1. Discovery of radio afterglow from the most distant cosmic explosion Poonam Chandra Royal Military College of Canada

2. Collaborators Dale Frail (NRAO), Derek Fox (PSU), Shri Kulkarni (Caltech), Edo Berger (Harvard), Brad Cenko (UCB), Douglas Bock (CARMA), Fiona Harrison (Caltech) and Mansi Kasliwal (Caltech) Publication: Chandra et al. “Discovery of Radio Afterglow from the Most Distant Cosmic Explosion” 2010 ApJ Letters 712, 31 2

3. GAMMA-RAY BURSTS (GRBs)  Most luminous events in the universe since big bang  Flashes of gamma-rays from random directions in sky  Few milliseconds to few seconds timescale  Even 100 times more energetic than supernovae  Brightest sources of cosmic gamma-ray photons in the universe  In universe roughly 1 GRB is detected per day  Short duration ( 2sec)  Most luminous events in the universe since big bang  Flashes of gamma-rays from random directions in sky  Few milliseconds to few seconds timescale  Even 100 times more energetic than supernovae  Brightest sources of cosmic gamma-ray photons in the universe  In universe roughly 1 GRB is detected per day  Short duration ( 2sec)

4. AVERAGE REDSHIFT = 1.5

5. SWIFT AVERAGE REDSHIFT = 2.7

6. Prospects for future observations 6

7. GRB are bright. Detectable at high redshift 7 GRB 080319B, Bloom et al. (2009)

8. Hubble Space Telescope Realm of the Galaxies  /  ~ 1e5

9. The First Stars (aka Population III) Initially formed from dark matter mini-halos at z=20-30 before galaxies Metal-free gas is unable to collapse via dust or atomic line cooling H 2 molecules form via gas phase reaction. Cools gas. Many uncertainties but a top-heavy IMF is a robust prediction Pop III: M~100 M , L~10 5 L , T~10 5 K, Lifetime~2-3 Myrs Single or double stars form Dominant mode of SF below 10 -3.5 Z  9 Turk, Abel, O”Shea (2009)

10. Observing the first stars. One star at a time Direct detection of first stars or surroundings may be impossible Prospects are good for the direct detection of stellar death The First Supernovae Pair Instability Supernovae (PISNe) – 140 M  < M * <260 M  – Bright, long-lived light curves – Returns metal-rich material Core Collapse Supernovae – 25 M  260 M  – Direct collapse to BH – BHs may be seeds for first quasars – Ideal properties to produce long duration gamma-ray bursts

11. First Supernovae. Observational diagnostics Low metalicity and the absence of dust extinction – NIR spectroscopy – X-ray photoelectric absorption Hyper-energetic explosion Low magnetic field Low density HII region – Strong radiation pressure from Pop III star – creates low density (1 cm -3 ) constant density region (10 pc) 11

12. Open questions in the high z universe How did the first stars, black holes and galaxies form? How and when did the metals get added? How was the universe re-ionized? 12 Barkana and Loeb (2007)

13. Negative k-correction at radio wavelengths Effect was first noted by Ciardi & Loeb (2000) Steep synchrotron self-absorption ( ν 2 ) partially counteracts d L 2 diming Time dilation (1+z) helps to probe the early epoch of reverse shock From z=2 to z=10 flux density drops only 40% 13 Frail et al. (2006)

14. Only 3 GRBs with z > 6 GRB 050904 GRB 080913 GRB 090423

15. Discovery of a GRB Afterglow at redshift 8.26 15 Swift. April 23, 2009 T+73 s. X-ray begins T+109 s. Optical begins No optical transient T+20 min. UKIRT begins. Faint K band transient discovered Y-band (1 um) dropout Too steep for dust  Gunn-Petterson Photo z from GROND, Gemini and VLT Spectro-z from VLT/SINFONI and ISAAC Photo-z: 8.06±0.25 Spectro-z 8.23±0.08 Tanvir et al. 2009 ISAAC Spectroscopy

16. Re-cap. Redshifts of important objects Object NameRedshift Milky Wayz=0.0 Virgo Clusterz=0.004 Quasar 3C273z=0.158 “Era of Galaxy formation”z=1-2 Most distant quasarz=6.43 Most distant galaxy (firm)z=6.96 GRB 090423z=8.2 First Stars appearz=20-30 Cosmic Microwave Background (CMB)z=1089 Big Bangz∞z∞ 16

17. Chris Carilli (NRAO)‏ Berlin June 29, 2005

18. A brief pause for some perspective Redshift z=8.2 is 630 millions years after the Big Bang. The Universe is 13.7 Billion old. The Earth is 4.6 Billion years old What was happening on Earth 630 Myrs ago? Ediacaran Period. Last extensive glaciation over (Cryogenian). Cambrian (explosion) hasn’t yet happened. No fish, no insects, no amphibians, etc. Soft-bodied fossils 18

19. Radio observations of GRB 090423 Detections - VLA: 8.5 GHz on Apr 25-Jun 27. – 74 +/- 22 uJy at Δ t~8 d – 2-hr integrations every 2 days – Data sets averaged (in UV plane) to improve detection sensitivity – Undetectable after Δ t~65 d PdBI: 95 GHz on Apr 23-24 – Castro-Tirado et al. report a secure source detection of 200 uJy (no error bar given) Non-Detections - WSRT: 4.9 GHz on May 22-23 CARMA: 95 GHz on Apr. 25 IRAM 30-m: 250 GHz on Apr 25 19

24. 24 Chandra et al. (2009) Last Chandra measurement

25. 25 Chandra et al. (2009) Last Chandra measurement -1.35 -1.10 Peak flux

26. Broadband modeling What can be learned from simple afterglow theory? – Peak flux NIR ~ Peak flux radio (F max =const.): ISM – Time to peak NIR=0.08 d, Radio=50 d. ( ν m α t -3/2 ) – No obvious jet break until later than day 50 – α NIR -α X ~0.25, i.e. cooling break (3-3p)/2(=-1.10) to (3-2p)/2(=-1.35) gives p=2.46 –Host extinction is low, Av<0.08 (from Tanvir et al.) 26

27. Broadband modeling Now fit broadband data to a detailed afterglow model –Model fitting is from Yost et al. –Constant density circumburst medium –Fit for E k, n o, θ j,ε B and ε e (keep p fixed) 27

28. Best Fit Parameters for p=2.46 ParametersIsotropicJet (t>45 d) E(Gamma) ergs1x10 53 >2.2x10 51 E(Kinetic) ergs3.8x10 53 >8.4x10 51 Density cm -3 0.9 --- e 0.02 --- eBeB 0.28 ---

29. Previous high redshift GRB 050904 z=6.26 29 Haislip et al. 2006; Kawai et al. 2006; Gou, Fox & Meszaros (2006)

30. Previous high redshift GRB 050904 z=6.26 30 Haislip et al. 2006; Kawai et al. 2006; Gou, Fox & Meszaros (2006) DENSITY> 100-680 cm -3

31. Comparisons with other afterglows 31 Chandra et al. (2010)

32. Comparisons with other afterglows Afterglow Properties – – GRB 050904 (z=6.26). Both are hyper-energetic (>10 51 erg) but they exploded in very different environments. (in situ) – Large energy predicted for Pop III. Not unique. – Low, constant density predicted for Pop III. Not unique. – No predictions for θ j, ε B, ε e & p High Energy Properties – – No quantitative predictions for luminosity, fluence, duration, and radiated energy for high z GRBs – Argument by Salvaterra et al. for Z>0.04 Z o not robust 32 Neither the high energy nor the afterglow properties are sufficiently unique to distinguish GRB 090423 from other GRBs

33. A rumination on the early radio emission Flux on first VLA detection (t rest ~1 d) exceeds FS predictions Excess, early emission is rather common for radio afterglows and is attributed to reverse shocks ( Kulkarni et al. 1999, Soderberg & Ramirez-Ruiz 2003, Nakar & Piran 2004) We predict that the 100 GHz detection on day 1-2 by Castro- Tirado et al. is dominated by a RS mm emission from RS is bright, redshift-independent (no extinction or scintillation). ALMA will be ideal. 33 Chandra et al. (2009)

34. A rumination on the early radio emission Flux on first VLA detection (t rest ~1 d) exceeds FS predictions Excess, early emission is rather common for radio afterglows and is attributed to reverse shocks ( Kulkarni et al. 1999, Soderberg & Ramirez-Ruiz 2003, Nakar & Piran 2004) We predict that the 100 GHz detection on day 1-2 by Castro- Tirado et al. is dominated by a RS mm emission from RS is bright, redshift-independent (no extinction or scintillation). ALMA will be ideal. 34 Inoue, Omukai, Ciardi (2007)

35. A seismic shift in radio afterglow studies The VLA gets a makeover! More bandwidth, better receivers, frequency coverage 20-fold increase in sensitivity Capabilities start in 2010 GRBs at higher frequencies where ISS is reduced Measure polarization and rotation measures Absorption lines possible (CO; see Inoue et al. 2007) z=2.5, EVLA 3σ, Δt=1 hr z=8.5, EVLA 3σ, Δt=1 hr

36. Conclusions We have detected the radio afterglow of GRB 090324 at z=8.3 The best-fit broad-band afterglow model is a quasi-spherical ( θ j >12 o ), hyper-energetic (10 52 erg) explosion in a constant, low density (n=1 cm -3 ) medium. GRB 050904 (z=6.26) was also hyper-energetic but it exploded into a high density medium (100X larger than GRB 090423) The high energy and afterglow properties of GRB 090423 are not sufficiently different from GRBs at moderate redshift to suggest a different type of progenitor model (e.g. Pop III). EVLA and ALMA are coming (soon). They will be important tools for both detecting and studying the first generations of stars in the early universe 36