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Orphan Afterglows

Orphan Afterglows. Daniel Perley Astro 250 3 April 2007. Gamma-Ray Burst Review. GRB Review. Burst of gamma-ray emission lasting 0.01 – 1000 s Typical fluence 10 -7 to 10 -3 erg/cm 2 Isotropic (extragalactic); 0.01 < z < 7 Observed rate ~1-3/day

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Orphan Afterglows

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  1. Orphan Afterglows Daniel PerleyAstro 2503 April 2007

  2. Gamma-Ray Burst Review GRB Review Burst of gamma-ray emission lasting 0.01 – 1000 s Typical fluence 10-7 to 10-3 erg/cm2 Isotropic (extragalactic); 0.01 < z < 7 Observed rate ~1-3/day Less for instruments with limited sensitivity or field of view Followed by long-wavelength afterglow counterpart Lasts for days to weeks to months Astro 250: Orphan Afterglows

  3. GRB vs. Afterglow GRB Review GRB: Internal Shocks Afterglow: Forward Shock Collision/interaction of shells of ultra-relativistic ejected material Intrinsic to progenitor Relativistic shock moving into circumburst medium Depends on environment Astro 250: Orphan Afterglows

  4. GRBs are Relativistic GRB Review "compactness problem" GRBs have short-timescale structure →small GRBs are extremely distant → extreme energy density Very large photon densities result in pair production (g g → e+ e-) opacity But, • Spectrum is non-thermal • Extremely high-energy photons observed for some bursts Solution: relativistic motion towards observer (energy density in source frame can be much less due to Doppler/light-travel/relativistic effects) Astro 250: Orphan Afterglows

  5. GRBs are Anisotropic GRB Review ... probably Isotropic-derved energy requirements are phenomenal Most theoretical models favor jet geometry Eiso≡ (Sg) × 4 π dL2 gamma-rays only GRB971214 (z=3.42): Eiso = 2.1 × 1053 erg GRB990123 (z=1.60): Eiso = 1.4 × 1054 erg GRB050904 (z=6.29): Eiso = 3.8 × 1053 erg M .c2 = 1.78 × 1054 erg Astro 250: Orphan Afterglows

  6. GRB Jets GRB Jets Assume that GRBs are jetted.Fairly good evidence this is the case. Recently the details are becoming controversial again. How does a jet geometry affect the appearance and evolution of a GRB and its afterglow? Astro 250: Orphan Afterglows

  7. The Relativistic Beam GRB Jets An emitter moving at v ~ c, even if emitting isotropically in its rest frame, will strongly beam its radiation as seen by the observer. v = 0Γ = 1 Beaming angle : Ω = 4π Beaming angle : Ω = πθ2θbeam ~ 1 / Γ v ~ cΓ >> 1 1 / Γ Astro 250: Orphan Afterglows

  8. Prompt Emission Beaming GRB Jets Initially, the GRB emission is strongly beamed towards the observer.(and anyone else within the jet beam) t = 10 sec internal shocks (prompt) θjet Γinit we see only a small part of the emitting region θbeam = 1 / Γinit intrinsic burst parameters: θjet - jet opening angle Eiso - 4π × energy per steradian Γinit – initial Lorentz factor Assume for simplicity a uniform ("top-hat") jet throughout this analysis. Astro 250: Orphan Afterglows

  9. Afterglow Beaming GRB Jets Once the afterglow sets in, the shock has swept up significant quantities of circumburst material and is decelerating. t = 100 sec external shock θjet Γ θbeam = 1 / Γ Astro 250: Orphan Afterglows

  10. Afterglow Beaming GRB Jets Each part of the afterglow shock is therefore beaming its emission into a larger area. t = 100 sec θjet Γ θbeam = 1 / Γ Astro 250: Orphan Afterglows

  11. Afterglow Beaming GRB Jets Each part of the afterglow shock is therefore beaming its emission into a larger area. However, this does not affect the flux viewed from Earth: emission from other areas is now beamed in our direction, replacing the lost flux. t = 100 sec θjet Γ 1 / Γ the apparent emitting region becomes a larger fraction of the jet (specifically, θE = θbeam = 1/Γ) 1 / Γ Astro 250: Orphan Afterglows

  12. Jet Break GRB Jets Eventually, the shock slows down enough that the apparent emitting region is the entire jet. t = 1 day Occurs when θE = θjet 1/Γ = θjet Γ = 1/θjet Γ θjet Note that up until this point, the GRB evolution is indistinguishable from a spherical explosion – points outside the jet are out of causal contact. Astro 250: Orphan Afterglows

  13. Post-Jet Break Evolution GRB Jets After this point, the increasingly outward-beamed radiation is no longer compensated by seeing more of the jet, and the observed flux drops rapidly. t = 10 day Γ θjet Astro 250: Orphan Afterglows

  14. Sideways Spreading GRB Jets At about the same time, the jet itself becomes "aware" of its finite extent, and it begins to expand outward.Its forward motion rapidly decelerates. Γ t = 100 day Γ ~ e-r/rsedov~ t-1/2 Astro 250: Orphan Afterglows

  15. Transition to Non-Relativistic GRB Jets Eventually the shock becomes nonrelativistic and unbeamed. Γ ~ 1 t = 500 day Astro 250: Orphan Afterglows

  16. Jet Light Curves GRB Jets t -(3/4)(p-1) t -(3/4)p Quantitative predictions are somewhat in dispute... Generally agreed-upon predictions: • First break occurs when Γ = 1/θjet Hydrodynamics: Γ(t) ~ 6 (Eiso/nISM)1/8 t-3/8 tjet ~ 0.25d (E/n)1/3(θjet/0.1)8/3 • Break should be achromatic No change in the spectrum; just viewing geometry log F optical / x-ray t -p tjet tspread tNR Eiso in units of 1052 erg nISM in units of cm-3 t in units of day t 1/2 const t -1/4 ? t -1/3 log F t -9/10 radio tjet tspread tNR log t Astro 250: Orphan Afterglows

  17. Observed Jet Breaks GRB Jets Jet breaks have been seen. Maybe. GRB 030226 If the Swift jet break crisis is real, all of this may be bunk. still unbroken at15 days Astro 250: Orphan Afterglows

  18. Off-Axis Prompt Emission Off-Axis Jets What if the jet is not directed at us? GRB emission is completely absent outside the physical jet beam (the relativistic beam is much narrower at this stage) Γinit θbeam = 1 / Γinit θjet Astro 250: Orphan Afterglows

  19. θjet Off-Axis Early Afterglow Off-Axis Jets The early afterglow is also invisible. Γ Astro 250: Orphan Afterglows

  20. θjet Off-Axis Jet Break Off-Axis Jets Sometime after the "break" time, however, the jet becomes visible Jet center is visible when θbeam ~ θobs Γ(t) ~ 1/θobs θobs Once the full jet is visible the afterglow will be (about) as bright as if we were on-axis at this time Γ Astro 250: Orphan Afterglows

  21. Off-Axis Light Curves Off-Axis Jets t -(3/4)(p-1) t -(3/4)p Off-axis light curve similar to on-axis, except that there is nearly no flux until Γ(t) ~ 1/θobs log F optical / x-ray t -p There is an afterglow without a GRB: "orphan afterglow" t -1/4 ? t 1/2 const t -1/3 log F radio tjet tspread tNR log t Astro 250: Orphan Afterglows

  22. Off-Axis Light Curves Off-Axis Jets Some more precise predictions: Optical (Granot et al. 2002) Radio (Totani & Panaitescu 2000) Astro 250: Orphan Afterglows

  23. Another Pathway: Dirty GRBs Dirty GRBs One can also generate an on-axis orphan afterglow by surpressing the GRB (gamma-ray) flux. This can be achieved with an energetic but low-Γinit burst outflow: will be opaque to pair-production (remember compactness problem) and not generate gamma rays, but will create a "normal" afterglow. Γinit I will ignore this completely in the remainder of the lecture. Astro 250: Orphan Afterglows

  24. The Orphan Afterglow Rate The Orphan Afterglow Rate For any burst, the post-jet afterglow is beamed towards a much larger angle than the early afterglow, so the rate of orphan afterglows should be very large. θobs θjet However, GRBs are much brighter and easier to detect than orphan afterglows, and can be seen to much greater distances / less energetic events. Typically, θjet ~ 5° θobspotentially can be the whole sky Astro 250: Orphan Afterglows

  25. The Orphan Afterglow Rate Afterglow Rate Estimate Procedure Intrinsic questions: • What is the GRB rate as a function of redshift? • What is the GRB luminosity function? • What is the jet angle distribution? • What is the distribution of Γinit and nISM? • How does afterglow luminosity relate to GRB luminosity? Survey questions: • What is the survey sky area? • What is the survey flux limit? • Can the survey distinguish OAs from other transients? Astro 250: Orphan Afterglows

  26. Optical Orphan Afterglow Rate Optical Orphan Afterglow Rate Optical orphan afterglows: "near misses" Optical flux drops precipitously after the jet break (the soonest possible time an off-axis afterglow can be observed) Therefore, only near-axis events will be seen. But these events can be quite bright and visible to large (cosmological) distances: intrinsic luminosity limited. Astro 250: Orphan Afterglows

  27. Optical Orphan Afterglow Rate A First-Cut Estimate Rate of GRBs: 1000 / year Fraction of GRBs with afterglow detectable (R<23) to t ~ 10 tjet: 3% Observable for ~10 days For an R<23 survey: # on-axis detectable afterglows:30 / sky / year = 1 / sky now # off-axis afterglows: 32 this many300 /sky/year = 10 / sky now Need to survey the equivalent of 1/10 of the sky (4000 sq deg.) to find an orphan afterglow.Can repeat if the interval is >~ 10 days. Astro 250: Orphan Afterglows

  28. Totani & Panaitescu (2002) Nakar et al. (2002) Zou et al. (2007) Optical Orphan Afterglow Rate More Precise Estimates Estimates in the literature vary wildly. • Uncertain rate of very low-luminosity GRBs • Uncertainty in actual light curve of an off-axis event • Uncertain distribution of GRB afterglow luminosities (dark bursts, etc) Astro 250: Orphan Afterglows

  29. Totani & Panaitescu (2002) Nakar et al. (2007) Zou et al. (2002) Optical Orphan Afterglow Rate Optical Survey Design Shallow and wide, or narrow and deep? Hypothetical Lick survey limiting rate of a survey: Rlimit = 4π/Asurvey ~ Flimit-2 limiting flux of a telescope: Flimit texp-1/2 texp Flimit-2 area of a survey: Asurvey = (Tsurvey / texp) × FOV texp-1 Flimit2 but, expect R  F-1→ shallow survey Astro 250: Orphan Afterglows

  30. Radio Orphan Afterglow Rate Radio orphan afterglows: low-z GRB remnants Radio emission most easily detected during late (non-relativistic, ~isotropic) stage of GRB evolution. Intrinsic energy output is ~constant (~1051 erg). → Late-time radio luminosity should also be constant: radio OA's are distance-limited. Astro 250: Orphan Afterglows

  31. Radio Orphan Afterglow Rate A First-Cut Estimate Rate of GRBs: 1000 / year Observable for ~1 year 970508: 70 μJy at 250 days (8 GHz) z = 0.835 VLA sensitivity limit: 50 μJy in 10 min ~20% of GRBs are at z < 1 → 200 detectable on-axis radio afterglows / sky / yr 200 × 100 = 20,000 detectable radio afterglows / sky / yr 20,000 detectable radio afterglows / sky now Frail et al. 2000 ~1000 x number of current optical afterglows Astro 250: Orphan Afterglows

  32. 107 106 105 N (F > Flim) 104 103 1000 100 10 1 0.1 Flim (μJy) Radio Orphan Afterglow Rate More Precise Estimates Also variable; depends on low-z GRB rate, beaming fraction, and ISM density NR ~ 20 (fn / 5 mJy)-3/2Levinson et al. 2002 Levinson et al. 2002 Astro 250: Orphan Afterglows

  33. 107 106 105 N (F > Flim) 104 103 1000 100 10 1 0.1 Flim (μJy) Radio Orphan Afterglow Rate Radio Survey Design Wide or deep? limiting rate of a survey: Rlimit = 4π/Asurvey  Flimit-2 Hypothetical VLA survey expect R  F-3/2→ wide/shallow Astro 250: Orphan Afterglows

  34. Orphan Afterglow Surveys Real Surveys Searches for long-wavelength high-z transients are still in their early stages. Some preliminary results: FIRST / NVSS Bower survey CFHT-WF surveyMany others I am ignoring Upcoming surveys: ATA, SDSS, Pan-STARRS, LSST, SKA Astro 250: Orphan Afterglows

  35. Orphan Afterglow Surveys NVSS-FIRST Transient Search Comparison of two VLA wide-field radio surveys (FIRST, NVSS) FIRST: Galactic caps, 3 minute integrations, 21 cm, B-configuration (5" resolution) NVSS: Entire northern sky, 21 cm, D-configuration (45" resolution) Flim ~ 5 mJy Asurvey ~ 2400 sq. deg Two epochs, variable interval 9 transient candidates identified 1 radio supernova, others not identifiable (lack of information) No orphan afterglows detected (survey sensitivity is low; any OA would be coincident with a bright galaxy) θjet < 15° Astro 250: Orphan Afterglows

  36. Orphan Afterglow Surveys Bower Transient Survey 22-year, extensively repeated survey of a blank field in far northern sky 5-8 GHz Flim ~ 500 μJy A ~ 0.02 sq. deg 944 epochs Aeff ~ 10 sq.deg 10 transients detected 1 probable radio supernova1 radio supernova or GRB afterglow (orphan or otherwise) 8 unknown sources Astro 250: Orphan Afterglows

  37. Orphan Afterglow Surveys CFHTLS-VWS Canada France Hawaii Telescope Legacy Survey (Very Wide Survey) A ~ 490 sq. deg mlim = 22.5 (r' - band) 14 epochs 1067 transients 1066 variable stars / asteroids / KBOs 1 orphan afterglow candidate Malacrino et al. 2007 Astro 250: Orphan Afterglows

  38. Pan-STARRS and LSST Future Surveys Extremely wide, deep future optical surveys Pan-STARRS PS1 1.8m telescope, FOV of 7 sq. deg. Entire visible sky to R~23 each week OA rate ~ 100 / year Completion 2007 PS4 2.2m telescope, FOV of 28 sq. deg? 1 OA per day? LSST Large Synoptic Survey Telescope 8.4m telescope, FOV 3.5 sq. deg Entire visible sky to R~24 in 3 nights OA rate ~ many / night Completion 2012? Astro 250: Orphan Afterglows

  39. ATA and SKA Future Surveys Upcoming radio instruments ATA Allen Telescope Array 4 sq. deg FOV Visible sky to 1 mJy each week OA rate ~ 1 / night SKA Square Kilometer Array Large telescope to be completed in effectively indefinite future Many times ATA transient rate Astro 250: Orphan Afterglows

  40. The Future Future Surveys The future for these orphans may not be so grim after all! Astro 250: Orphan Afterglows

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