When Neutron Stars Collide Page 32

AMATEUR ASTRONOMY: ENIGMATIC JEWELS: S&T TEST REPORT: Mega 50 Years at Riverside Observing Variable Nebulae Monochrome CCD Camera PAGE 64 PAGE 22 PAGE 58

THE ESSENTIAL GUIDE TO ASTRONOMY

GRAVITY & LIGHT: WHEN NEUTRON STARS COLLIDE PAGE 32

Astronomy from 100,000 Feet PAGE 14

How To Collimate Your Schmidt-Cass Scope PAGE 28

FEBRUARY 2018 February’s Missing Moon PAGE 84 Targets for the Hunter skyandtelescope.com PAGE 54 GRAVITATIONAL WAVES by Govert Schilling WHEN NEUTRON STARS COLLIDE

After decades of hard work, astronomers have caught their fi rst spacetime ripples from the smash-up of two dead stars. CREDIT.LEFT XXXXXXXXXXXXXXX CREDIT.LEFT

32 FEBRUARY 2018 • SKY & TELESCOPE n August 17th, a new age of astronomy began. That 400 day, the Advanced Laser Interferometer Gravita- 300 tional-Wave Observatory (LIGO) registered tiny 200 O ripples in spacetime, produced by a pair of frantically orbiting neutron stars right before they collided. But the rea-

100 (Hz) Frequency son that they herald a new age is that they didn’t come alone: Telescopes on the ground and in space detected the cosmic smash-up and the fading glow of its radioactive fi reball all across the electromagnetic spectrum (S&T: Jan. 2018, p. 10). Seconds before collision Astronomers have known neutron star binaries exist since –10 –8 –6 –4 –2 0 1974, when Russell Hulse and Joseph Taylor (then at the p THE CHIRP This spectrogram combines the signals from both LIGO University of Massachusetts, Amherst) discovered the fi rst detectors to show the characteristic sweeping “chirp” signal of a merger. one, PSR 1913+16. The two objects have an average separa- As the neutron stars came closer to each other, circling faster, they pro- tion of less than a million kilometers and an orbital period of duced higher-frequency gravitational waves, shown by the greenish line 7.75 hours. But that separation and period are shrinking with sweeping upwards, until eventually they merged (not shown). time. In fact, the binary’s very slow decrease in orbital period, measured over subsequent years by Taylor, Joel Weisberg (now How History Was Made at Carleton College), and others, perfectly matches Einstein’s Rumors about the neutron star event had circulated since prediction for energy loss due to the emission of gravitational August 18th, when Craig Wheeler (University of Texas, Aus- waves. Some 300 million years from now, the two neutron tin) tweeted: “New LIGO. Source with optical counterpart. stars in the Hulse-Taylor binary will collide and merge. Blow your sox off!” Then, on September 27th, the LIGO and This slow death dance provided the fi rst hard evidence, Virgo collaborations announced the detection of GW170814, even if it was indirect, that gravitational waves were real, and the gravitational-wave signal of a black hole merger. The dis-

2017 the discovery of PSR 1913+16 ultimately earned Hulse and covery led some to assume that the earlier rumors had been Taylor the 1993 Nobel Prize in Physics. The objects’ inexora- just hype: Because these colliding black holes shouldn’t give

NATURE ble inspiral also gave a huge boost of conﬁ dence for physi- off any light, you wouldn’t expect an optical counterpart.

AL. / cists such as Rainer Weiss (MIT) and Kip Thorne (Caltech), But in a speech October 3rd after his co-reception of the who were designing the fi rst prototypes of LIGO-like laser 2017 Nobel Prize in Physics (shared with Thorne and former interferometers. If one binary neutron star would coalesce in LIGO director Barry Barish of Caltech), Weiss confi rmed 300 million years, others might do so tomorrow — and the another announcement was coming — and wouldn’t say energetic burst of gravitational waves the collision produced should be detectable with extremely sensitive instruments 45° here on Earth. 30° On August 17th, tomorrow arrived. “We’ve been wait- 15° ing for this for 40 years,” says Ralph Wijers (University of Amsterdam, The Netherlands). 0° “I couldn’t believe my eyes,” adds LIGO lead astrophysi- cist Vicky Kalogera (Northwestern University). Back when –15° LIGO caught its fi rst event in September 2015, many team members didn’t believe it was real (S&T: Sept. 2017, p. 24). –30° But in the neutron stars’ case, it was immediately clear that 12h here was the thing they’d all been waiting for. “It’s a lot more exciting than the fi rst gravitational-wave detection.” –45° 16h Astronomers around the world share Kalogera’s elation. 8h Observing both gravitational waves and electromagnetic –60° radiation from the catastrophic coalescence of two hyper- dense neutron stars provides astronomers with a wealth –75° of new, detailed information. The new buzzword is multi- messenger astronomy, the study of objects or phenomena in h the universe using fundamentally different types of emis- 4h 20 sion. The detection of neutrinos from supernova 1987A had provided a tantalizing glimpse of this future, but as Edo p FINDING GW170817 Gravitational-wave data homed in on a Berger (Harvard) comments, “2017 August 17 will always be banana-shaped region in the Southern Hemisphere (blue lines), more remembered as the singular moment when multi-messenger specifi c than the region specifi ed by gamma-ray data (red lines). The

ILLUSTRATION: NSF / LIGO / SONOMA UNIVERSITY STATE / A. SIMONNET; CHIRP: LSC / ALEX NITZ; SKY SOURCE: MAP: IAIR DATA ARCAVI ET astronomy was born.” black dot marks the location of the kilonova, in the galaxy NGC 4993.

skyandtelescope.com • FEBRUARY 2018 33 Gravitational Waves

what. Thirteen days later on October 16th, at a large press fl ash of the most energetic electromagnetic radiation in conference at the National Press Club in Washington, D.C., nature. The European Space Agency’s Integral gamma-ray astronomers and physicists fi nally revealed their secret. observatory confi rmed the outburst. Here’s what happened. On Thursday, August 17th, at Short gamma-ray bursts were already thought to be pro- 12:41:04 UT, LIGO bagged its fi fth confi rmed gravitational- duced by colliding neutron stars. The merger would blast two wave signal, now designated GW170817. But this signal had a narrow, energetic jets of particles and radiation into space much longer duration than the fi rst four: Instead of a second (probably perpendicular to the neutron stars’ orbital plane). or less, like the earlier detections, the spacetime ripples were If one of the jets were directed toward Earth, we would see seen for roughly 100 seconds, increasing in frequency from a gamma-ray burst lasting less than two seconds or so. The a few tens of hertz to above 600 Hz before disappearing into natural question was, could GRB 170817A possibly be related the detectors’ noise. to the LIGO event that was observed just 1.7 seconds before? This is the gravitational-wave signal expected from closely Initially, astronomers had doubts. Gamma-ray bursts usu- orbiting neutron stars, with masses of about 1.2 and 1.6 ally occur at distances of billions of light-years. GRB 170817A times the mass of the Sun. Eventually, they whirled around looked about as bright to Fermi as other GRBs, so if this each other many hundreds of times per second (faster than 2-second burst had indeed occurred at a mere 130 million your kitchen blender), a fair fraction of the speed of light. light-years distance, it must have been unusually wimpy. As the “Einstein waves” emitted by the accelerating masses In principle, one might think researchers could answer drained the system of orbital energy, the neutron stars drew the question by simply looking to see if the two signals came closer together. Ultimately, the two merged. (This fi nale went from the same place on the sky. But astronomers unfortu- undetected by LIGO: The waves’ frequency was too high.) nately couldn’t precisely pinpoint the source of the gamma- From the LIGO data, astronomers determined that the colli- rays. Fermi’s “error box” measured a few tens of degrees in sion took place roughly 130 million light-years from Earth. diameter (the full Moon is only half a degree wide), and A mere 1.7 seconds after the gravitational-wave event, NASA’s Swift satellite, which sometimes can catch a Fermi at 12:41:06 UT, NASA’s Fermi Gamma-ray Space Telescope event with its more precise X-ray telescope, didn’t see any detected a 2-second gamma-ray burst — a brief, powerful X-ray emission immediately after the GRB.

Discovery Timeline

Gravitational wave LIGO, Virgo

Gamma ray Fermi, Integral, Astrosat, IPN, Inssight-HXMT, Swift, AGILE, CALET, H.E.S.S., HAWC, Konus-Wind

X-ray Swift, MAXI / GSC, NUSTAR, Chaandra, Integral

Ultraviolet Swift, HST

Optical Swope, DECam, DLT40, REM-ROOS2, HST,T, Las Cumbres, SkSkyMapper,yMapper, VISTA, MASTER, MagMagellan,ellan, Subaru, Pan-STARRS1, HCT, TZAC, LSGT, T17, Gemini-South, NTT, GRONDGROND, SOAR, ESO-VLT, KMTNet, ESO-VSV T, VIRT, SALT, CHILESCOPE, TOROS, BOOTES-5, Zadko, iTelescope, AAT, Pi of the Sky, AST3-2, ATLAS, Danish Tel, DFN, T80S, EABA ASTROPHYSICAL

Infrared REM-ROS2, VISTA, Gemini-Southh, 2MASSS, Spitzer, NTT, GROND, SOAR, NOT, ESO-VLT, Kanata Telecope, HST

Radio ATCA, VLA, ASKAP, GMRT, MWAA, LOFAR, LWA, ALMA, OVRO, EVN, e-MERLIN, MeerKAT, Parkes, SRT, Effelsberg

–100 –50 0 50 0.01 0.1 1 10 Seconds from collision Days from collision 2017 (848:L12, DOI 10.3847) (848:L12, 2017

SEQUENCE OF DISCOVERIES This timeline breaks down the discovery and follow-up observations of the neutron-star merger, relative to the inferred collision time. After the initial gravitational-wave and gamma-ray detections, the time scale is logarithmic. The colored dots represent observations from each wavelength range, with areas approximately scaled by brightness; the lines indicate when the source was detectable by at least one

telescope in that band. The names of the relevant instruments or teams appear in each section. GREGG DINDERMAN SOURCE: ABBOTT / S&T, B. P. ET AL. / JOURNAL LETTERS

34 FEBRUARY 2018 • SKY & TELESCOPE However, once the LIGO team dug the gravitational-wave signal out of the data from both 75° Hanford detectors — it took a while before the Livingston 60° signal was retrieved from the data stream because of a technical glitch — the researchers used the 45° 3-millisecond difference in arrival time to trace 30° 12h the origin of the waves back to somewhere within 15° two thin, banana-shaped strips of sky in the south- 22h 20h 18h 16h 14h 10h 8h 6h 4h 2h 0° ern celestial hemisphere. These “bananas” were extremely narrow in this particular case, thanks to –15° the long duration of the event. And one overlapped –30° with Fermi’s error box. –45°

Virgo to the Rescue –60° –75° Finding an optical counterpart to the gamma-ray burst would settle the issue, because the debris from colliding neutron stars should glow at other wavelengths. But on the basis of the LIGO and 75° Livingston gamma-ray data alone, astronomers could only 60° narrow the search area to some 60 square degrees 45°

— still far too much area to search effectively. 30° Luckily, a third gravitational-wave detector was 12h up and running: Europe’s Virgo observatory, in 15° 22h 20h 18h 16h 14h 10h 8h 6h 4h 2h Italy, had been observing in tandem with LIGO 0°

since August 1st. Using the differences in a signal’s –15° arrival time at three detectors makes it possible for –30° scientists to identify the source’s location much more precisely than with just two. In fact, they –45° had used this technique three days before, to trace –60° the black hole merger GW170814 back to a large –75° region on the border of the southern constellations Horologium and Eridanus (S&T: Jan. 2018, p. 10). Yet surprisingly, Virgo had not “triggered” on 75° Virgo GW170817. The Einstein-wave signal of the coalesc- 60° ing neutron stars arrived 22 milliseconds earlier at 45° Virgo than it did at Livingston, but it almost doesn’t show up in Virgo’s data stream — even though the 30° 12h European instrument shouldn’t have had any prob- 15° lem detecting it, given its amplitude. 22h 20h 18h 16h 14h 10h 8h 6h 4h 2h 0° It soon became clear why. Laser interferometers like LIGO and Virgo can detect gravitational –15° waves from nearly every direction. But because of –30°

their design, there are four regions of sky on the –45° instrument’s local horizon for which the detection –60° sensitivity is much lower than average. At the very –75° center of those regions are blind spots. It turned out that the source of the spacetime ripples nearly coincided with one of Virgo’s blind spots. p BLIND LEADING THE BLIND Each of the gravitational-wave detectors By combining the LIGO data with the weak has four blind spots across the sky, but the three patterns do not match. Virgo signal, astronomers were able to fence off a Based on the time delay between the signal’s arrival at LIGO’s two sites, much smaller, elongated part of the sky, with an the source of GW170817 lay somewhere along the large gray circle’s area of just some 28 square degrees. The sector lay edge; the signal’s strength narrowed the possibilities to the two green regions. Because the signal was so weak in Virgo’s data, researchers in southern Virgo and eastern Hydra and smack in realized that the source lay near one of that observatory’s blind spots, but the overlap region between LIGO’s thin “banana” not near LIGO’s — and one of Virgo’s spots abuts the region LIGO’s data

REED ESSICK / LIGO and Fermi’s error box. favored (three-site localization in purple). The red circle is the real location.

skyandtelescope.com • FEBRUARY 2018 35 Gravitational Waves

August 17, 2017 August 21, 2017 200

150

100

Brightness 50 (in millions of Suns) 0 1234 Days (since merger)

p FIRST LIGHT Left: These composite images of NGC 4993 show the kilonova (marked by yellow arrows) upon discovery on August 17th and four days later, on August 21st, when it had dramatically reddened. Both images use data from the Swope and Magellan telescopes, taken in different fi lters. The left-hand image contains the fi rst optical photons received from the afterglow, called SSS17a. Right: The kilonova reddened and faded by a factor of more than 20 in just a few days, as shown here in data from Las Cumbres Observatory telescopes.

Counterpart Search the binary star Gamma Hydrae. The source was surprisingly Now the hunt was on. Over recent years, the LIGO-Virgo bright, enough for experienced amateur astronomers to have Collaboration had signed a formal agreement with about 100 picked it out with large (16-inch) telescopes. The galaxy’s red- teams of astronomers all over the world to share this kind of shift puts it at a distance of 130 million light-years, the same information under strict embargo, meaning they couldn’t go distance as inferred from the gravitational waves. public with it before a specifi ed date. The alert system would Without doubt, here was the optical counterpart of both enable the teams to search for electromagnetic counterparts the neutron star collision that produced the gravitational- of any gravitational-wave signals with telescopes on the wave signal and the short gamma-ray burst. ground and in space, preferably right after the detection. In the subsequent days and weeks, dozens of ground- With the latest coordinates of the search area for GW170817 based telescopes and space observatories observed that point, in hand, some 70 teams trained their instruments at the including the Hubble Space Telescope, Gemini South, Keck, suspected crime scene. the European Southern Observatory’s Very Large Telescope, The 1-meter Henrietta Swope Telescope at the Las Campa- ALMA, the Chandra X-ray Observatory (it picked up X-rays nas Observatory in northern Chile was the fi rst to strike gold. some 9 days after the event), and the Very Large Array (radio The team’s success depended on a clever strategy. The LIGO waves 16 days after the crash). Researchers even searched for data provided them with a rough indication of the source’s high-energy neutrinos in data from the IceCube neutrino distance, and within the search area there were only a few detector in Antarctica and the Pierre Auger Observatory in dozen galaxies at this distance range. Astronomers with the Argentina, but they found no matches. Swope Supernova Survey rapidly checked the galaxies one by “I would think this is the most intensely observed astro- one, in order of probability, to see if they could fi nd an optical nomical event in history,” Kalogera says. The paper describing transient in any of them. the follow-up observations (unoffi cially known as the “multi- 2017 (848:L12, DOI IMAGES: 10.3847); SANTA (848:L12, CRUZ / U.C. 1M2H AND 2017 CARNEGIE Around 23:33 UT, they found a 17th-magnitude point of messenger paper”) is coauthored by some 3,600 physicists light some 10 arcseconds (7,000 light-years) northeast of the and astronomers from more than 900 institutions. According core of the lenticular (S0) galaxy NGC 4993, which lies near to some estimates, a whopping 15% of the worldwide astro- nomical community are on the author list. And it’s only one of many dozens of papers on GW170817 released on October 16th, in journals including Physical Review Letters, The Astro- physical Journal Letters, Science, and Nature. , SOURCE: SARAH WILKINSON / LCO S&T ASTROPHYSICAL JOURNAL LETTERS Striking Gold Astronomers have now observed the fading aftermath of the neutron star collision at every possible electromagnetic wavelength. The aftermath phenomenon is known as a kilonova — a bright, transient event less luminous than a supernova, but about a thousand times as bright as a normal nova and some 100 million times more luminous than the Sun. Only once before, in June 2013, have astronomers found a possible p FIRST IMAGES These are the fi rst six observations of the kilonova (three left-hand columns), all taken within 12 hours of the gravitational- kilonova in conjunction with a short gamma-ray burst, but wave signal. On the right are the fi rst detection in X-rays 9 days later that one was extremely faint, due to its distance of some 4 MULTIWAVELENGTH THUMBNAILS: ABBOTT B. P. ET AL. / (top) and in radio 16 days later. billion light-years (S&T: Nov. 2013, p. 12). OBSERVATORIES FOLEY; / RYAN GRAPH: LEAH TISCIONE /

36 FEBRUARY 2018 • SKY & TELESCOPE The kilonova — a term coined in 2010 by Vahe Petrosian David Schramm and his then-PhD student James Lattimer (Stanford), Brian Metzger (Columbia University), and oth- (Stony Brook University). ers — is basically the sizzling fi reball from the neutron star For example, Harvard’s Berger once calculated that a run- smash-up. Chunks of hot, dense nuclear matter are hurled of-the-mill neutron star merger might produce some 10 times into space, in all possible directions, with velocities easily the mass of the Moon in pure gold. Gijs Nelemans (Radboud reaching 20% or 30% the speed of light. Liberated from the University, The Netherlands) thinks it may well be much neutron stars’ extreme gravity, the debris expands, rapidly higher, up to at least a few Earth masses. Metzger agrees. losing its ultra-high density. This debris is primarily neutrons “From the optical light curve of the kilonova,” he says, “it but has some protons, too. The neutrons and protons in the appears that the collision ejected some 5% of a solar mass of resulting thermonuclear cauldron quickly combine into heavy material into space, more than enough for the formation of atomic nuclei. These nuclei capture more neutrons, making many Earth masses’ worth of gold.” them unstable and, therefore, highly radioactive. The extra So apparently, with the discovery of the counterpart of neutrons decay more slowly into protons, releasing the energy GW170817, scientists also literally struck gold. According to that makes the ejecta glow. What remains is an incredibly hot Edward van den Heuvel (University of Amsterdam), a retired expanding shell, loaded with some of the heaviest elements in expert on compact binary star evolution, astronomers have the periodic table. discovered 16 binary neutron stars so far in the Milky Way. Spectroscopic observations by the X-shooter instrument at “From this number, I estimate that neutron star collisions the Very Large Telescope and other instruments have indeed occur once every 50,000 years or so in our Milky Way Gal- indicated the existence of heavy rare earth elements (also axy,” he says. “Over the age of the Milky Way, that amounts known as lanthanides) in the fi reball that resulted from the to a few hundred thousand of these gold-spawning events in neutron star merger. According to Metzger, “It would take just one galaxy. That’s a lot of gold.” improbable fi ne-tuning to not also produce much heavier elements.” The observations thus appear to confi rm the theory To Be Determined that the majority of elements more massive than iron are A few mysteries remain, though. One is the nature of the produced by the decay of nuclear matter in the aftermath of gamma-ray signal observed by Fermi. If GRB 170817A was neutron star collisions, rather than in supernova explosions a regular gamma-ray burst, one of its jets must have been — a possibility fi rst suggested way back in 1974 by the late aimed at our home planet in order for us to see it. But in that

1 2 H Periodic Table of Cosmic Origins He

3 4 Merging neutron stars Dying low mass stars 5 6 7 8 9 10 Li Be Exploding massive stars Exploding white dwarfs B C N O F Ne 11 12 Big Bang Cosmic ray ﬁssion 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

55 56 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

87 88 Fr Ra

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

89 90 91 92

, SOURCE: JENNIFER JOHNSON / SDSS / CC BY 2.0 Ac Th Pa U ELEMENTS IN THE SOLAR SYSTEM Astronomers think that the elements in our

S&T planetary system have different cosmic origins, with many of the heaviest coming from neutron-star mergers (dark purple). Those with more than one source are divided according to the approximate proportions from each process. Technetium (Tc), pro- methium (Pc), and elements heavier than uranium don’t have stable isotopes and are

LEAH TISCIONE / / LEAH TISCIONE therefore blacked out or excluded.

skyandtelescope.com • FEBRUARY 2018 37 38 Gravitational Waves FEBRUARY 2018 surprisingly and close relatively also 980425, was GRB which burst gamma-ray long ofthe behavior strange the explain to scenario asimilar putforward had colleagues his and Wijers atEarth. straight pointed ajet from seen than level weaker atamuch rays, gamma the emit would itself cocoon butthe off-axis, still is jet the setup, this In ejecta. collision infl jets the that material of cocoon athick in orpermanently) (either temporarily stuck get jets the which in scenario, ofthis version complex down. itslowed as broaden to itstarted after jet burst gamma-ray energetic the by produced are waves radio fi radioactive the from radiation thermal is kilonova ofthe glow infrared and optical the While ber. Septem- early showupuntil didn’t which source, the from waves radio forthe holds same The angle. an from jet the at looking when natural be —would later 9days observed — X-rays in delay the that says Troja also burst. gamma-ray ofthe weakness for the explanation likely most the is that X-ray follow-up, think the Troja led who Goddard), (NASA Eleonora and Kalogera including astronomers, side? Many the from slightly burst gamma-ray the observed we maybe So detected. not was which X-ray emission, prompt produced have also should jets the Moreover, itwas. howclose given detected, they than rays gamma in powerful more times 10,000 atleast be itto expected have would astronomers case, Mansi Kasliwal (Caltech) and colleagues suggest a more amore suggest (Caltech) colleagues and Kasliwal Mansi until November2017. GW170817, itwasnotvettedandannounced black holemerger—althoughdetectedbefore See page11fortherecently announcedfi neutron-star mergerliesinthegapbetween. 5 solarmasses.Theobjectproduced bythe whereas knownblackholesgenerallylieabove Most neutron starsliebelow2solarmasses, while arrows pointupfrom alowerlimit). ranges (lineswithoutdotsare massranges, and stellar-mass blackholes,withuncertainty Shown are theknownmassesforneutron stars The Stellar Graveyard • SKY &TELESCOPE SKY X-ray blackholes ated as they drilled through the the through drilled they as ated reball, the X-rays and and X-rays the reball, fth Neutron stars detect hints of the collision’s “ring-down phase,” a brief phase,” abrief collision’s “ring-down ofthe hints detect could LIGO hole crashes, black earlier the With observed. fi The answer: tive moved out of LIGO’s detection range before the two neutron neutron two the before range outofLIGO’smoved detection hole. black merged of the fi the estimate able to were astronomers ring-down, ofthis characteristics the From zero. to dwindles rapidly waves gravitational ofthe amplitude the which in period stars. The merger remnant might be extremely informative. extremely be might remnant merger The stars. ofthese physics forthe implications have might that limit upper —an masses 2solar above just in weigh that stars tron neu- afew detected only have hole? black Astronomers mass astellar- into collapse they or did masses, solar ofafew star neutron ahyper-massive into merge stars city-sized two the Did rest? the to happened butwhat space, into ejected was mass combined oftheir fraction Asmall stars? neutron two ofthe fate the was What mystery: as-yet-unsolved another on light shed also could catastrophe cosmic ofthe site the of observations future And issue. the solve eventually may 48 hours. within wavelengths blue red to from 170817A ofGRB counterpart optical ofthe transition for the accounts neatly model this that SN 1998bw. notes also Wijers as known explosion asupernova with coincided and weak, But in the case ofGW170817, case the But in frequency wave rising the Unfortunately, the LIGO data can’t provide adefi can’t provide data LIGO the Unfortunately, observations kilonova existing ofall analysis A detailed nal stages of the merger event weren’t weren’t event merger ofthe stages nal Gravitational-wave black holes GW170817 nal nal mass ni- 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Masses

LEAH TISCIONE / S&T, LIGO SOURCES: LIGO AND VIRGO COLLABORATIONS NEUTRON STARS: F. ÖZEL AND P. FREIRE / ANNUAL REVIEWS OF ASTRONOMY AND ASTROPHYSICS 2016 BLACK HOLES: GRZEGORZ WIKTOROWICZ AND CHRIS BELCYNSKI / STELLARCOLLAPSE.ORG NASA AND ESA tion,” Lattimer says. tion,” Lattimer informa- much so yield can event one single that remarkable so-called This forth. so and stiffness, material’s the depth, with changes density its way the star, ofthe structure interior the about something physicists tells deformations resulting ofthe nitude mag- The forces. tidal mutual by squeezed and stretched be will they closer, and closer draw stars neutron two the As detail. in fi the from waves high-frequency the when especially structure, neutron-star on GW170817 as information more such provide should on Earth. alaboratory in tions condi- extreme such recreate We can’t yet millimeter. cubic ofone avolume into ofmatter tons thousand ahundred pack easily which remnants, stellar hyper-dense ofthese properties material the in interested particularly are Physicists stars. ofneutron properties the and environments, extreme in ter ofmat- behavior the relativity, general synthesis, element heavy evolution, star binary bursts, on gamma-ray revelations offuture iceberg ofthe tip proverbial the be outto turn may are, they as spectacular GW170817The observations, History Making ofasecond. afraction hole after ablack into collapsing further before rotation, fast incredibly its upby held masses, solar 2.8 of some object ahyper-massive into coalesced have fi may stars neutron two the Instead, ejecta.” much so expect youwouldn’t hole, ablack into collapse immediate X-rays,” itin hesays. detected have would we and hot, now, extremely be itwould right there star neutron massive ahyper- was there “If hole. black anew produced have must precisely). very known not are stars neutron two ofthe masses (the individual masses solar 2.8 to of 2.7 order on the mass system atotal indicate observations LIGO the though even object, merged ofthe properties the strain con- to data observational strong have donot astronomers So Kalogera. says lost, was signal the and collided, actually stars cal details are still pretty sketchy ( sketchy pretty still are details cal theoreti- although cores, their within deep ofmatter forms extreme contain may they that itis likely more the are, stars neutron smaller The across. 22kilometers than smaller be cannot they that indicates of evidence line another diameter; in 27 atmost kilometers are stars neutron the that follows it mass, ejecta estimated mass.” more the From eject and powerfully more collide they “As hesays. aresult, coalesce,” they before together closer get can stars “More compact stars. neutron two ofthe deformations tidal on the constraints fi expanding relativistically amassive, such on Earth. laboratories in experiments nuclear from constraints with consistent be to appears everything far, But so Lattimer. says GW170817 current observations, ofthe basis In principle, a detailed study of gravitational-wave signals signals ofgravitational-wave study adetailed principle, In an been had “But,” there “if headds, agrees. Metzger confi is Nelemans Moreover, he adds, the fact that the merger produced produced merger the that fact the headds, Moreover, equation of state nal stages of the merger can also be observed observed be also can merger ofthe stages nal dent enough to claim that the collision collision the that claim to enough dent has not yet been determined on the on the determined been yet not has S&T: July 2017, July p. 16). “It’s reball puts some some puts reball rst ¢ p University Press in2017. Waves, ofAstronomy and theFuture His latestbookis lives intheNetherlandsbutloves toexplore hishomeplanet. The orangedotupperleftofthegalaxy’s centeristhekilonova. which liesneartheborder oftheconstellationHydra,SeaSerpent. beyond my wildest dreams.” my wildest beyond It’s really years. 40 past the over become have observations X-ray as routine as just be may measurements tional-wave or so, gravita- 20years “Butwithin heexplains. nucleus, atomic an than less by changed arms Virgo’sand detector ofLIGO length the passed, waves Einstein the When says. hard,” he incredibly are measurements “These breakthrough. spectacular next the see to can’t wait Heuvel Van den tivity. sensi- higher even atan run, observing another yet start will Virgo and of2018, LIGO both fall the In too. fast, emerging fi emerging an as science gravitational-wave establishes discovery new the GW170817 announced, was to signifi expect astronomers observations, future With measurements. existing with line in —nicely megaparsec per second per ters 82 62 kilome- and between aHubble constant yields velocity, 4993’s NGC amplitude), with wave combined recession Einstein- observed on the (based distance galaxy’s host of the measure independent an And ofrelativity. theory of Einstein’s —confi aquadrillion in parts afew within to oflight speed atthe propagate ripples spacetime that lows itfol- waves, gravitational the and rays gamma ofthe time

NGC 4993 Sky &Telescope As 2017 Nobel laureate Barry Barish noted when when noted Barish Barry 2017As laureate Nobel arrival near-simultaneous the From there’s more. And cantly crank up the precision in this estimate. this in precision upthe crank cantly skyandtelescope.com ThisHubbleimageshowsthelenticulargalaxyNGC4993, Ripples inSpacetime:Einstein, Gravitational Contributing Editor , publishedby Harvard GOVERT SCHILLING rming predictions predictions rming

eld. And it’s And eld. • FEBRUARY 2018