Revisiting the Gamma-Ray Burst Classification in the Fermi Era

1. Revisiting the Gamma-Ray Burst Classification in the Fermi Era 张 富 文 紫金山天文台/桂林理工大学 合作者： 邵 琅 范一中 韦大明

2. Open Questions in GRB Physics(Zhang 2011) • Classification • Progenitor • Central engine • Ejecta composition • Energy dissipation and particle acceleration mechanism • Radiation mechanism • Long term engine activity • External shock afterglow physics • Origin of high energy emission • Cosmological setting

3. Contents • Previous classification results • Fermi data analysis • Conclusion and discussion

4. Duration and hardness ratio as classification discriminators • long/soft class 3/4 • short/hard class 1/4 Kouveliotou et al. 1993

5. A firmer footing afterglow and host galaxy observations • Long/soft GRBs -------------- massive stellar explosions (Type Ic Supernovae association; star-forming region; galaxy center ) • Short/hard GRBs ------------ compact object mergers (elliptical or early type host galaxies, little star formation; large offset)

6. Challenge and question GRB 060614 and GRB 060505 are two long duration nearby GRBs • have no bright SN associations • Share similar properties to short GRBs (Fynbo et al. 2006; Della Valle et al. 2006).

7. High-z Bursts • GRB 080913 z = 6.7 T90 = 8 s • GRB 090423 z =8.2 T90 = 12 s • GRB 090429B z ∼ 9.4 T90 = 5.5 s (Cucchiara et al. 2011) T90 ~ 1 sec in the rest frame, but their properties similar to long bursts

8. GRB 090426 z = 2.609 T90 = 1.28 s host galaxy is a blue, luminous and star-forming galaxy (Levesque et al. 2010) (Thone et al. 2011 )

9. the circumburst particle number density (about 10 cm−3) is much higher, similar to classical long-duration GRBs (Xin et al. 2011) The half-opening angle of the suspected jet as well as the luminosity indicate the death of a massive star rather than to the merger of two compact objects (Guelbenzu et al. 2011)

10. Hardness vs. T90 BAT data Sakamoto et al. 2011

11. Energy Ratio Distribution (Goldstein et al. 2010) Their results confirm the presence of two GRB classes as well as heavily suggesting two different GRB progenitor types.

12. certain observation properties do not always refer to certain types of progenitor massive star origin (compact star origin) ( Zhang et al. 2009)

13. Low -ε Type I (compact star origin) High -ε Type II (massive star origin) (Lv et al. 2010)

14. Revisit the classification with Fermi GRB data Sample: 1. Fermi GRB catalogue (Nava et al. 2011) Sample 1: GRBs with curved spectra(Band or CPL spectra) (316) Sample 2: GRBs with power-law spectra (110) 2. All GRBs with well measured peak energies and redshifts

15. Duration distribution Curved spectra (Band or CPL) 2 s Power-law spectra All Fermi GRBs

16. HR vs Duration

17. Peak energy/alpha vs duration

18. Epeak/fluence energy ratio distribution short long

19. Ep-Eiso Correlation

20. Ep-Liso correlation

21. Conclusion and discussion • Both the duration and the energy ratio, two well known classification discriminator, all show a obvious bimodality, but no robust separation line • The picture that bursts could be divided to two classes is confirmed again. • The correlation between hardness ratio and duration strongly depend upon the spectral shape of GRBs. • Short and long GRBs could follow different peak energy-isotropic energy correlation, but follow the same peak energy-luminosity correlation.

22. Energy Dependence of duration Bissaldi et al. 2011

23. T90 (average) vs. Energy Bissaldi et al. 2011

24. Fong et al. 2010 Offset

25. energy comparison Nysewander et al .2009

26. However, the fundamental questions remains unanswered: • How to class a individual burst only based on one classifier • What are the progenitors of short and long GRBs? Thanks！