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A First Targeted Search for Gravitational-Wave Bursts from Core-Collapse Supernovae in Data of First-Generation Laser Interferometer Detectors

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A First Targeted Search for Gravitational-Wave Bursts from Core-Collapse Supernovae in Data of First-Generation Laser Interferometer Detectors A First Targeted Search for Gravitational-Wave Bursts from Core-Collapse Supernovae in Data of First-Generation Laser Interferometer Detectors LIGO Scientific Collaboration and Virgo Collaboration (Dated: 3 March 2014) We present results from a search for gravitational-wave bursts in coincidence of 4 core collaplse optical supernovae observed during the fifth and sixth LIGO scientific runs, the Virgo second sci- ence run and astrowatch data. The analysis searches for transients of duration . 1 s over the frequency band 64{2048 Hz, without other assumptions on the signal waveform, polarization or specific occurrence time in the onsource window. PACS numbers: 04.80.Nn, 07.05.Kf, 95.30.Sf, 95.85.Sz I. INTRODUCTION example of such constraints. Satellite detectors observe unpredictable bursts of soft gamma-rays from neutron stars with extreme magnetic fields, known as magnetars. The physics of core-collapse supernovae cannot be fully The hypothesis that a GW arrives within ±2 s of such understood from electromagnetic signatures because the a gamma-ray burst was tested by looking for transient photons tipically leave the source after being absorbed excess power in the GW data within this signal region, and emitted many times. An important alternative a and comparing to the background [? ]. On this basis source of information can be provided by transient grav- the hypothesis of a GW signal in the data was rejected. itational waves (of duration of 1 s) which are expected . The loudest transient event in the signal region was then to be emitted by core-collapse supernovae [? ]. compared to simulated GW signals predicted by models This paper reports on a search for GW bursts occurring of the damping of non-radial global stellar modes, in coincidence of nearest optically observed Core Collapse constraining those models via estimation of upper limits supernovae explosions during the S5, S6 runs from the on emitted GW energy. LIGO scientific collaboration and VRS1 VRS2 runs from the VIRGO scientific collaboration. More specifically S5 So long as the signal region duration t is much lasted from 04 November 2005 to 30 September 2007, S6 s less than the from July 7th, 2009 and Oct. 20, 2010, VSR2 started characteristic duration of the time windows of contin- with S6 and ended on Jan.8, 2010, and VSR3 started on uous data aquisition t , model exclusion from individual Jun. 26, 2010 and ended with S6. d null detections can proceed as described above. However, This search makes use of the observed source sky loca- when ts td the situation is more complicated. In this tion, and uses constraints on the signal arrival time. case, the detectors may have outages during the signal As explained in [? ] and the references within, the region, and the data will have gaps. A short burst of gravitational wave signal to be expected from a core signal falling into a gap will be missed. collapse event is uncertain. Its overall strength and char- For this reason a new strategy different from the stan- acteristics (duration, frequency content, polarization, dard All-sky and targeted searches needs to be emplyed etc.) will depend on the dominant emission process and in order to search for GW transients in correspondence the complex structure, rotation, and thermodynamics with CCSN EM triggers. of the progenitor star. It is impossible to robustly predict the signal's detailed time series, since most of the processes driving the emission have stochastic character II. TRIGGERS: CONSIDERED OPTICAL (e.g., convection/turbulence, rotational instabilities). CORE-COLLAPSE SUPERNOVAE This is true even if the full set of parameters influencing the overal signal characteristics were known, understood, [TODO: CDO: Much of this section is incorrect and and the space spanned by them covered with detailed must be fixed by CDO. PLEASE DO NOT TOUCH.] computational models. More than 100 core-collapse supernovae were discov- Even if indirect evidence of gravitational waves (GW) ered in the optical by amateurs and professional as- has been observed in pulsars like PSR B1913+16 tronomers (e.g., [? ]) during the S5/S6 LIGO and the The global network of gravitational wave (GW) laser VSR2/VSR3 Virgo data taking periods. interferometers [? ] has yet to directly detect a signal. For our search it is important to have an estimate of Nevertheless coincident searches of GW signal in laser the time of core collapse in a considered core-collapse interferometers data when triggers from other observa- supernova. This time coincides (within ∼1 − few s; e.g., tion channels were available have already constrained [? ]) with the time of strongest GW emission. The better astrophysical models. Magnetar flares provide an the estimate of the core collapse time, the smaller the on- 2 TABLE I: Core-collapse supernovae selected as triggers for the GW search carried out in this study. Distance gives the best current estimate for the distance to the host galaxy. t1 and t2 are the UT dates delimiting the on-source window. ∆t is the temporal extent of the on-source window. LIGO/Virgo run indicates during which data taking campaign the supernova exploded, and Detectors list the first-generation interferometers active at least during parts of the on-source window. See text for details and references. [TODO: add detector coverage fractions!] SN Type Host Galaxy Distance t1 t2 ∆t LIGO/Virgo run Detectors coverage [Mpc] [days] 2011dh IIb M51 8:40 ± 0:70 2011 May 30.37 2011 May 31.89 1.52 S6E/VSR3 G1,V1 36% 2008bk IIP NGC 7793 3:53 +0:41 −0:29 2008 Mar 13.50 2008 Mar 25.14 12.64 A5 G1,H2 2008ax IIb NGC 4490 9:64 +1:38 −1:21 2008 Mar 2.19 2008 Mar 3.45 1.26 A5 G1,H2 2007gr Ic NGC 1058 10:55 ± 1:95 2007 Aug 10.39 2007 Aug 15.51 5.12 S5/VSR1 H1,H2,L1,V1 75.9% source window of detector data that must be searched ∼10−15 Mpc. Since GWs from core-collapse supernovae and the smaller the confusion background due to non- are most likely very weak and because the observable GW Gaussian non-stationary detector noise. amplitude scales with one-over-distance, nearer events The time of core collapse can be estimated based on are greatly favored. (ii) Well constrained time of explo- estimates of the explosion time and the radius of the sion leading to an uncertainty in the time of core collapse progenitor. The explosion time is defined as the time of less than ∼2 weeks. (iii) At least partial availability of at which the supernova shock breaks out of the stel- science-quality data from more than one interferometer lar surface and the EM emission of the supernova sets. in the on-source window. Basic information about the progenitor can be obtained The core-collapse supernovae making these cuts are SN from the lightcurve and spectrum of the supernova (e.g., 2007gr, SN 2008ax, SN 2008bk, and SN 2011dh. Table I [? ]). Much more information can be obtained if pre- summarizes key properties of these supernovae and we explosion imaging of the progenitor is available (e.g., [? discuss each in more detail in the following. ]). A red supergiant progenitor with a typical radius of SN 2007gr, a Type Ic supernova, was discovered on few 100 − 1500 R produces a Type IIP supernova and 2007 August 15.51 UT [? ]. A pre-discovery empty image has an explosion time of order 1 day after core collapse [? taken by KAIT [? ] on August 10.44 UT provides a base- ? ]. A Wolf-Rayet star progenitor, giving rise to a Type line constriant on the explosion time. The progenitor of Ib/c supernova, has been stripped of its hydrogen (and this supernova was a compact stripped-envelope star [? helium) envelope by stellar winds or binary interactions ?? ] through which the supernova shock propagated and has a radius of only few − 10 R and shock breakout within tens of seconds. In order to be conservative, we occurs within ∼10 − 100 s of core collapse [?? ]. add an additional hour to the interval between discovery The breakout of the supernova shock through the sur- and last non-detection and arrive at a GW on-source win- face of the progenitor star leads to a short-duration high- dow of 2007 August 10.39 UT to 2007 August 15.51 UT. luminosity burst of EM radiation with a spectral peak de- The sky location of SN 2007gr is R:A: = 02h43m27s:98, pendent on the radius of the progenitor. The burst from Decl:= +37◦2004400:7 [? ]. The host galaxy is NGC 1058. shock-breakout preceeds the rise of the optical lightcurve Schmidt et al. [? ] used the EPM to determine the dis- which occurs on a timescale of days after shock break- tance to SN 1969L, which exploded in the same galaxy. out [? ]. With the exception of very few serendipitous They found D = (10:6 + 1:9 − 1:1) Mpc. This is broadly discoveries of shock breakout bursts (e.g., [?? ]), core- consistent with the more recent Cepheid-based distance collapse supernovae are usually discovered days after ex- estimate of D = (9:29 ± 0:69) Mpc to NGC 925 by [? ]. plosion and their explosion time is constrained by one This galaxy is in the same galaxy group as NGC 1058 and or multiple of (i) the most recent non-detection, i.e., thus presumed to be in close proximity. For the purpose by the date of observation of the host galaxy without of this study, we use the conservative combined distance the supernova present, (ii) by comparison of observed estimate of D = (10:55 ± 1:95 Mpc). lightcurve and spectra with those of other supernovae for SN 2008ax, a Type IIb supernova [? ], was discov- which the explosion time is well known, (iii) by lightcurve ered by KAIT on 2008 March 3.45 UT [? ].
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