze” (13 February 2012), http://planck.caltech.edu/news20120213.html.

5. P. L. Nolan et al., Astrophys. J. Suppl. Ser. 199, 31 (2012).6. See P. Ray, Public List of LAT-detected Gamma-Ray

Pulsars (5 January 2012), https://confluence.slac.stanford.edu/display/GLAMCOG/Public+List+of+LAT-Detected+Gamma-Ray+Pulsars.

7. M. S. Briggs et al. Geophys. Res. Lett. 38, L02808 (2011).8. M. Ackermann et al., Phys. Rev. Lett. 108, 011103 (2012).9. M. Ackermann et al., Phys. Rev. Lett. 107, 241302

(2011).10. Fermi Science Support Center, http://fermi.gsfc.nasa

.gov/ssc/.11. P. F. Michelson, W. B. Atwood, S. Ritz, Rep. Prog. Phys.

73, 074901 (2010). ■

November 2012 Physics Today 45

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