SONG Targets of Opportunity: Searching for Pulses in Gamma-Ray Burst Afterglows

1. SONG Targets of Opportunity: Searching for Pulses in Gamma-Ray Burst Afterglows Jon Hakkila

2. SONG’s primary mission : asteroseismology and searching for extrasolar planets. SONG also has the potential to observe occasional, special secondary Targets of Opportunity (TOOs), provided these do not interfere with the primary mission. TOO observations, coordinated by the SONG TAC, should take advantage of SONG’s 24-hour observations, but should not be disruptive over long time periods. The search for pulses in selected gamma-ray burst afterglows is one example of TOO science for which SONG is a preferred instrument.

3. General GRB background: • Bright flashes of primarily -radiation • Nonthermal spectra peak at ~270 keV • Durations 10-3 s to 103 s; highly variable • Cosmological sources • Spectra evolve, but no previously known pattern of time history evolution • Beamed radiation • Indicative of relativistic shocks • Afterglows observed for many GRBs; afterglow physics is well-understood • Prompt emission is still poorly-understood

4. Standard synchrotron shock model (e.g. Rees & Meszaros, ApJL, 1994, 430, 94). GRB Prompt Emission Light Curves Spectra  Epk  25-50 keV, 50-100 keV, 100-300 keV, 300 keV-1 MeV

5. Hypernova Central Engine Model of Long GRBs Merging Compact Objects Central Engine Model of Short GRBs GRB Classes Short Long Intermediate? Intermediate class’s existence appears to be statistical, rather than representing a source population.

6. GRB bulk properties do not indicate simple behaviors, even though some (e.g. lag, Epk, variability) are luminosity indicators. • GRB complexity results from overlapping pulses. • Long and Short GRBs appear inherently different. • Relativistic cosmology alters GRB observed properties: • Inverse square law: distant GRBs appear fainter than similar nearby ones • Time dilation: distant GRBs have durations and temporal structures stretched more than nearby ones. • Energy shift: distant GRBs have their fluxes shifted to lower observed energies than similar nearby ones. • Energy shift: distant GRBs have their fluxes shifted to lower observed energies than similar nearby ones. GRB pulse properties appear to be simpler • Inherent time asymmetry (longer decay than rise rates), • Hard-to-soft spectral evolution, and • Pulse lengthening at lower energies

7. Pulse extraction via semi-automated pulse-fitting algorithm.Uses the Bayesian Blocks methodology (Scargle 1998), a dual timescale threshold definition (Hakkila et al. 2003), and a 4-parameter pulse model (Norris et al. 2005). • Pulse peak flux (p256) - peak flux of summed multichannel data (black) measured on 256 ms timescale. • Pulse duration - time interval between times when flux is e-3 of pulse peak flux. • Pulse peak lag- time interval between channel 3 peak (100-300 keV; green) and channel 1 peak (25-50 keV; red). • Fluence - time-integrated flux. • Hardness - ratio of channel 3 fluence to channel 1 fluence. • Asymmetry - pulse shape measure; 0 is symmetric and 1 is asymmetric.

8. 300 keV - 1 MeV 100 keV - 300 keV 25 keV - 1 MeV 50 keV - 100 keV 3 2 1 25 keV - 50 keV Pulse-fitting example: GRB 950325a (BATSE 3480)

9. 300 keV - 1 MeV 100 keV - 300 keV 50 keV - 100 keV 1 2 3 4 25 keV - 1 MeV 25 keV - 50 keV Pulse-fitting example: GRB 910930 (BATSE 0840)

10. 100 keV - 300 keV 50 keV - 100 keV 1 2 25 keV - 1 MeV 25 keV - 50 keV Pulse-fitting example: GRB 930123 (BATSE 2600)

11. original burst CCF; short lag 1 2 25 keV - 1 MeV CCF of reconstructed pulse 1: long lag CCF of reconstructed pulse 2: short lag CCF of reconstructed pulses 1+2: short lag GRB 930123 (BATSE 2600) What does GRB lag measure (as obtained from the CCF)? The CCF lag is dominated by large amplitude, narrow pulses with short lags. Longer-lag pulses can smear out this behavior. The GRB peak flux is not generally the pulse peak flux of the brightest pulse, due to pulse overlap.

12. Peak luminosity vs. duration (w) and peak luminosity vs. lag for BATSE GRBs: Pulse relations replace bulk prompt emission relations (Hakkila et al. 2008, ApJ 677, L81). Pulse Property Correlations (1390 pulses in 646 BATSE GRBs) The GRB lag vs. luminosity relation (Norris et al. 2000) is actually a pulse lag vs. pulse luminosity relation. Furthermore, pulse duration is highly correlated with pulse lag, so pulse duration also indicates luminosity.

13. More Pulse Property Correlations

14. Long GRB Pulse Correlations Short GRB Pulse Correlations Correlations among GRB pulse properties are unmistakable!

15. BATSE 0111; z ≈ 0.9 BATSE 0332; z ≈ 0.9 Some low-z BATSE bursts BATSE 0563; z ≈ 0.8 BATSE 1406; z ≈ 0.8 Correlated pulse properties can be used to estimate GRB redshifts (Hakkila, Fragile, & Giblin, 2009, AIP Conf. 1133, 479) . BATSE 0214; z ≈ 4.3 BATSE 0237; z ≈ 5.3 BATSE 0594; z ≈ 5.4 BATSE 0803; z ≈ 4.6 Some high-z BATSE bursts

16. Supportive Observations Arimoto et al. (2010, PASJ, in press) verify the pulse lag and pulse duration vs. pulse luminosity relations using HETE-2. They find that pulse curvature can explain the observed energy dependence (see also Lu, Qin, and Zhang 2006, MNRAS, 367, 275).

17. Peng et al. (ApJ in press) Pulse Physics • Pulses appear to start simultaneously at all energies (Hakkila and Nemiroff 2009, ApJ 705, 372). • Pulse Epk values decay from the moment the pulse begins. Model: Kinematic energy injection into a medium via relativistic shock; the medium cools. Standard spectral model has been either a synchrotron spectrum or a thermal plus power law spectrum.

18. Pulse Physics Boci, Hafizi, & Mochkovitch (2010, A&A, submitted) predict the pulse lag and pulse duration vs. pulse luminosity relations from pulse properties near peak flux. They find, however, that these correlative relations cannot be obtained as a direct and simple consequence of the standard synchrotron shock model (Rees & Meszaros, ApJL, 1994, 430, 94).

19. GRB 070311 Connecting Prompt Emission to the Afterglow X-ray pulses typically start seconds to minutes after the prompt emission and end within minutes to hours.

20. Connecting Prompt Emission to the Afterglow Optical flares typically start minutes to hours after the prompt emission and end within hours to days, with 10 ≤ R ≤ 20.

21. Hypothesis: afterglow flares are late, low energy GRB pulses. • If so, then the properties of these late pulses must be correlated as they are for the prompt pulses, (lags roughly minutes long)--> a testable hypothesis. Many follow-up questions can be addressed if this hypothesis is validated: • Are late pulses limited to GRBs with specific morphologies (e.g. Long vs. Short, many pulses vs. few pulses)? • What constraints can late pulses place on the GRB energy mechanism? • What are the limiting amplitudes of late pulses? • How common are late pulses? How often can they happen within a single GRB afterglow?

22. SONG’s temporal resolution and sampling completeness are excellent for making multiwavelength observations of GRB afterglow flares. Standard TOO observing: • Schedule a TOO to break away from current SONG target upon receipt of an appropriate GCN notice. Alternate TOO strategies: • Have a standing TOO request to respond to a GCN notice with an offline SONG node. • Near the end of an observing campaign on the current target, have an increasing prioritization to divert SONG nodes in response to a GCN notice.

23. Conclusions • Pulses are the basic building blocks of GRB prompt emission. Pulse properties correlate with one another. • GRB bulk properties are constructed by combining and smearing out pulse characteristics in ways that potentially lose valuable information. • Pulse physics models indicate simple pulse geometries but may not support the standard synchrotron shock model. • SONG observations can ascertain whether afterglow flares are part of the afterglow, or instead late shocks having the correlated properties of GRB pulses. • JH acknowledges support by SC Space Grant, NASA Swift, NASA AISR, NASA ADAP, and SC Space Grant Palmetto Scholars, and graciously thanks the SONG conference organizers.