Connecting LIGO-Virgo to Observational Astronomy Neil Gehrels NASA-GSFC Open Data Gravitational Wave Astronomy October 27, 2011 Outline → Transient sky → Gravitational wave counterpart needs → Fermi & Swift observatories → X-ray follow-up of GW triggers Fermi → GW follow-up of gamma-ray triggers Concentrate on NS-NS mergers and short gamma-ray bursts The Variable Gamma-ray Sky Fermi LAT GW Electromagnetic Counterparts Electromagnetic observations are essential to LIGO/Virgo science • Confirmation of GW triggers - Early GW detections are likely to be low significance - Coincidence detection of photons will lock in the discovery • Augmentation of LIGO-Virgo sensitivity - Knowing time & position of transient reduces GW search space - Detection of photons can confirm a low-significance GW blip • Physical and astronomical understanding of GW events - Arcsecond positions provided by electromagnetic observations - Redshifts, spectra and lightcurves are essential for source understanding Trigger & Follow-up Counterpart identification between GW and EM can go both ways: • Multi-wavelength (gamma-ray) trigger for deep GW searches • Multi-wavelength observations of GW event positions Fermi Gamma Ray Mission Large Area Telescope (LAT) Gamma-ray Burst Monitor (GBM) GBM Field: 8 sr Positioning: ~5˚ LAT - 20 MeV - >300 GeV GBM - 8 keV - 40 MeV Launch June 2008 Swift Mission UVOT XRT 3 instruments Rapid slewing spacecraft Field: 2 sr GRBs: 100 yr-1 BAT X-ray / optical follow-up Rapid target upload Launch November 2004 Short GRB Long GRB Short vs Long GRBs GRB 050724 - Swift GRB 990123 - SAX elliptical host SF dwarf host Chandra XRT GRB In non-SF In SF and SF galaxies galaxies No SNe detected Accompanied by SNe Possible merger model Collapsar model BH well supported Short GRB Info • long GRBs Data on jet opening angles • short GRBs are poor and confusing GRB 050724 Typical values: θjet ~ 20˚ short θjet ~ 5˚ long GRB 050709 • • GRB 051221A GRB 060614 • log Eγ (erg) Implications for LIGO-Virgo Range for short GRB rates is -3 -1 - 10 - 30 Gpc yr based on BAT - GBM For 20˚ beaming angle, 1/fb ~ 15. Assuming all short GRBs are due to NS-NS mergers, merger rate is 150 to 450 Gpc-3 yr-1 With sensitivity distance of ~300 Mpc (0.1 Gpc3), ALIGO-Virgo detection rate will be 15 to 45 yr-1 Note: The ALIGO-Virgo NS-NS merger detection limit is 445 Mpc on-axis event (Abdie et al. 2010). For random orientation ~200 Mpc. k. Thorne Angular Distribution of GWs For mergers, GWs are more intense along the orbital axis of the merger, which is the GRB beaming direction. Relative amplitudes of two polarizations: h_plus = (1+cos2(incl)) / 2 h_cross = cos (incl) RMS total amplitude = sqrt [ (h_plus2+h_cross2) ] (comm: P. Sutton) Angular Containment of GWs with Amp^3/2 weighting ALIGO-Virgo rate 15 – 45 yr-1 1 – cos( i ) 46% With GRB ~3 – 8 yr-1 23% 17% With axis <40˚ 6% ~7 – 20 yr-1 Angular Containment of GWs 20˚ Observer in beam 17% of time Follow-up of LIGO-Virgo Triggers Short GRB X-ray Afterglows – Swift / XRT 3 hours Sakamoto et al. 2008 ALIGO merger detection distance ~300 Mpc (on axis) or z~0.07 Typical short GRB distance is z~0.5 XRT detection at z=0.5 at 3 hours is 20σ XRT detection at z=0.07 at 3 hours is 1000σ highly significant detections Off-Axis Afterglow Off-axis emission of electromagnetic radiation is currently under study. Afterglow signal on-axis for ALIGO-Virgo events is so intense that prospects are good for detection of off-axis afterglow for most events. Fermi GBM for Short GRB Triggers Gamma-ray Burst Monitor • Views entire unocculted sky • 12 NaI: 8 keV – 1 MeV • 2 BGO: 150 keV – 40 MeV GBM Rates Best instrument for short GRB triggers for GWs - 300 GRBs per year - 25% short GRBs - 5˚ positions are adequate Duration (sec) Conclusions • Short GRBs are thought to be due to NS-NS mergers - Uncertainties remain on origin, rates & beaming • Estimates of rates and beaming imply ALIGO-Virgo rates of 15 – 45 yr-1 • GW signal is maximum in same direction as γ-ray beam • Roughly 17% of GW detections will have accompanying GRBs • With rapid (few hour) follow-up, afterglow will be bright - Probably true even for off-axis events. • Rapid announcements of GW transients is essential for follow-up and full science exploitation. .