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

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

Outline Short GRBs : A Review Population synthesis predictions 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? 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

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/0611128] Many sources at limit of detector (BATSE) LIGO-XXX

Short GRBs: A Review Merger motivation? No SN structure in afterglow In both old, young galaxies Occasional host offsets GRB 050709 (Fox et al Nature 437 845) GRB 051221 (Soderberg et al 2006) Young NSs are some (known) Energetics suggest not all 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 + 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

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

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

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

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/0610076 LIGO-XXX

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 584 985 (2003) Kim et al astro-ph/0608280 Kim et al ASPC 328 261 (2005) Kim et al ApJ 614 137 (2004) 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 effects Applied to reconstruct Ntrue from Nseen Example: Lmin correction: One seen --> many missed LIGO-XXX

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

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

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

Can short GRBs be mergers? Key Solid: 25-75% 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

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

Can short GRBs be mergers? Key Solid: 25-75% 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

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

Short GRBs: Where are we with Swift? See Nakar 2007 astro-ph/0701748 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/0611128] LIGO-XXX

(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/0610076) (mature draft to be submitted; “raw data” [totally unconstrained distributions] shown) Enhanced LIGO (3x single-IFO range of 15 Mpc??) R(NS-NS) ~ 0.05 - 1 / 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

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

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

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

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

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

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

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

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/0603257\ From old 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 LIGO-XXX

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