The Long and the Short of Gamma-Ray Bursts

Kevin Hurley

UC Berkeley

Space Sciences Laboratory

EXAMPLE

“…I was asked to give this talk in Italy, on a small Venetian island called San Servolo. It sounded so good, I couldn’t refuse. But the long and the short of it is, that when I got there, I discovered that the place used to be an insane asylum!*”

*http://www.sanservolo.provincia.venezia.it/english/news/museoManicomio.asp

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-DOUBLE LOGNORMAL FIT

THE BATSE GRB DURATION DISTRIBUTION

(McBreen et al. 1994)

THE IPN AND SWIFT GRB DURATION DISTRIBUTIONS

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836 IPN BURSTS

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WHICH DISTRIBUTION IS CLOSEST TO THE TRUE ONE?

• Distributions are instrument-dependent:

– Trigger criteria, detector sizes, efficiencies as a function of energy, sensitivities

• IPN is a mix of many instruments, and sensitivity is limited (not good)

• Swift is very sensitive, has good trigger criteria, but its high energy coverage is limited by both the detector and the mask efficiencies (better, but not perfect)

• BATSE had good trigger criteria, good high energy efficiency, and observed many bursts. Their distribution is probably closest, but by no means without strong selection effects

BASED ON DURATION >25 keV ALONE, WHAT IS THE DIVIDING LINE BETWEEN LONG AND SHORT GRBS?

• The lognormal fit to the two distributions predicts that a burst with T90<5 s has a probability <10-3 of being a long duration burst

• This is roughly a 3σ criterion

• A 10-6 probability criterion would be T90<0.9 s

• But duration should not be used by itself if more data are available

~~

~

WHY ARE THE SHORT AND LONG BURST DISTRIBUTIONS SO BROAD?

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MODE=0.5

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• Define the relative width as

• Then the relative width of the short burst distribution is 2.95, and the relative width of the long burst distribution is 2.91

• They are equally broad, in relative terms

• (Same conclusion using variance0.5/mean)

• Is this because we are using the “raw” durations, i.e. uncorrected for redshift and systematics?

FWHMMode

DURATION DISTRIBUTIONS OF 70 GRBs WITH MEASURED T90 AND REDSHIFTS

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BEFORE REDSHIFT CORRECTION

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MEAN STANDARD DEVIATION

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116 s 335 s

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65 s 312 s

1. The mean is smaller after redshift correction, as expected

2. The standard deviation does not change substantially afterredshift correction!

IS THE WIDTH OF THE DURATION DISTRIBUTION MAINLY INTRINSIC?

• A variable has a lognormal distribution when it is the outcome of many multiplicative factors

• For GRB durations, this might include progenitor mass, surrounding ISM, details of accretion onto the black hole

• But this is not the only effect that shapes the distributions

– S/N considerations for the long duration bursts may introduce a factor ~2 uncertainty in T90 (Bonnell et al. 1997)

– a simple 1+z correction ignores other factors (e.g. pulse width as a function of energy)

OTHER WAYS OF DETERMINING WHETHER A BURST IS LONG OR SHORT, IN ORDER OF INCREASING COMPLEXITY

(Donaghy et al. astro-ph/0605570)

1. Duration

2. Spectral hardness

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•Hardness is good for distinguishing short SGR bursts (not giant flares) from GRBs

•But it is a weak criterion for distinguishing long and short GRBs

1. Duration

2. Spectral hardness

3. Extended soft emission

OTHER WAYS OF DETERMINING WHETHER A BURST IS LONG

OR SHORT, IN ORDER OF INCREASING COMPLEXITY (Donaghy et al. astro-ph/0605570)

Mazets et al.2002

•Present in some BATSE bursts (Lazzati et al. 2001; Connaughton 2002)

•Present in some Konus-Wind bursts (Mazets et al. 2002; Frederiks et al. 2004)

•Present in some Swift and HETE bursts (Barthelmy et al. 2005; Villasenor et al. 2005)

•Possibly a defining characteristic of short bursts, but requires good low energy coverage and/or intense burst to detect it

OTHER WAYS OF DETERMINING WHETHER A BURST IS LONG OR SHORT (Donaghy et al. astro-ph/0605570)

1. Duration

2. Spectral hardness

3. Extended soft emission

4. Spectral lag

Norris 2002Norris &Bonnell.2006

Lag vs. flux for 1429 long BATSE bursts Lag vs. flux for 260 short BATSE bursts Average lag ~ 50 ms Average lag ~0.1±0.5 s (consistent with 0)

• If a burst has small lag with good significance, it is probably short; if it has significant positive lag, it’s probably long

•If T90<2 s and lag is consistent with zero, it’s probably a short burst

1. Duration

2. Spectral hardness

3. Extended soft emission

4. Spectral lag

5. Pulse widths

OTHER WAYS OF DETERMINING WHETHER A BURST IS LONG

OR SHORT, IN ORDER OF INCREASING COMPLEXITY (Donaghy et al. astro-ph/0605570)

PULSE WIDTHS (Norris et al. 1996)

• Decompose the GRB time history into many individual pulses described by

I(t)=A exp[-( t-tmax / σrise)ν] , t < tmax

I(t)=A exp[-( t-tmax / σdecay)ν] , t > tmax

• Pulse widths for short bursts are tens of milliseconds long

• Pulse widths for long bursts are hundreds of milliseconds long

• Don’t try this at home!

1. Duration

2. Spectral hardness

3. Extended soft emission

4. Spectral lag

5. Pulse widths

6. Energy radiated in gamma-rays

OTHER WAYS OF DETERMINING WHETHER A BURST IS LONG

OR SHORT, IN ORDER OF INCREASING COMPLEXITY (Donaghy et al. astro-ph/0605570)

Eγ

• In general, short bursts radiate 1048 – 1049 erg, while long bursts radiate 1050 – 1051 erg

• Redshift obviously required

• Evidence for beaming useful, but not always required

SOME WELL-STUDIED SHORT BURSTS

DATE DETECTED BY

z Eγ, erg BEAMED? HOST,

LOCATION

790613 IPN 0.09?* 6x1049iso ? ?

050509B Swift 0.225?* 2.7x1048iso ? ELLIPTICAL?

050709B HETE 0.16 3x1048 14º SPIRAL,

OUTSKIRTS

050724 Swift 0.258 9.9x1049iso ? ELLIPTICAL,

OUTSKIRTS

050813 Swift 0.722 1.7x1050iso ? ?

050906 Swift 0.031?* 1.2x1047iso ? ?

060121 HETE 1.5/4.6?* 3/1.3x1049 2.3º/0.6º SPIRAL?

*Based on probability argument

1. Duration

2. Spectral hardness

3. Extended soft emission

4. Spectral lag

5. Pulse widths

6. Energy radiated in gamma-rays

7. Location within host galaxy

OTHER WAYS OF DETERMINING WHETHER A BURST IS LONG

OR SHORT, IN ORDER OF INCREASING COMPLEXITY (Donaghy et al. astro-ph/0605570)

LOCATION WITHIN HOST GALAXY

• Long bursts occur in star-forming regions

• Location in the outskirts older stellar population

• But a location within a galaxy wouldn’t necessarily contradict this

GRB050709B; Fox et al. 2005

GRB050724; Berger et al. 2005

1. Duration

2. Spectral hardness

3. Extended soft emission

4. Spectral lag

5. Pulse widths

6. Energy radiated in gamma-rays

7. Location within host galaxy

8. Type of host galaxy/age of stellar population

OTHER WAYS OF DETERMINING WHETHER A BURST IS LONG

OR SHORT, IN ORDER OF INCREASING COMPLEXITY (Donaghy et al. astro-ph/0605570)

SOME WELL-STUDIED SHORT BURSTS

DATE DETECTED BY

z Eγ, erg BEAMED? HOST,

LOCATION

790613 IPN 0.09?* 6x1049iso ? ?

050509B Swift 0.225?* 2.7x1048iso ? ELLIPTICAL?

050709B HETE 0.16 3x1048 14º SPIRAL,

OUTSKIRTS

050724 Swift 0.258 9.9x1049iso ? ELLIPTICAL,

OUTSKIRTS

050813 Swift 0.722 1.7x1050iso ? ?

050906 Swift 0.031?* 1.2x1047iso ? ?

060121 HETE 1.5/4.6?* 3/1.3x1049 2.3º/0.6º SPIRAL?

*Based on probability argument

• Host galaxy which is not undergoing star formation is probably a good indication of a short burst

• But short bursts can also occur in galaxies which are still forming stars

Gorosabel et al. 2006

CONCLUSIONS

• 10 years ago, distinguishing long and short bursts was easy – hardness and duration were the only criteria required

• Today, it’s much harder – up to 9 criteria should be used, and some are ambiguous

• We should probably avoid the terms “short-hard bursts” and “long-soft bursts” entirely

• Donaghy et al. (2006) divide the criteria into gold (lag, extended emission, location, host type), silver (duration, pulse width, Eγ), and bronze (spectral hardness) standards

• But the platinum standard may turn out to be detection of gravitational radiation

• And that’s the long and the short of it