The Biggest Bangs Since the Big One: New Perspectives on GRBs

Transcription

1 The Biggest Bangs Since the Big One: New Perspectives on GRBs Dr. Eli Rykoff University of Michigan March 19 th, 2007 University of Michigan

2 Outline Gamma-Ray Bursts The Swift satellite and the Robotic Optical Transient Search Experiment (ROTSE-III) telescopes GRB Progenitors The long-lived central engine High redshift GRBs and GRB cosmology High energy emission from GRBs GLAST and the future of ROTSE-III

3 Gamma-Ray Bursts

4 Gamma-Ray Bursts (GRBS) What are they? Energetic explosions, releasing over ergs in γ-rays in tens of seconds Cosmological distances, z ~ Variable, diverse, and rare About 1/day in the Universe visible to the Earth GRB GRB Schady, et al, 2006 Rykoff, et al, 2005 GRB as seen by BATSE

5 Gamma-Ray Bursts Detected by satellites such as Swift Two types of GRBs: long and short Variability on millisecond timescales Bursts have afterglows Broadband emission ~ 50% of bursts have detectable optical afterglows t + 8 hr t + days Beppo-Sax X-Ray afterglow of GRB970228

6 Swift ROTSE-III Designed for quick Designed for rapid optical follow-up On target in < 7 s 0.45 m telescopes Global coverage Australia, Texas, Namibia, Turkey 3.5 deg f.o.v. detection of GRBs and rapid follow-up BAT: kev, 2 steradian f.o.v. Slew to target in ~ 90 s XRT: kev UVOT 2

7 Compactness Problem The γ-ray spectrum is non-thermal Band spectrum (Band, et al., 1993) The millisecond variability GRB GRB timescale implies 10 1 R < cm Light-crossing time for coherent emission BATSE SD0 BATSE SD1 BATSE LAD0 BATSE SD4 Large energy OSSE confined in a small volume: COMPTEL Telescope COMPTEL Burst Mode EGRET TASC pair production is possible: γ+γ e + +e - This should Epeak create an optically thick plasma, and a thermal spectrum Flux (photons cm 2 s 1 MeV 1 ) E 2 N E (erg cm 2 s 1 ) Photon Energy (MeV) Briggs, et al, 1999

8 Compactness Problem The γ-ray spectrum is non-thermal Band spectrum (Band, et al., 1993) The millisecond variability timescale implies R < cm Light-crossing time for coherent emission Large energy confined in a small volume: pair production is possible: γ+γ e + +e - This should create an optically thick plasma, and a thermal spectrum

9 Ultrarelativistic Outflow Time dilation The ejected material is travelling near the speed of light: it is right behind the emitted photons The actual size of the GRB emission region is ~ ½cΓ 2 t ~ cm (70 A.U.) Emitted light is blue-shifted into the γ-rays Emission region is optically thin if bulk Lorentz factor Γ > 100 Some of the fastest bulk flows in the Universe

10 Fireball Model The internal/external shock model Burst emission is synchrotron emission from shock accelerated electrons Prompt γ-rays from internal shocks Afterglow from external forward shock in ambient environment Outflow is collimated We look down the center of a jet

11 Afterglows Forward external shock accelerates electrons with a power-law spectral energy distribution Relativistic electrons emit synchrotron emission Requires ambient magnetic field Ejecta decelerates into ambient medium; afterglow fades as a power-law Evolution of afterglow is self-similar 10 Magnitude GRB Time (s)

12 Long GRBs & Short GRBs Paciesas, et al., 1999 BATSE burst showed bi-modal duration distribution (Kouveliotou, et al., 1993) Short bursts also have a harder spectrum

13 Long GRB Progenitors Clues to long burst origins: Associated with star-forming regions and host galaxies (Bloom, et al, 2002) Some bursts associated with core-collapse supernovae (eg, Hjorth, et al, 2003) Fruchter, et al, 2006 Pian, et al, 2006

14 Collapsar Model Very massive (> 20 Msun) rapidly rotating star Core collapses into a black hole Stellar material (~ 0.1 Msun) accretes onto the black hole, with jets out the poles Simultaneous supernova Peaks after 1-2 weeks NASA

15 NASA Short GRBs: Merging Compact Objects Different hosts than long bursts Chandra X-ray image GRB z=0.16 Not necessarily associated with star-forming regions HST Optical Fox, et al. (2006) No associated supernovae Dimmer and closer than long GRBs Merging Compact Objects

16 GRB : A New Type of GRB? Swift detected a long, soft GRB (~ 100s) Light curve & spectrum typical of long GRB Low redshift (z=0.125) Gehrels, et al., 2006 No associated supernova: luminosity <100x seen for other long GRBs Cobb, et al. (2006), Schaefer & Xiao (2006) log F! in erg/sec/cm 2 /Ang!15!16!17!18!19!18.5 SN 1994I / 100, I SN 2002ap / 132, I SN 1994I / 100, V!19 SN 2002ap / 132, V!19.5 Swift V SSO R Watcher R Danish R HST I HST V suggest a chance coincidence with host 2!20!20! ! log time since GRB in s Gal-Yam, et al., 2006

17 Long Duration Central Engine

18 X-Ray Flares About ~50% of GRBs show X-Ray flares in the light curve Δt/t ~ 0.1 (eg, Burrows, et al, 2007, Chincarini, et al., 2007) Seen in long and short burst afterglows Flares could be... Brightening in the external shock (eg, density variations) Slow, late arriving shells Late ejections of shells from the central engine 0.3!10 kev flux (erg cm!2 s!1 ) 10!13 10!12 10!11 10!10 10!9 10!8 10!7 BAT time since burst (s) GRB a (long burst) XRT Burrows, et al. (2007) GRB (short burst) Barthelmy, et al. (2005)

19 Optical Flares? 10-6 GRB 60904b GRB060904B z=0.7 z= GRB GRB z= X-rays 10-8 Flux (erg cm -2 s -1 ) γ-rays Flux (erg cm -2 s -1 ) optical Time Since Burst (s) Time Since Burst (s) If X-ray flares from the external shock and the afterglow Expect corresponding optical flares Expect energy injected into afterglow These effects are not seen

20 Long-Lived Central Engines X-ray flares appear to have same Band spectrum as prompt γ-rays (Butler & Kocevski, 2007) Peak energy ~ 1 kev External shock cannot create small Δt/t (Lazzati & Perna, 2007) Late arriving shells (with small Γ contrast) also cannot create small Δt/t Late ejected shells can create arbitrarily small Δt/t Lazzati & Perna (2007)

21 La Parola, et al. (2006) Barthelmy, et al. (2005) What About Short GRBs? In merger, viscous timescale of the disk (< 1 s) sets short burst duration At least three short GRBs show late time X-ray flares Similarity to long burst flares suggests common origin: the accretion disk (eg, Perna, et al., 2006) Fragmentation of accretion disk? (eg, Chen, et al., 2006) GRB GRB

22 Long Duration Energy Injection Early Swift X-ray afterglows are generally flatter than predicted from afterglow models This has been taken as long duration energy injection into the shock (eg, due to late arriving slower shells) (eg, Nousek, et al., 2006) After this there is a normal decay Flux (erg cm -2 s -1 ) ± ± ±0.07 GRB GRB z=3.80 z=3.8 X-rays γ-rays Flux (erg cm -2 s -1 ) GRB GRB z=0.54 z= optical Time Since Burst (s) Time Since Burst (s)

23 Problems with Energy Injection Energy injection should affect both X-ray and optical afterglow Light curve breaks usually chromatic (eg, Panaitescu, et al., 2007) Flux (erg cm -2 s -1 ) GRB GRB z= ± ± ± Time Since Burst (s) The late energy injection implies Less kinetic energy at the early times Higher γ-ray production efficiency (> 90%) This is a challenge to the internal shock model Is the X-ray emission efficiency increasing with time? (eg, Granot, et al., 2007; Panaitescu, et al., 2007)

24 High Redshift GRBs

25 High Redshift Bursts Flux (erg cm -2 s -1 ) GRB GRB z= Time Since Burst (s) GRB z=6.3 Optical transients bright enough to be seen with a modest telescope Unique way to find high redshift starforming galaxies Probe epoch of reionization? Near infrared imaging is important Boër, et al., 2006

26 Are GRBs Standard Candles? Amati (2002) found a correlation between Epeak (peak energy of GRB spectrum) and Eiso (isotropic equivalent γ-ray energy) Amati, 2006 Schaefer (2003) found similar Epeak vs. L If L Γjet a and Ep Γjet b then L can be related to Ep

27 Jet Correction! jet "!1 "!1 GRBs are beamed Geometric effects create an achromatic light curve break at a characteristic time tjet Correcting for this jet break results in a tighter correlation between Epeak and Eγ (isotropic energy corrected for beaming) (Ghirlanda, et al., 2004)

28 Ghirlanda, et al., 2007 Jet Correction! jet "!1 "!1 GRBs are beamed Geometric effects create an achromatic light curve break at a characteristic time tjet Correcting for this jet break results in a tighter correlation between Epeak and Eγ (isotropic energy corrected for beaming) (Ghirlanda, et al., 2004)

29 Lack of Jet Breaks Most Swift bursts have not shown evidence for jet breaks Good coverage in X-rays Simple light curve consistent with jet break past 10 6 s eg, Sato, et al. (2006); Schady, et al. (2007) Flux (erg cm -2 s -1 ) GRB GRB z=1.26 z= Time Since Burst (s) Late optical coverage either insufficient or shows many [kev] src E peak 2 10 GRB GRB a XRF a bumps and wiggles Ghirlanda (2007) suggested a solution to the jet break issue E! src [10 erg] Sato, et al. (2006)

30 of 17 GRBs (N06). In the present sample of 33 events there is no outlier with respect to the Epeak Eγ and Epeak Eiso tjet correlations (besides GRB and GRB but see Ghisellini et al for the possibility that even these two GRBs are not outliers). Indeed, these correlations are strengthened by the new Swift bursts. In particular we added 6 firm bursts (i.e. with defined Epeak, z and tjet ) and 2 lower limits (on tjet ) which enlarge to 25 the original sample of pre Swift busts used to fit these correlations. With this sample we re analyzed the Epeak Eγ correlation in the two possible scenarios of a homogeneous and wind medium (HM and WM). The updated correlations are completely consistent with those found in the pre Swift era and their fits are statistically im10-6 proved (i.e. χ2 = 1.08 and χ2 = 0.89 in the HM and WM case, respectively). We also re analyzed the phenomenological Lack of Jet Breaks BAT and XRT light curve of GRB The responds to the double component model of W07 ameters listed in their table 1) with the two comesented by the long dashed lines. The value of derived by W07 is shown by the vertical long The dot-dashed line show our fit to the data (the omponents correspond to the dashed lines) by astopt = 0.6 days and by excluding the X ray flare optical R band data are shown (Monfardini et al. et al. 2006; Wozniak et al. 2006). The dotted verrks where the jet break is expected, based on the orrelation (in the WM case) and the shaded region l. The host galaxy and a possible supernova flux should be subtracted off. The break should be achromatic in the optical, but we should relax the requirement of a simultaneous break in the X ray light curve, since the X ray flux may be due to another component. If a simultaneous X ray and optical break is present, one should check that this is not the time break ending the plateau phase (Ta in Willingale et al. 2007). If this is the case, and the optical and X ray flux continue to track one another, it is possible that the component dominating the X ray flux is also contributing in the optical, hiding any jet break. A steep [i.e. F (t) t α, with α > 1.5] optical decay in a limited time interval is not necessarily an indication of a jet break occurred earlier. A steep decay can in fact be the tail of a flare. A densely sampled light curve is required to disentangle this case from a post break light curve. The bolometric luminosity should be calculated between 1 kev and 10 MeV in the rest frame of the GRB, using the spectral parameters. It is also useful, when a cut-off power law is used instead of the Band function, to give limits by using a Band model with a fixed β value. Sometimes the Swift/BAT and the Konus Wind data give different spectral fits. Given the limited energy range of BAT, one should take the result of spectral fitting the BAT data with care. An outlier for the Epeak Eγ correlation which is also an outlier for the Epeak -Eiso correlation, while being an outlier, is recognizable and should be put in the same category of GRB and GRB When deriving jet angles from the jet break times, the same values of the efficiency η should be used. One should also allow for a range in possible densities of the interstellar medium (taken in the range 1 10 cm 3 in GGL04), when this is not derived by other means. Of course this does not apply to the Epeak Eiso tjet correlation. 6 insufficient or shows many bumps and wiggles Ghirlanda (2007) suggested a solution to the jet break issue m the Amati to the Ghirlanda relation, especially bursts with similar Epeak. Note that the different Amati and Ghirlanda correlation imply that they ome small value of Eγ = Eiso corresponding to c bursts. The intersection value, however, depends mbursts density profile: it is below 1048 erg in the ller for the HM case. sion and conclusions multiple breaks of the X ray and optical afterurves the identification of the jet break time is Following these rules we have selected, among all Swift long bursts with firm redshift measurements (46 long GRBs up to Dec. 2006), those with measured peak energy and some information about a jet break in the optical light curve. We have found 16 Swift bursts that can be added to the pre Swift sample of 17 GRBs (N06). In the present sample of 33 events there is no outlier with respect to the Epeak Eγ and Epeak Eiso tjet correlations (besides GRB and GRB but see Ghisellini et al for the possibility that even these two GRBs are not outliers). Indeed, these correlations are strength- GRB z=1.26 GRB z= Time Since Burst (s) GRB src Most Swift bursts have not shown evidence for jet breaks Good coverage in X-rays Simple light curve consistent with jet break past 10 s eg, Sato, et al. (2006); Schady, et al. (2007) Late optical coverage either The jet break should be observed in the optical. The light curve in the optical should extend up to a time longer than the jet break time predicted by the Epeak Eγ G. Ghirlanda et al.: Spectral energy correlations in the Swift era correlation. E peak [kev] In the era of multiple breaks of the X ray and optical afterglow light curves the identification of the jet break time is complex. Even more so when flares (more often in the X rays, but sometimes also in the optical) occur, and the lightcurve is not densely sampled. In the following, for clarity, we list some points which we believe are particularly important for the correct identification of the jet break time. Flux (erg cm-2 s-1) 5. Discussion and conclusions GRB a 102 XRF a E! [1052 erg] Sato, et al. (2006) src

31 Other Luminosity Indicators!& Schaefer (2007), compiled a set of luminosity indicators for GRBs Based on relativistic effects Mostly empirical Using many indicators is more robust than a single indicator eg, Norris, et al. (2000); Fenimore & Ramirez-Ruiz (2000); Reichart, et al., (2001) Constructed a Hubble diagram for GRBs!"#(!,!"#&!*!"#(!,!%!$!#!"!&!%!$!#!"!&!%!$!#!" '% '$ '# " #!"#$! %&# '()*+,- Lag-Luminosity '% '$ '#!"#$%&'()*+ Variability-Luminosity '% '$ '# " #!"#$! %& '()*+,- Min. Rise Time-Luminosity

32 Other Luminosity Indicators '$ #" #* Concordance Cosmology Schaefer (2007), compiled a set of luminosity indicators for!"#(!,!&!%!$!# ):8<@AB6$,C7DED8$=F@G? #) #( #' ## #! #& #% #$!" GRBs Based on relativistic effects Mostly empirical Using many indicators is more robust than a single indicator!"#$%&##'($)*+!"+,!, $-$./012$3$-$45 eg, Norris, et al. (2000); Fenimore & Ramirez-Ruiz (2000); Reichart, et $ % &! # ' ( ) al., (2001) "6789:;<$=>? Constructed a Hubble diagram for GRBs 7. GRB Hubble diagram for the concordance cosmology. For the concordance!"#&!*!"#(!,!"!&!%!$!#!"!&!%!$!#!" '% '$ '# " #!"#$! %&# '()*+,- Lag-Luminosity '% '$ '#!"#$%&'()*+ Variability-Luminosity '% '$ '# " #!"#$! %& '()*+,- Min. Rise Time-Luminosity

33 High Energy Emission

34 High Energy Emission GRB emission has been seen to very high energies GRB What is the true high energy profile? Synchtrotron + Inverse Compton? Evidence for particle acceleration? Pe er & Waxman (2003), from Gonzales, et al. (2003) X-Ray Flares: Is there IC emission? Hurley, et al. (1994)

35 GLAST Gamma-Ray Large Area Space Telescope The Large Area Telescope (LAT) Silicon tracker + calorimeter + anticoincidence detector A γ-ray particle detector in space ~ 20 MeV GeV Arcminute GRB positions The Glast Burst Monitor (GBM) (8 kev - 25 MeV) Poor localizations Study AGNs, GRBs, pulsars, dark matter...

36 ROTSE-III And GLAST Optical counterparts are required to determine burst redshifts and broadband properties The LAT positions improve at higher energy Not clear how many high energy photons are created by GRBs Typical positions will be ~ 30 arcminutes Require wide-field optical follow-up

37 Conclusions Swift has made many GRB discoveries Short burst localizations Long-lived central engine Energy injection/difficulties with standard afterglow modeling GRBs might be used for cosmology Until we understand the prompt emission & afterglow emission, this is a challenge GLAST launch in November will answer questions and probably create more questions

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39 Outline Gamma-Ray Bursts (GRBs) Prompt Optical Emission from GRBs The ROTSE-III Telescope Array Recent Results