Rise and Fall of the X-ray flash 080330: an off-axis jet?

1. Rise and Fall of the X-ray flash 080330: an off-axis jet? C.Guidorzi 1,2,3 on behalf of a large collaboration of the Swift, Liverpool and Faulkes Telescopes, GROND, NOT, REM, TAROT teams and in particular S. Kobayashi and J. Granot 1INAF-Osservatorio Astronomico Brera, Italy 2Astrophysics Research Institute, Liverpool John Moores University, UK 3Physics Dept. University of Ferrara, Italy

2. Outline • X-Ray Flashes (XRFs) as a class of GRBs • XRF 080330: broadband data set • Observed properties: • prompt -ray emission • Broadband (X-ray,UV,B,V,r,i,z,J,H,Ks) afterglow (light curves, SED) • Interpretation of XRF 080330 properties Egypt 2009

3. X-ray Flashes (XRFs) A softer and less energetic version of classical GRBs (Heise et al. 2001; Kippen et al. 2001) e.g. Ep vs. Eiso relation (Amati et al. 2008) Egypt 2009

4. XRFs vs. c-GRBs = • Overall, same prompt temporal properties • On average, same afterglow properties(although see Sakamoto et al. 2008 and the Swift sample) • Associations (or lack of) with hypernovae ≠ • XRFs are softer, due to a lower Ep (≤ 30 keV), while c-GRBs have Ep of a few 100 keV. • In some cases, less energetic and smoother -ray light curves(e.g. 060218, Campana et al., 2006) Egypt 2009

5. F Peak Energy: Ep XRF XRR GRB Sakamoto et al. 2008 Egypt 2009

6. XRF 080330 • Swift-BAT (15-150 keV) detected and promptly localised it. • Swift-XRT and UVOT promptly followed it up and began at 77 sec post trigger time  X-ray and UV afterglow. • Several robotic facilities promptly reacted and discovered the rising optical counterpart: in particular, 2-m class telescopes (GROND, and LT). Egypt 2009

7. -ray prompt emission Egypt 2009

8. -ray prompt emission • Swift-BAT (15-150 keV) detected and promptly localised it. • Ep < 35 keV • 4 pulses • Marginal soft-to-hard evolution, from 2 to 1.5 • S(15-150 keV)= 3.6 x 10-7 erg cm-2 • Eiso < 2.2 x 1052 ergs XRT Egypt 2009

9. Panchromatic Light curves (from 30 s out to a few days post burst) Egypt 2009

10. X-ray  NIR Light Curve Shallow optical rise Fo(t) t+0.5 Egypt 2009

11. Spectral Energy Distributions: 1 , 2 Typical Band fit ox = 0.74 ± 0.03 Egypt 2009

12. X-ray  NIR Light Curve Plateau at every  Egypt 2009

13. SED 3: a single unextinguished PL! ox = 0.79 ±0.01 Egypt 2009

14. Decay and late-time red Bump (at 1 day) Egypt 2009

15. At 1 day it got redder! Red bump o = 1.05 ± 0.06 Egypt 2009

16. -Did you measure z?-Yes, we did. Egypt 2009

17. NOT: absorption spectrum z = 1.51 Taken at t=46 min Egypt 2009

18. Multi-band simultaneous Modeling F(t)  t- 1 -0.6 2 0.15 3 1.1 4 3.5 F(t)  t- 1 -0.4 2 2.0 t1 600 s t2 34 ks Egypt 2009

19. Main Properties: Summary • Soft, long 4-pulsed event. • X-ray steep decay is high-latitude emission of the last pulse  end of the prompt emission • Long plateau (typical X-rays, not so much in optical), single PL spectrum with almost no dust: Av<0.02 • Rise-plateau-decay is ACHROMATIC • Red bump at 1 day Egypt 2009

20. Interpretation(s) Egypt 2009

21. Interpretation(s) • Does the optical rise mark the afterglow onset? No, too slow! • Achromatic evolution  geometry  jet(s) • How many jets? • With just one jet, red bump is the reverse shock of a late energy injection episode. • 2 is also OK (e.g. see 080319B), but more contrived! Mind Okkham’s razor! Egypt 2009

22. Off-axis jet obs  2 0 , (0 few degrees) Granot et al. 2005 Egypt 2009

23. Why does the Sphynx look so tiny? Maybe you’re an off-axis observer… Egypt 2009