1 / 30

Short GRBs and Mergers: Astrophysical constraints on a BH-NS and NS-NS origin

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. Context?. If you want to… Test GR soon (=LIGO) Need high SNR, clean merger waveforms

oswald
Download Presentation

Short GRBs and Mergers: Astrophysical constraints on a BH-NS and NS-NS origin

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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) • Need high SNR, clean merger waveforms • EM coincidence helps search (orientation, time) • Understand shortGRBs • 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 • 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? LIGO-XXX

  4. 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 [Berger et al, astro-ph/0611128] 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 050709 (Fox et al Nature 437 845) GRB 051221 (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 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 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 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 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 mergerrate) • 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 LIGO-XXX

  11. Milky Way Binary Pulsars 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) 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! • 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 Ntruefrom Nseen Example: Lmincorrection: One seen --> many missed LIGO-XXX

  12. Popsyn and Universe From recent 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) Merging after 10 Gyr Merging after 2nd supernova Plot: Birth time for present-day mergers LIGO-XXX

  13. Can short GRBs be mergers? Short GRBs • 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 NS-NS LIGO-XXX

  15. 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

  16. Can short GRBs be mergers? Result: Distributions which agree = mostly at low redshift 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] O’Shaughnessy et al (in prep) LIGO-XXX

  17. Can short GRBs be mergers? • Matching redshifts • Observed NS-NS • (Milky Way) • All agree? • - possible • - special parameters • needed (~1/100) Key Solid: 25-75% Dashed: 10-90% Dotted: 1%-99% Test 2: Are they distributed consistently in redshift? (NS-NS shown) • Predictions & observations 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 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/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

  20. What will LIGO see? Enhanced LIGO (3x single-IFO range of 15 Mpc??) R(NS-NS) ~ 0.05 - 1 / year 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 NS-NS 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 LIGO-XXX

  24. Supporting slides follow • LIGO and short GRBs : Nondetection still useful • Swift detection biases LIGO-XXX

  25. 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 LIGO-XXX

  26. Accepted models Constraint-satisfying volume 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 Plot: Birth time for present-day mergers From recent ancient SFR = ellipticals (mergers, …) From old 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 Nagamine et al astro-ph/0603257\ 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. Conclusions • Broader model space • Polar kicks? • Different maximum NS mass • [important: BH-NS merger rate sensitive to it!] • Different accretion physics 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) Goal: - show predictions robust to physics changes - if changes matter, understand why (and devise tests to constrain physics) LIGO-XXX

More Related