1. 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
LIGO-XXX

2. Context? If you want to… Test GR soon (=LIGO) Understand short GRBs
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?
LIGO-XXX

3. Outline Short GRBs : A Review Population synthesis predictions
Intersection with LIGO
Population synthesis predictions
Milky Way astro-ph/ ;
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?
And how we can test this idea, preferably with gravitational waves, but for the time being with existing electromagnetic observations of the bursts and of merging compact objects.
LIGO-XXX

4. Short GRBs: A Review Short GRBs dP/dL ~ L-2 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
[Berger et al, astro-ph/ ]
Many sources at limit
of detector (BATSE)
LIGO-XXX

5. Short GRBs: A Review Merger motivation? No SN structure in afterglow
In both old, young galaxies
Occasional host offsets
GRB (Fox et al Nature )
GRB (Soderberg et al 2006)
Young NSs are some (known)
Energetics suggest not all
LIGO-XXX

6. 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
+ about our parameterized models for how stars form from gas,
evolve as binary stars, supernovae, and eventually end up (or not) as
compact binaries,
+ how we test that understanding against the milky way,
to calibrate our understanding of the evolution of the most massive binary stars
+ how we extend our understanding to a realistic heterogeneous universe,
consisting of both old and young stellar populations,
+ what our best understanding of a heterogeneous universe leads us to expect for short GRBs,
if indeed short GRBs arise from merging binaries,
+ and whether that understanding agrees with short GRB observations,
whether when we combine our understanding of binary evolution with
a heterogeneous universe we can explain observations of short GRBs
in terms of mergers of compact objects.
I’m also here to talk about gravitational wave astronomy, because -- as I hope I’ll demonstrate -- I think gravitational waves are
+ the best way to improve our understanding of binary mergers and
+ the *only* way (short term) to unambiguously investigate the central
engines of short GRBs and test the merger hypothesis
LIGO-XXX

7. 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
LIGO-XXX

8. StarTrack and Population 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
Plot: Distribution of mass efficiencies seen
in simulations
More binaries/mass
O’Shaughnessy et al (in prep)
LIGO-XXX

9. StarTrack and Population 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
Specifically:
Popsyn case: Uniform priors
GRB case
Priors are the set of *simulated* systems…which because of irregular v sampling don’t agree with *either* the first paper (which used the truncated region v2>200>v1) or the second (which uses the whole region, uniformly)
Plot: Probability that a random binary
merges before time ‘t’, for each model
Merging after 2nd
supernova
Merging after
10 Gyr
O’Shaughnessy et al (in prep)
: changed priors since last paper
LIGO-XXX

10. 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
+ about our parameterized models for how stars form from gas,
evolve as binary stars, supernovae, and eventually end up (or not) as
compact binaries,
+ how we test that understanding against the milky way,
to calibrate our understanding of the evolution of the most massive binary stars
+ how we extend our understanding to a realistic heterogeneous universe,
consisting of both old and young stellar populations,
+ what our best understanding of a heterogeneous universe leads us to expect for short GRBs,
if indeed short GRBs arise from merging binaries,
+ and whether that understanding agrees with short GRB observations,
whether when we combine our understanding of binary evolution with
a heterogeneous universe we can explain observations of short GRBs
in terms of mergers of compact objects.
I’m also here to talk about gravitational wave astronomy, because -- as I hope I’ll demonstrate -- I think gravitational waves are
+ the best way to improve our understanding of binary mergers and
+ the *only* way (short term) to unambiguously investigate the central
engines of short GRBs and test the merger hypothesis
astro-ph/
LIGO-XXX

11. Milky Way Binary Pulsars
Observations
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 (2003)
Kim et al astro-ph/
Kim et al ASPC (2005)
Kim et al ApJ (2004)
Rate estimate Kim et al ApJ (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 effects
Applied to reconstruct Ntrue from Nseen
Example: Lmin correction:
One seen --> many missed
LIGO-XXX

12. Popsyn and Universe 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 recent
Merging after
10 Gyr
Plot:
Birth time for
present-day mergers
Merging after 2nd
supernova
+ about our parameterized models for how stars form from gas,
evolve as binary stars, supernovae, and eventually end up (or not) as
compact binaries,
+ how we test that understanding against the milky way,
to calibrate our understanding of the evolution of the most massive binary stars
+ how we extend our understanding to a realistic heterogeneous universe,
consisting of both old and young stellar populations,
+ what our best understanding of a heterogeneous universe leads us to expect for short GRBs,
if indeed short GRBs arise from merging binaries,
+ and whether that understanding agrees with short GRB observations,
whether when we combine our understanding of binary evolution with
a heterogeneous universe we can explain observations of short GRBs
in terms of mergers of compact objects.
I’m also here to talk about gravitational wave astronomy, because -- as I hope I’ll demonstrate -- I think gravitational waves are
+ the best way to improve our understanding of binary mergers and
+ the *only* way (short term) to unambiguously investigate the central
engines of short GRBs and test the merger hypothesis
LIGO-XXX

13. Can short GRBs be mergers?
Few observations
Minimum luminosity
~ unknown
Observed number
--> rate upper bound
Binary pulsars
Many (isolated) observed
Minimum luminosity ~ known
Observed number
--> rate (+ ‘small’ error)
Plots:
Cartoon on Lmin
observed
Conclusion:
The number (rate) of short GRB observations is
a weak constraint on models
LIGO-XXX

14. 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
+ about our parameterized models for how stars form from gas,
evolve as binary stars, supernovae, and eventually end up (or not) as
compact binaries,
+ how we test that understanding against the milky way,
to calibrate our understanding of the evolution of the most massive binary stars
+ how we extend our understanding to a realistic heterogeneous universe,
consisting of both old and young stellar populations,
+ what our best understanding of a heterogeneous universe leads us to expect for short GRBs,
if indeed short GRBs arise from merging binaries,
+ and whether that understanding agrees with short GRB observations,
whether when we combine our understanding of binary evolution with
a heterogeneous universe we can explain observations of short GRBs
in terms of mergers of compact objects.
I’m also here to talk about gravitational wave astronomy, because -- as I hope I’ll demonstrate -- I think gravitational waves are
+ the best way to improve our understanding of binary mergers and
+ the *only* way (short term) to unambiguously investigate the central
engines of short GRBs and test the merger hypothesis
NS-NS
LIGO-XXX

15. Can short GRBs be mergers?
Key
Solid: %
Dashed: 10-90%
Dotted: 1%-99%
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)
LIGO-XXX

16. 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
[95% Komogorov-Smirnov given GRBs]
[consistent selection effects]
Result:
Distributions
which agree
= mostly
at low redshift
O’Shaughnessy et al (in prep)
LIGO-XXX

17. Can short GRBs be mergers?
Key
Solid: %
Dashed: 10-90%
Dotted: 1%-99%
Test 2: Are they distributed consistently
in redshift? (NS-NS shown)
Predictions & observations
Matching redshifts
Observed NS-NS
(Milky Way)
All agree?
- possible
- special parameters
needed (~1/100)
O’Shaughnessy et al (in prep)
LIGO-XXX

18. 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
-Observations: Galactic NS-NS
High merger rate
Expect
High merger rate in spirals
Solid: Unconstrained
Dashed:
Dotted:
Plot: fs : fraction of mergers in spirals (z=0)
Consistent so far
Mostly in
ellipticals
O’Shaughnessy et al (in prep)
Mostly in
spirals
LIGO-XXX

19. Short GRBs: Where are we with Swift?
See Nakar 2007
astro-ph/
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 energies
Surprises
Afterglows look odd
Classification no longer trivial (e.g., long bursts w/ short spikes; long close bursts w/ no SN; etc)
GLAST will help
- less biased selection
- improve Swift triggering
(e.g., lower flux thresholds)
-> more statistics
[Berger et al, astro-ph/ ]
LIGO-XXX

20. (3x single-IFO range of 15 Mpc??)
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/ ) (mature draft to be submitted; “raw data”
[totally unconstrained distributions] shown)
Enhanced LIGO
(3x single-IFO range of 15 Mpc??)
R(NS-NS) ~ / year
Initial
NS-NS
The rate prediction uses binary fraction smoothing (crudely/by eye, from 1/2 to 1)
As well as eyballing the merger rate density limits
Advanced
BH-NS
LIGO-XXX

21. Detection: A scenario for 2014
Scenario: (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
LIGO-XXX

22. 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
LIGO-XXX

23. 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
+ about our parameterized models for how stars form from gas,
evolve as binary stars, supernovae, and eventually end up (or not) as
compact binaries,
+ how we test that understanding against the milky way,
to calibrate our understanding of the evolution of the most massive binary stars
+ how we extend our understanding to a realistic heterogeneous universe,
consisting of both old and young stellar populations,
+ what our best understanding of a heterogeneous universe leads us to expect for short GRBs,
if indeed short GRBs arise from merging binaries,
+ and whether that understanding agrees with short GRB observations,
whether when we combine our understanding of binary evolution with
a heterogeneous universe we can explain observations of short GRBs
in terms of mergers of compact objects.
I’m also here to talk about gravitational wave astronomy, because -- as I hope I’ll demonstrate -- I think gravitational waves are
+ the best way to improve our understanding of binary mergers and
+ the *only* way (short term) to unambiguously investigate the central
engines of short GRBs and test the merger hypothesis
LIGO-XXX

24. Supporting slides follow
LIGO and short GRBs : Nondetection still useful
Swift detection biases
+ about our parameterized models for how stars form from gas,
evolve as binary stars, supernovae, and eventually end up (or not) as
compact binaries,
+ how we test that understanding against the milky way,
to calibrate our understanding of the evolution of the most massive binary stars
+ how we extend our understanding to a realistic heterogeneous universe,
consisting of both old and young stellar populations,
+ what our best understanding of a heterogeneous universe leads us to expect for short GRBs,
if indeed short GRBs arise from merging binaries,
+ and whether that understanding agrees with short GRB observations,
whether when we combine our understanding of binary evolution with
a heterogeneous universe we can explain observations of short GRBs
in terms of mergers of compact objects.
I’m also here to talk about gravitational wave astronomy, because -- as I hope I’ll demonstrate -- I think gravitational waves are
+ the best way to improve our understanding of binary mergers and
+ the *only* way (short term) to unambiguously investigate the central
engines of short GRBs and test the merger hypothesis
LIGO-XXX

25. Outline Predictions and Constraints: Milky Way Why Ellipticals Matter
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
LIGO-XXX

26. Accepted models Constraint-satisfying volume 9% of models work 7d grid
7d volume:
Hard to visualize!
Extends over ‘large’ range:
characteristic extent(each parameter):
0.091/7~0.71
9% of models work
7d grid
= 7 inputs to
StarTrack
LIGO-XXX

27. Outline Predictions and Constraints: Milky Way Why Ellipticals Matter
Two-component star formation model
Predictions and Constraints Revisited
Prior predictions
Reproducing Milky Way constraints
LIGO-XXX

28. Importance of early SFR
Long 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
ancient SFR
= ellipticals
(mergers, …)
From recent
Plot:
Birth time for
present-day mergers
Nagamine et al astro-ph/ \
From old
LIGO-XXX

29. 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
LIGO-XXX

30. Belczynski et al. (in prep)
Conclusions
Future (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
Final:
Early days
Lots yet to be done to confirm and flesh out, but …
It looks like we understand
Goal:
- show predictions robust to physics changes
- if changes matter, understand why
(and devise tests to constrain physics)
LIGO-XXX