Connecting LIGO-Virgo to Observational Astronomy · Observational Astronomy Neil Gehrels NASA-GSFC Open Data Gravitational Wave Astronomy October 27, 2011 → Transient sky → Gravitational

Connecting LIGO-Virgo to Observational Astronomy

Neil Gehrels

NASA-GSFC

Open Data Gravitational Wave Astronomy

October 27, 2011

→ Transient sky

→ Gravitational wave counterpart needs → Fermi & Swift observatories → X-ray follow-up of GW triggers → GW follow-up of gamma-ray triggers

Outline

Fermi

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

Gamma-ray Burst Monitor (GBM)

Large Area Telescope (LAT)

LAT - 20 MeV - >300 GeV GBM - 8 keV - 40 MeV

Fermi Gamma Ray Mission

GBM

Field: 8 sr

Positioning: ~5˚

Launch June 2008

3 instruments Rapid slewing spacecraft Field: 2 sr GRBs: 100 yr-1

X-ray / optical follow-up Rapid target upload

Swift Mission

BAT

XRT UVOT

Launch November 2004

Short vs Long GRBs Long GRB

In non-SF and SF galaxies

No SNe detected

Possible merger

model

In SF galaxies

Accompanied by

SNe

Collapsar model well supported

GRB 990123 - SAX SF dwarf host

GRB

Short GRB

XRT

Chandra

GRB 050724 - Swift elliptical host

BH

Short GRB Info

• • GRB 051221A GRB 060614

• GRB 050709

GRB 050724

log Eγ (erg)

long GRBs short GRBs •

• Data on jet opening angles are poor and confusing Typical values:

θjet ~ 20˚ short θjet ~ 5˚ long

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.

Implications for LIGO-Virgo

k. Thorne

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 Distribution of GWs

Angular Containment of GWs

1 – cos( i )

with Amp^3/2 weighting

6%

17%

46%

23%

ALIGO-Virgo rate 15 – 45 yr-1

With GRB ~3 – 8 yr-1

With axis <40˚ ~7 – 20 yr-1

Angular Containment of GWs

20˚

Observer in beam 17% of time

Follow-up of LIGO-Virgo Triggers

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

Sakamoto et al. 2008

Short GRB X-ray Afterglows – Swift / XRT

3 hours

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.

Off-Axis Afterglow

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

Duration (sec)

Best instrument for short GRB triggers for GWs - 300 GRBs per year - 25% short GRBs - 5˚ positions are adequate

• 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.

Conclusions