aa r X i v : . [ g r- q c ] M a y MULTIMESSENGER ASTRONOMY
N.L. CHRISTENSEN a for the LIGO Scientific Collaboration and the Virgo Collaboration Physics and Astronomy, Carleton College,Northfield, Minnesota 55057, USA
Multimessenger astronomy incorporating gravitational radiation is a new and exciting fieldthat will potentially provide significant results and exciting challenges in the near future.With advanced interferometric gravitational wave detectors (LCGT, LIGO, Virgo) we willhave the opportunity to investigate sources of gravitational waves that are also expected to beobservable through other messengers, such as electromagnetic ( γ -rays, x-rays, optical, radio)and/or neutrino emission. The LIGO-Virgo interferometer network has already been usedfor multimessenger searches for gravitational radiation that have produced insights on cosmicevents. The simultaneous observation of electromagnetic and/or neutrino emission could beimportant evidence in the first direct detection of gravitational radiation. Knowledge of eventtime, source sky location, and the expected frequency range of the signal enhances our abilityto search for the gravitational radiation signatures with an amplitude closer to the noise floorof the detector. Presented here is a summary of the status of LIGO-Virgo multimessengerdetection efforts, along with a discussion of questions that might be resolved using the datafrom advanced or third generation gravitational wave detector networks. The era of gravitational wave (GW) astronomy has begun. The LIGO 1 and Virgo 2 GWinterferometric detectors have demonstrated their ability to operate at or near their initialdesign sensitivities. LIGO’s sixth scientific run, S6, and Virgo’s third scientific run, VSR3, wererecently completed; GEO 600 3 also acquired data during this period. The LIGO ScientificCollaboration (LSC) and the Virgo Collaboration have been working together in their effort todetect binary inspiral4 ,
5, burst6 ,
7, continuous wave8 ,
9, and stochastic background10 signals, aswell as GWs associated with electromagnetic (EM) events (such as a γ -ray burst, GRB) 11 , a [email protected] ulse 14, and subsequent observations by Taylor and Weisberg 15 have shown that the decayof the orbit matches perfectly with what is predicted via energy loss by GW emission. GWdetectors, like LIGO and Virgo, hope to observe GWs produced by astrophysical sources. Theobservation of these GWs will provide information about the astrophysical event. LIGO, Virgo,and other detectors will not be just GW detectors, they will also be the new generation of astro-nomical observatories. It is possible that some sources of GWs may not emit EM radiation; forexample, imagine the oscillations of a newly formed black hole. Other sources, like a supernova,will likely emit both EM radiation and GWs, and the observation of the GWs in coincidencewith EM observations could give new insight about the source. EM observations of the universeare done with radiation having frequencies above 10 MHz. On the other hand, GW observationswill be from frequencies below 10 kHz; this should provide very different information aboutthe universe. Since GWs are weakly interacting, any waves produced will traverse the universewithout being scattered or absorbed; this gives another unique opportunity for scientists to see new phenomena in our universe. In this article we discuss how LIGO and Virgo are searchingfor GW signals in coincidence with EM events. This is an example of multimessenger astron-omy. Searches are conducted for GWs at times of observed EM events (the external triggerstrategy) 11 ,
12. Since GW data from LIGO and Virgo is non-stationary 19 ,
20, finding a GWsignal candidate in coincidence with an EM transient will increase confidence that the signal isastrophysically produced, and not a spurious noise event.LIGO and Virgo have developed another strategy for finding GW events in association withEM transients. During a period of joint data collection directional information was sent to EMobservatories soon after outlier events were observed in the LIGO-Virgo data; these initial teststook place from Dec 17 2009 to Jan 8 2010, and Sep 4 to Oct 20 2010. When interesting GWtriggers were generated, numerous EM observatories have been notified within 30 minutes aspart of an EM follow-up effort 21.There are a number of possible sources for an EM signal accompanying a GW. Long GRBsare likely associated with massive star collapse 22, producing γ -rays then subsequent x-ray andoptical afterglows. A double neutron star (NS) or NS/blackhole merger could be the source ofshort GRBs 23 (with prompt γ -rays and maybe weak, isotropic afterglows). Other interestingphenomena include soft gamma repeater (SGR) flares; these are highly magnetized (10 G )neutron stars that emit γ -ray flares sporadically 29.In addition, many astrophysical events will produce detectable high and low energy neutrinos;neutrino events will be another important multimessenger area. LIGO and Virgo are currentlyworking with IceCube 24 ,
25 and ANTARES 26 ,
27 in the search for GW signals at the timethese neutrino observatories register events. It is suspected that high energy neutrinos could beemitted from long GRBs 22, short GRBs 23, low-luminosity GRBs 28, or even choked
GRBs 30.Core collapse supernovae have prompt low energy neutrino emission (along with delayed opticalsignals). In the future, with the advanced detectors, it will be fruitful to search for GWs incoincidence with low energy neutrinos from supernovae 31.Multimessenger observations could help to address and perhaps resolve a number of openquestions in astrophysics 32. For example: * What is the speed of GWs? (subluminal or superluminal?) * Can GW detectors provide an early warning to EM observers? (to allow the detection of earlylight curves.) * What is the precise origin of SGR flares? (what is the mechanism for GW and EM emissionand how are they correlated?) * What happens in a core collapse supernova before the light and neutrinos escape? * Are there electromagnetically hidden populations of GRBs? * What GRB progenitor models can we confirm or reject? * Is it possible to construct a competitive Hubble diagram based on GW standard sirens? 33 , LIGO and Virgo have already published astrophysically important multimessenger papers; whileno GWs were observed, the upper limits that have been set do provide significant constraintson the systems in question 11 , , , ,
37. Virgo and LIGO have developed methods wherebysearches are conducted for GWs at times of GRBs. By constraining the GW search to a relativelyshort period (typically tens to hundreds of seconds) the background rejection is improved, andthe sensitivity for GW detection is increased. Long GRB events are assumed to be produced bymassive star collapse, and GW searches by LIGO and Virgo use their unmodeled burst searchpipelines 12 , , ,
37. The coalescence of a neutron star - neutron star, or neutron star - blackhole binary system is suspected to be the source of the short GRBs; the LIGO-Virgo compactbinary coalescence and burst pipelines are both used to search for GWs from short GRBs 11.Even by not seeing a GW signal in association with a GRB, important astrophysical state-ments can be made. For example, LIGO and Virgo were able to set lower limits on sourcedistances for 22 short GRBs during LIGO’s fifth and Virgo’s first scientific runs (S5, VSR1)based on the assumption that these were neutron star - neutron star, or neutron star - blackhole binary coalescences 11. For the same S5/VSR1 period, LIGO and Virgo were able to setupper limits on the amplitude of GWs associated with 137 GRBs, and also place lower boundson the distance to each GRB under the assumption of a fixed energy emission in GWs; thesearch was conducted for burst waveforms ( < s ) with emission at frequencies around 150 Hz,where the LIGO - Virgo detector network had its best sensitivity 37. The average exclusiondistance for the set of GRBs was about 15 Mpc.The short-duration, hard-spectrum GRB 070201 had an EM determined sky position coin-cident with the spiral arms of the Andromeda galaxy (M31). For a short, hard GRB as thiswas, possible progenitors would be the merger of two neutron stars, a neutron star and a blackhole, or a SGR flare. No GW candidates were found in LIGO data within a 180 s long windowaround the time of this GRB 38. The results imply that a compact binary progenitor of GRB070201 was not located in M31.SGRs intermittently emit brief ( ≈ . s ) intense bursts of soft γ -rays, often with peak lumi-nosities up to 10 erg/s ; intermediate bursts with greater peak luminosities can last for seconds.Rare giant flare events can even be 1000 times brighter than common bursts 39. SGRs couldbe good sources of GWs. These magnetars are likely neutron stars with exceptionally strongmagnetic fields (up to 10 G ). The SGR bursts may be from the interaction of the stars mag-netic field with its solid crust, with crustal deformations, catastrophic cracking, excitation of thestars nonradial modes, and then emission of GWs 40. The sources are also potentially close by.LIGO has conducted searches for short-duration GWs associated with SGR bursts. There wasno evidence of GWs associated with any SGR burst in a sample consisting of the 27 Dec 2004giant flare from SGR 1806-20 36 , and 190 lesser events from SGR 1806-20 and SGR 1900+14 41.An innovative technique was also used to look for repeated GW bursts from the storm of flaresfrom SGR 1900+14; the GW signal power around each EM flare was stacked , and this yieldedper burst energy limits an order of magnitude lower than the individual flare analysis for thestorm events 42. Electromagnetic Transients
There are numerous scenarios where one could expect a GW signal to appear at the same time asan EM event. LIGO and Virgo have recently pursued two strategies to try and find coincidentGW and EM events. One is to look for GWs in LIGO and Virgo data at times when EMobservatories have registered a transient signal. In the other, LIGO and Virgo have sent timesand sky locations to numerous EM observatories with a 30 minute latency; these correspond toLIGO and Virgo triggers that have been determined to be statistically significant.
Presently there is a search of recent data from LIGO’s sixth scientific run (S6) and Virgo’s secondand third scientific runs (VSR2 and VSR3) for GWs in association with GRBs; LIGO and Virgoare examining events recorded by Swift 43 and Fermi 44. Because the time and sky position ofthe GRB are known, this has the effect of reducing the background noise, and improving thesensitivity of the GW search. LIGO and Virgo have also commenced with an effort to find GWsin association with GRBs where the GW signal extends for a time scale of many seconds, toweeks 46; the search for these intermediate duration signals has not been previously attempted.For long GRBs 22 LIGO and Virgo use their unmodeled burst pipeline 6 , , − s to +60 s . LIGO and Virgo resultsfor GRB events during S6-VSR2/3 will for forthcoming soon. During two recent periods (17 Dec 2009 to 8 Jan 2010, and 4 Sep 2010 to 20 Oct 2010, withinS6-VSR2/3) LIGO and Virgo worked with a number of EM observatories, testing a new methodwhereby GW data was rapidly analyzed 21. The time and sky location of statistically signif-icant GW triggers were sent to EM observatories within 30 minutes. Wide EM field of viewobservations are important to have, but sky location information that is as accurate as possibleis also necessary. For this effort the start of the pipeline consisted of triple coincident (fromthe two LIGO detectors and Virgo) unmodeled burst, or compact binary coalescence triggers.Within a period of 10 minutes it was determined whether the events were statistically significantor not, and whether the quality of the data from the GW observatories was good. The signif-icance above threshold for an event was determined via comparisons with background events.The target false alarm rates were 1 event per day for the initial test period, then reduced to0.25 event per day for the second test period (excluding Swift 43 and the Palomar TransientFactory 47, where the rate was 0.1 event per day). Information on known globular cluster andgalaxy locations were then used to further restrict the likely sky position of the potential source;only sources out to a distance of 50 Mpc were considered to be possible. Within 30 minutes ofthe initial registration of the potential GW event, the significant triggers were manually vettedby on-call scientific experts, and scientific monitors in the the observatory control rooms. If apotential GW trigger passed all of the tests the direction information was then sent to variousEM observatories, including a number of optical observatories: The Liverpool telescope 48, thePalomar Transient Factory 47, Pi of the Sky 53, QUEST 59, ROTSE III 54, SkyMapper 55,AROT 56, and the Zadko Telescope 57. Trigger information was also sent to the Swift X-rayobservatory 43 ,
58, and the radio network LOFAR 49. Part of the research work from LIGOand Virgo has also involved the development of image analysis procedures able to identify theEM counterparts. In the initial S6-VSR2/3 test period there were 8 potential GW events wherethe information was passed onto the EM observatories, and observations were attempted for4 of them; for the second test period there were 6 potential GW events, and 4 of them hadEM observations attempted. The full results from this EM follow-up effort will be published inthe near future. This EM follow-up effort during S6-VSR2/3 was a successful milestone, and apositive step toward the advanced detector era where the chances of GW detections will be veryenhanced, and these rapid EM observations, when coupled with the GW data, could provideimportant astrophysical information on the sources.Long and short GRB afterglows peak a few minutes after the prompt EM/GW emission50 , Kilo-novae model afterglows peak about a day after the GW emission 52, so EM observations aday after the GW trigger would be an important validation for these type of events. In orderto discriminate between the possible EM counterpart (to the GW source) from contaminatingtransients repeated observations over several nights are necessary to study the light curve.
Many of the energetic astrophysical events that could produce GWs are also expected to emitneutrinos. LIGO and Virgo are currently investigating methods to use observations of high andlow energy neutrinos to aid in the effort to observe GWs.
High energy neutrinos (HENs) are predicted to be emitted in astrophysical events that alsoproduce significant amounts of GWs, and by using the time and sky location of observed HENsthe ability to confidently identify GWs will be improved. HENs should be emitted in long GRBs;in the prompt and afterglow phases, HENs (10 − GeV ) are expected to be produced byaccelerated protons in relativistic shocks 22. HENs can also be emitted during binary mergersinvolving neutron stars 23. HENs and GWs could both come from low luminosity GRBs; thesewould be associated with an energetic population of core-collapse supernovae 28. There is aclass of events where GWs and HENs might be observed in the absence of a GRB observation,namely with choked GRBs; these could plausibly come from baryon-rich jets. Because theenvironment could be optically thick, the choked GRB events may be hidden from conventionalEM astronomy, and HENs and GWs will be the only messengers to reveal their properties 30.LIGO and Virgo are presently working with IceCube 24 ,
25 and ANTARES 26 ,
27 to see ifthere are HEN events in coincidence with GW signals in LIGO (S5 and S6) and Virgo (VSR1,VSR2 and VSR3) data. The HEN event time, sky position, and reconstructed energy informationenhance the sensitivity of the GW search. During S5 and VSR1 IceCube had 22 of its strings inoperation, while ANTARES had 5 strings. IceCube reached its full complement of 86 strings nearthe time of the end of S6 and VSR3, while ANTARES reached 12 strings. IceCube can providea neutrino trigger sky location to about 1 degree squared accuracy; then by using catalogs ofgalaxy positions, including distance, the trigger information from the LIGO and Virgo datacan provide a joint test statistic, and reduced false alarm rate. For example, there would be afalse alarm rate of about 1 in 435 years for a one-second coincidence time window and spatialcoincidence p-value threshold of 1% 60 ,
61. The size of the time window to be used about theneutrino trigger is a critical parameter in the search, and will need to be larger than 1 s; takinginto account the physical processes that could result in neutrino, γ -ray, and GW emission, itas determined that a conservative ± s time window would be appropriate 62. The resultsof this research effort be published soon.A potential problem for a neutrino - GW search occurs with long GRBs, where HENs fromrelativistic shocks might be emitted between a few hours (internal shocks 28) to a few days(external shocks 22) after the GW emission caused by core bounce 60. For these events a largertime window will be necessary (days) which will increase the false alarm rate. Better sky positionaccuracy, either through an improved neutrino detector or an expanded GW detector network(for example with the coming network of advanced detectors), would help to address this issue. Low energy neutrinos (LENs) will be an important multimessenger partner to GWs for corecollapse supernovae (CCSN). LIGO and Virgo are developing search methods involving LENs,especially for the advanced detector era. A range of 3 to 5 Mpc is admittedly at the edge ofdetectability for aLIGO and Super-K 63; at this distance the supernovae rate becomes about1/year 64. A weak coincident signal in both GWs and LENs may be convincing, especially ifthere were also an optical signal. For a galactic supernova, the neutrino signal will be large,and LIGO and Virgo would do a standard external trigger search (GRB search) with a tightcoincidence window. A CCSN produces 10-20 MeV neutrinos (all flavors) over a few 10s ofseconds. It is expected that all three neutrino flavors would be created; GWs and neutrinoswould be emitted promptly in the CCSN, while EM radiation could be delayed. The neutrinoand GW information would truly provide a probe of the physics of the core collapse 65. Theonset of the signal could probably be determined to better than 1 s. Detectors, such as Super-K 63, would detect of order 10 neutrinos for a CCSN at the galactic center. The optical (EM)signature of a CCSN could be obscured; for example, SN 2008iz in M82 was missed via opticalobservations 66. With just EM information the exact time of the core collapse bounce could beuncertain to many hours. A tight coincidence window provided by neutrino observations couldbe used to establish a correlation with GWs. In the advanced GW detector era the sensitivityrange of GW and neutrino detectors will be similar, and it is a research goal of LIGO and Virgothat LEN information will be used in association with data from the advanced GW detectors. There is an active effort by LIGO and Virgo to find GWs in coincidence with EM or neutrinocounterparts. Numerous studies have already been conducted using LIGO and Virgo data fromthe initial generation of detectors, and more results will be forthcoming soon. AdV17, aLIGO16,and GEO-HF (an upgraded GEO, with improved high frequency response)3 ,
67 should be on-linein 2014 and start trying to achieve their enhanced sensitivities. LCGT 18 could be operatingin 2015. A global network of advanced detectors will be simultaneously observing in the secondhalf of this decade, and multimessenger techniques using EM and neutrino event informationwill improve the probability for detecting GWs. By using GW, EM and neutrino observationsall together there will be a tremendous opportunity to decipher the astrophysics pertaining tomany different types of cataclysmic events in the universe.
Acknowledgments
This document has been assigned LIGO Document Number P-1100053. The work was fundedby NSF Grant PHY-0854790. The authors gratefully acknowledge the support of the UnitedStates National Science Foundation for the construction and operation of the LIGO Laboratory,the Science and Technology Facilities Council of the United Kingdom, the Max-Planck-Society,nd the State of Niedersachsen/Germany for support of the construction and operation of theGEO600 detector, and the Italian Istituto Nazionale di Fisica Nucleare and the French CentreNational de la Recherche Scientifique for the construction and operation of the Virgo detec-tor. The authors also gratefully acknowledge the support of the research by these agenciesand by the Australian Research Council, the Council of Scientific and Industrial Research ofIndia, the Istituto Nazionale di Fisica Nucleare of Italy, the Spanish Ministerio de Educaci´on yCiencia, the Conselleria d’Economia Hisenda i Innovaci´o of the Govern de les Illes Balears, theFoundation for Fundamental Research on Matter supported by the Netherlands Organisationfor Scientific Research, the Polish Ministry of Science and Higher Education, the FOCUS Pro-gramme of Foundation for Polish Science, the Royal Society, the Scottish Funding Council, theScottish Universities Physics Alliance, The National Aeronautics and Space Administration, theCarnegie Trust, the Leverhulme Trust, the David and Lucile Packard Foundation, the ResearchCorporation, and the Alfred P. Sloan Foundation.
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