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 - 10 - 30 Gpc-3 yr-1 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.