Short GRBs and Mergers: Astrophysical constraints on a BH-NS and NS-NS origin
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Transcript of Short GRBs and Mergers: Astrophysical constraints on a BH-NS and NS-NS origin
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LIGO-XXX
Short GRBs and Mergers: Astrophysical constraints on
a BH-NS and NS-NS origin
Richard O’Shaughnessy[V. Kalogera, C. Kim, K. Belczynski, T. Fragos]
PSU, May 14, 2007
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LIGO-XXX
Context?
If you want to…• Test GR soon (=LIGO)
– Need high SNR, clean merger waveforms– EM coincidence helps search (orientation, time)
• Understand short GRBs– May not all be mergers– Merger waves or absence distinguishes
• Improve models for binary stellar evolution– Merger rate is constraint– Short GRB observations could be a constraint…– Consistency with non-GRB observations?
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LIGO-XXX
Outline• Short GRBs : A Review
– Intersection with LIGO
• Population synthesis predictions– Milky Way astro-ph/0610076; 0609465
– Universe
• Could short GRBs be mergers? – Detection rates consistent?– Redshift distribution, hosts?
• Where are we now? What happens next? – EM: Swift + GLAST and biases– GW: Initial and Enhanced LIGO?
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LIGO-XXX
Short GRBs: A Review
Short GRBs• One of two (?) classes• Cosmological distances
– Low redshift selection effect?• Hard: often peaks out of band• Flux power law
dP/dL ~ L-2
--> most (probably) unseen
Many sources at limitof detector (BATSE)
[Berger et al, astro-ph/0611128]
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LIGO-XXX
Short GRBs: A ReviewMerger motivation?• No SN structure in afterglow
• In both old, young galaxies
•Occasional host offsets
GRB 051221 (Soderberg et al 2006)• Young NSs are some (known) Energetics suggest not all
GRB 050709 (Fox et al Nature 437 845)
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LIGO-XXX
Short GRBs: Review
• Gravitational waves essential– Central engine? : Certainty requires gravitational waves
• See inspiral• Check masses
– Coincident observation powerful [e.g., merger-burst delay time; opening angle constraints; masses; NS radius; …]
– Nondetection still useful[e.g., find fraction of short bursts from NS alone nearby]
• Short GRBs : potentially powerful tool?– Constrain channels: Short GRBs >> 10/yr; #(NS-NS)=4
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LIGO-XXX
But are Short GRBs Mergers?Direct test from one event?
EM: Fireball problem -- can’t see central engine Enormous merger model diversity (e.g opening angles)
GW: LIGO range short vs usual GRB distance [see later]
Statistics - look for obvious inconsistencies?Predict #, distribution of mergers Implies #, distribution of short GRBs Compare + reject inconsistent models
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LIGO-XXX
StarTrack and Population SynthesisPopulation synthesis:• Evolve representative sample• See what happens
Variety of results Depending on parameters used…• Range of number of binaries per input mass
Priors matter a priori assumptions about what parameters likely influence expectations
O’Shaughnessy et al (in prep)
More binaries/mass
Plot: Distribution of mass efficiencies seen in simulations
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LIGO-XXX
StarTrack and Population SynthesisPopulation synthesis:• Evolve representative sample• See what happens
Variety of results Depending on parameters used…• Range of number of binaries per input mass • Range of delays between birth and merger
Priors matter a priori assumptions about what parameters likely influence expectations
O’Shaughnessy et al (in prep)
Merging after 2ndsupernova
Merging after 10 Gyr
Plot: Probability that a random binary merges before time ‘t’, for each model
: changed priors since last paper
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LIGO-XXX
Popsyn and Milky Way
Population synthesis• Controlled uncertainties --> wide but limited range of predictions
Milky Way: A test• ~ steady state system (average merger rate)
• Compare to observations (several Kim et al) (NS-NS binaries + known selection effects)
– Observation: shaded– Theory: dotted curve– Systematics : dark shaded
• Limited set (9%) consistent– Complicated, extended 7d volume– Lots of physics can be mined
More binaries/mass
astro-ph/0610076
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LIGO-XXX
Milky Way Binary PulsarsObservations
– 7 NS-NS binaries – 4 WD-NS binaries
Selection effects“How many similar binaries exist, given we see one?”
Examples• Lifetime :
– age + merger time < age of universe• Lifetime visible :
– time to pulsar spindown, stop?• Fraction missed - luminosity:
– many faint pulsars Distribution of luminosities ~ known
• Fraction missed - beaming: – Not all pointing at us!
Kim et al ApJ 584 985 (2003)Kim et al astro-ph/0608280Kim et al ASPC 328 261 (2005)
Kim et al ApJ 614 137 (2004)
Example: Lmin correction:
One seen --> many missed
Rate estimate Kim et al ApJ 584 985 (2003)
(steady-state approximation)
Number + ‘lifetime visible’ + lifetime + fraction missed
=> birthrate + error estimate (number-> sampling error)
Note: • Only possible because many single pulsars seen:
Lots of knowledge gained on selection effectsApplied to reconstruct Ntrue
from Nseen
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LIGO-XXX
Inhomogeneous universe: The reality
– Time-dependent, multicomponent SFR
– Use delay time distribution
(dP/dt ~ 1/t)
– Long delays matter
Sample multicomponent predictions:
• Merger rate in spirals
(NS-NS)
From recentPlot:
Birth time forpresent-day mergers
Popsyn and Universe
Merging after 2ndsupernova
Merging after 10 Gyr
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LIGO-XXX
Can short GRBs be mergers?Binary pulsars
• Many (isolated) observed• Minimum luminosity ~
known• Observed number --> rate (+ ‘small’ error)
Short GRBs• Few observations• Minimum luminosity ~ unknown• Observed number --> rate upper bound
Conclusion:The number (rate) of short GRB observations is
a weak constraint on models
observed
Plots:Cartoon on Lmin
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LIGO-XXX
Can short GRBs be mergers?
Test 1: Are there enough mergers? … so far, usually yes:• Plot: All-sky detection rate vs predictions, if
+ No bursts fainter than seen+ All sky coverage & no beaming
… but surprising if detectors
“fine-tuned” many should be missed
BH-NS
NS-NS
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LIGO-XXX
Can short GRBs be mergers?
Test 2: Are they distributed consistently
in redshift? (BH-NS shown)
BH-NS?:• Predictions:
– 500 pairs of simulations
– Range of redshift distributions
• Observations:– Solid:
certain
– Shaded:
possible
O’Shaughnessy et al (in prep)
KeySolid: 25-75%Dashed: 10-90%Dotted: 1%-99%
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LIGO-XXX
Can short GRBs be mergers?
Test 2: Are they distributed consistently
in redshift? (BH-NS shown)
• Predictions that agree?– Compare cumulative distributions:
maximum difference < 0.48 everywhere
– Compare to well-known GRB redshifts since 2005• dominated by low redshift
O’Shaughnessy et al (in prep)
[95% Komogorov-Smirnov given GRBs]
[consistent selection effects]
Result:Distributions which agree = mostly at low redshift
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LIGO-XXX
Can short GRBs be mergers?
Test 2: Are they distributed consistently
in redshift? (NS-NS shown)
• Predictions & observations
KeySolid: 25-75%Dashed: 10-90%Dotted: 1%-99%
O’Shaughnessy et al (in prep)
• Matching redshifts
• Observed NS-NS (Milky Way)
• All agree? - possible - special parameters needed (~1/100)
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LIGO-XXX
Can short GRBs be mergers?
NS-NS?:• Physical interpretation
– Observations : GRBs • Dominated by recent events
– Expect: • Recent spirals dominate or
• or Ellipticals dominate, with long delays
O’Shaughnessy et al (in prep)
-Observations: Galactic NS-NS• High merger rate
-Expect-High merger rate in spirals
• Consistent so farMostly inellipticals
Mostly inspirals
Plot: fs : fraction of mergers in spirals (z=0)
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LIGO-XXX
Short GRBs: Where are we with Swift?
Good: No longer clueless• Hosts : variety, most star forming• Redshifts : Mostly nearby
Bad• Afterglow searches biased against high redshift (Berger 2007)• Swift search biased against short bursts (Gehrels, Ringberg)
– Few events– Detection rate hard to interpret
• Narrow, strange sky coverage• No peak energiesSurprises• Afterglows look odd• Classification no longer trivial (e.g., long bursts w/ short spikes; long close bursts w/ no
SN; etc)
See Nakar 2007
astro-ph/0701748
[Berger et al, astro-ph/0611128]
GLAST will help
- less biased selection
- improve Swift triggering (e.g., lower flux thresholds) -> more statistics
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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LIGO-XXX
What will LIGO see?
When will LIGO see mergers? [supporting slide] [based on single-IFO NS range (e.g., 15 Mpc)]
Extrapolating from Milky Way: Whole universe: about the same(revised version astro-ph/0610076) (mature draft to be submitted; “raw data”
[totally unconstrained distributions] shown)
Initial
Advanced
NS-NS
BH-NS
Enhanced LIGO
(3x single-IFO range of 15 Mpc??)
R(NS-NS) ~ 0.05 - 1 / year
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LIGO-XXX
Detection: A scenario for 2014Scenario: (Advanced LIGO)• Observe n ~ 30 BH-NS events [reasonable]• Rate known to within d log R ~2[1/n1/2ln(10)]~ 0.16
• Relative uncertainty down by factor d log R/ log R ~ 0.16/1
16% ~ 9% : Comparable to all EM observations
Repeat for BH-BH, NS-NS• Independent channels (each depends differently on model params)->
Volume [0.09 (0.16)3] ~ (3 x 10-4) !!Params [0.09 (0.16)3]1/7 ~ 0.32
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LIGO-XXX
LIGO Nondetections Useful
SGRs (=B-driven bursting neutron stars) are GRBs • Known galactic/nearby source : SGR 1806• Unknown (small?) contribution to nearby short GRB rate
LIGO can “distinguish”:• Short GRB nearby (e.g., <15 Mpc)
– Merger : Detectable– SGR : Marginally/not detectable
• Application– Assist host galaxy searches (i.e., minimum distance to merger)– estimate SGR contribution
critical to apply spiral fraction as constraint
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LIGO-XXX
Conclusions
• Useful comparison method despite large uncertainties
• Preliminary results
– Via comparing to pulsar binaries in Milky Way
– Via comparing to short GRBs?• Conventional popsyn works : weak constraints-> standard model ok • Expect GRBs in either host : spirals form stars now
– Spirals now favored; may change with new redshifts!
• Short GRBs = NS-NS? easier : few consistent ellipticals
• Short GRBs = BH-NS? harder : fewer observations
• Observational recommendations
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LIGO-XXX
Supporting slides follow• LIGO and short GRBs : Nondetection still useful
• Swift detection biases
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LIGO-XXX
Outline
• Predictions and Constraints: Milky Way– Observations (pulsars in binaries) and selection effects
– Prior predictions versus observations
– Constrained parameters
– Physics behind comparisons : what we learn
– Revised rate predictions
– What if a detection?
• Why Ellipticals Matter• Predictions and Constraints Revisited
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LIGO-XXX
Accepted modelsConstraint-satisfying volume
7d volume: • Hard to visualize!• Extends over ‘large’ range: characteristic extent(each parameter): 0.091/7~0.71
7d grid
= 7 inputs to StarTrack
9% of models work
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LIGO-XXX
Outline
• Predictions and Constraints: Milky Way• Why Ellipticals Matter
– Two-component star formation model
• Predictions and Constraints Revisited– Prior predictions– Reproducing Milky Way constraints
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LIGO-XXX
From recentPlot:Birth time for
present-day mergers
Importance of early SFRLong delays allow mergers in ellipticals now• Merger rate from starburst: R ~ dN/dt~1/t• SFR higher in past:
• Result: – Many mergers now occur in ancient binaries Nagamine et al astro-ph/0603257\
From oldancient SFR = ellipticals (mergers, …)
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LIGO-XXX
Outline
• Predictions and Constraints: Milky Way
• Why Ellipticals Matter
• Predictions and Constraints Revisited
• GRBs– Review + the short GRB merger model
– Short GRB observations, the long-delay mystery, and selection effects
– Detection rates versus Lmin
– Predictions versus observations:• If short GRB = BH-NS
• If short GRB = NS-NS
– Gravitational waves?
• Conclusions
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LIGO-XXX
ConclusionsFuture (model) directions:• More comparisons
– Milky Way• Pulsar masses• Binary parameters (orbits!)• Supernova kick consistency?
– Extragalactic• Supernova rates
Some examples:Belczynski et al. (in prep)
• Broader model space–Polar kicks?–Different maximum NS mass
[important: BH-NS merger rate sensitive to it!]
–Different accretion physics
Goal: - show predictions robust to physics changes - if changes matter, understand why (and devise tests to constrain physics)