orphan afterglows daniel perley astro 250 3 april 2007

Orphan Afterglows

Daniel Perley

Astro 250

3 April 2007

3 April 2007Astro 250: Orphan AfterglowsDaniel Perley

Gamma-Ray Burst Review

Burst of gamma-ray emission lasting 0.01 – 1000 s

Typical fluence 10-7 to 10-3 erg/cm2

Isotropic (extragalactic); 0.01 < z < 7

Observed rate ~1-3/day

Less for instruments with limited sensitivity or field of view

Followed by long-wavelength afterglow counterpart

Lasts for days to weeks to months

GRB Review


GRB vs. AfterglowGRB Review

GRB: Internal Shocks Afterglow: Forward ShockCollision/interaction of shells of ultra-relativistic ejected material

Intrinsic to progenitor

Relativistic shock moving into circumburst medium

Depends on environment


GRBs are Relativistic

GRBs have short-timescale structure → small

GRBs are extremely distant → extreme energy density

Very large photon densities result in pair production (→ e+ e-) opacity

But,

• Spectrum is non-thermal

• Extremely high-energy photons observed for some bursts

Solution: relativistic motion towards observer (energy density in source frame can be much less due to Doppler/light-travel/relativistic effects)

"compactness problem"

GRB Review


GRBs are Anisotropic... probably

Isotropic-derved energy requirements are phenomenal

GRB971214 (z=3.42): Eiso = 2.1 × 1053 erg

GRB990123 (z=1.60): Eiso = 1.4 × 1054 erg

GRB050904 (z=6.29): Eiso = 3.8 × 1053 erg

Eiso ≡ (S) × 4 π dL2

M . c2 = 1.78 × 1054 erg

gamma-rays only

Most theoretical models favor jet geometry

GRB Review


GRB Jets

Assume that GRBs are jetted.Fairly good evidence this is the case.

Recently the details are becoming controversial again.

How does a jet geometry affect the appearance and evolution of a GRB and its afterglow?

GRB Jets


The Relativistic Beam

An emitter moving at v ~ c, even if emitting isotropically in its rest frame, will strongly beam its radiation as seen by the observer.

GRB Jets

v = 0Γ = 1

v ~ cΓ >> 1

Beaming angle : Ω = 4π

Beaming angle : Ω = π θ2

θbeam ~ 1 / Γ

1 / Γ


Prompt Emission Beaming

Initially, the GRB emission is strongly beamed towards the observer.(and anyone else within the jet beam)

θjet

intrinsic burst parameters:

θjet - jet opening angle

Eiso - 4π × energy per steradian

Γinit – initial Lorentz factor

Assume for simplicity a uniform ("top-hat") jet throughout this analysis.

θbeam = 1 / Γinit

internal shocks (prompt)

t = 10 sec

Γinit

we see only a small part of the emitting region

GRB Jets


Afterglow Beaming

Once the afterglow sets in, the shock has swept up significant quantities of circumburst material and is decelerating.

external shock

θbeam = 1 / Γ

Γθjet

t = 100 sec

GRB Jets


Afterglow Beaming

Each part of the afterglow shock is therefore beaming its emission into a larger area.

t = 100 sec

Γθjet

θbeam = 1 / Γ

GRB Jets


Afterglow Beaming

the apparent emitting region becomes a larger fraction of the jet

(specifically, θE = θbeam = 1/Γ)

1 / Γ

1 / Γ

Each part of the afterglow shock is therefore beaming its emission into a larger area.

t = 100 sec

Γθjet

GRB Jets

However, this does not affect the flux viewed from Earth: emission from other areas is now beamed in our direction, replacing the lost flux.


Jet Break

Eventually, the shock slows down enough that the apparent emitting region is the entire jet.

t = 1 day

Γθjet

Note that up until this point, the GRB evolution is indistinguishable from a spherical explosion – points outside the jet are out of causal contact.

Occurs when θE = θjet

1/Γ = θjet

Γ = 1/θjet

GRB Jets


Post-Jet Break Evolution

After this point, the increasingly outward-beamed radiation is no longer compensated by seeing more of the jet, and the observed flux drops rapidly.

θjet

Γ

t = 10 day

GRB Jets


Sideways Spreading

At about the same time, the jet itself becomes "aware" of its finite extent, and it begins to expand outward.Its forward motion rapidly decelerates. t = 100

day

Γ

Γ ~ e-r/rsedov

~ t-1/2

GRB Jets


Transition to Non-Relativistic

Eventually the shock becomes nonrelativistic and unbeamed.

Γ ~ 1t = 500 day

GRB Jets


Jet Light Curves

Quantitative predictions are somewhat in dispute...

log

F

log t

t -(3/4)(p-1)

t -(3/4)p

t -p

log

F

tjet

t 1/2

t -1/3

t -9/10

constt -1/4 ?

optical / x-ray

radio

tjet tspread

tspread

tNR

tNR

Generally agreed-upon predictions:

• First break occurs when Γ = 1/θjet

Hydrodynamics: Γ(t) ~ 6 (Eiso/nISM)1/8 t-3/8

tjet ~ 0.25d (E/n)1/3(θjet/0.1)8/3

• Break should be achromatic

No change in the spectrum; just viewing geometry

Eiso in units of 1052 erg

nISM in units of cm-3

t in units of day

GRB Jets


Observed Jet Breaks

Jet breaks have been seen. Maybe.

GRB 030226

If the Swift jet break crisis is real, all of this may be bunk.still unbroken at

15 days

GRB Jets


Off-Axis Prompt Emission

What if the jet is not directed at us?

θjet

Γinit

GRB emission is completely absent outside the physical jet beam (the relativistic beam is much narrower at this stage)

Off-Axis Jets

θbeam = 1 / Γinit


Off-Axis Early Afterglow

The early afterglow is also invisible.

θjet

Γ

Off-Axis Jets


Off-Axis Jet Break

Sometime after the "break" time, however, the jet becomes visible

θjet

Jet center is visible when θbeam ~ θobs

Γ(t) ~ 1/θobs

θobs

Γ

Once the full jet is visible the afterglow will be (about) as bright as if we were on-axis at this time

Off-Axis Jets


Off-Axis Light Curves

Off-axis light curve similar to on-axis, except that there is nearly no flux until Γ(t) ~ 1/θobs

log

F

t -(3/4)(p-1)

t -(3/4)p

t -poptical / x-ray

log t

log

F

tjet

t 1/2

t -1/3

constt -1/4 ?

radio

tspread tNR

Off-Axis Jets

There is an afterglow without a GRB: "orphan afterglow"


Off-Axis Light Curves

Some more precise predictions:

Radio (Totani & Panaitescu 2000)

Optical (Granot et al. 2002)

Off-Axis Jets


Another Pathway: Dirty GRBs

One can also generate an on-axis orphan afterglow by surpressing the GRB (gamma-ray) flux.

This can be achieved with an energetic but low-Γinit burst outflow: will be opaque to pair-production (remember compactness problem) and not generate gamma rays, but will create a "normal" afterglow.

Γinit

Dirty GRBs

I will ignore this completely in the remainder of the lecture.


The Orphan Afterglow Rate

For any burst, the post-jet afterglow is beamed towards a much larger angle than the early afterglow, so the rate of orphan afterglows should be very large.

θobs

Typically, θjet ~ 5°

θobs potentially can be

the whole sky

θjet

However, GRBs are much brighter and easier to detect than orphan afterglows, and can be seen to much greater distances / less energetic events.



Afterglow Rate Estimate Procedure

Intrinsic questions:• What is the GRB rate as a function of redshift?• What is the GRB luminosity function?• What is the jet angle distribution?

• What is the distribution of Γinit and nISM?

• How does afterglow luminosity relate to GRB luminosity?

Survey questions:• What is the survey sky area?• What is the survey flux limit?• Can the survey distinguish OAs from other transients?



Optical Orphan Afterglow Rate

Optical orphan afterglows: "near misses"


Optical flux drops precipitously after the jet break (the soonest possible time an off-axis afterglow can be observed)

Therefore, only near-axis events will be seen. But these events can be quite bright and visible to large (cosmological) distances: intrinsic luminosity limited.


A First-Cut Estimate

Rate of GRBs: 1000 / year

Fraction of GRBs with afterglow detectable (R<23) to t ~ 10 tjet: 3%

Observable for ~10 days

For an R<23 survey:

# on-axis detectable afterglows:30 / sky / year = 1 / sky now

# off-axis afterglows: 32 this many300 /sky/year = 10 / sky now

Need to survey the equivalent of 1/10 of the sky (4000 sq deg.) to find an orphan afterglow.Can repeat if the interval is >~ 10 days.



More Precise Estimates

Estimates in the literature vary wildly.• Uncertain rate of very low-luminosity GRBs• Uncertainty in actual light curve of an off-axis event• Uncertain distribution of GRB afterglow luminosities (dark bursts,

etc)

Totani & Panaitescu (2

002)

Zou et al. (2

007)

Nakar et a

l. (2002)



Optical Survey Design

Shallow and wide, or narrow and deep?

Totani & Panaitescu (2

002)

Zou et al. (2

002)

Nakar et a

l. (2007)

but, expect R F-1 → shallow survey

limiting flux of a telescope:

Flimit texp-1/2

texp Flimit-2

area of a survey:

Asurvey = (Tsurvey / texp) × FOV texp

-1

Flimit2

limiting rate of a survey:

Rlimit = 4π/Asurvey

~ Flimit-2

Hypothetical Lick survey



Radio Orphan Afterglow Rate

Radio orphan afterglows: low-z GRB remnants

Radio emission most easily detected during late (non-relativistic, ~isotropic) stage of GRB evolution.

Intrinsic energy output is ~constant (~1051 erg).→ Late-time radio luminosity should also be constant: radio OA's are distance-limited.


A First-Cut Estimate

Rate of GRBs: 1000 / year

Observable for ~1 year

~1000 x number of current optical afterglows

970508: 70 μJy at 250 days (8 GHz)

z = 0.835

VLA sensitivity limit: 50 μJy in 10 min

~20% of GRBs are at z < 1

→ 200 detectable on-axis radio afterglows / sky / yr

200 × 100 = 20,000 detectable radio afterglows / sky / yr

20,000 detectable radio afterglows / sky now


Frail et al. 2000


More Precise Estimates

Also variable; depends on low-z GRB rate, beaming fraction, and ISM density

Flim (μJy)

10 1 0.11001000

103

104

105

106

107

N (

F >

Flim

)

NR ~ 20 (f/ 5 mJy)-3/2

Levinson et al. 2002


Levinson et al. 2002


Radio Survey Design

Wide or deep?

Flim (μJy)

10 1 0.11001000

103

104

105

106

107

N (

F >

Flim

)

limiting rate of a survey:

Rlimit = 4π/Asurvey

Flimit-2

Hypothetical VLA surveyexpect R F-3/2

→ wide/shallow



Real Surveys

Searches for long-wavelength high-z transients are still in their early stages.

Some preliminary results:

FIRST / NVSS

Bower survey

CFHT-WF surveyMany others I am ignoring

Upcoming surveys:

ATA, SDSS, Pan-STARRS, LSST, SKA

Orphan Afterglow Surveys


NVSS-FIRST Transient Search

Comparison of two VLA wide-field radio surveys (FIRST, NVSS)

FIRST: Galactic caps, 3 minute integrations, 21 cm, B-configuration (5" resolution)

NVSS: Entire northern sky, 21 cm, D-configuration (45" resolution)

Flim ~ 5 mJy

Asurvey ~ 2400 sq. deg

Two epochs, variable interval

9 transient candidates identified

1 radio supernova, others not identifiable (lack of information)

No orphan afterglows detected (survey sensitivity is low;any OA would be coincident with a bright galaxy) θjet < 15°



Bower Transient Survey

22-year, extensively repeated survey of a blank field in far northern sky

5-8 GHz

Flim ~ 500 μJy

A ~ 0.02 sq. deg

944 epochs

Aeff ~ 10 sq.deg

10 transients detected

1 probable radio supernova1 radio supernova or GRB afterglow (orphan or otherwise)

8 unknown sources



CFHTLS-VWS

Canada France Hawaii Telescope Legacy Survey (Very Wide Survey)

A ~ 490 sq. deg

mlim = 22.5 (r' - band)

14 epochs

1067 transients

1066 variable stars / asteroids / KBOs

1 orphan afterglow candidate


Malacrino et al. 2007


Pan-STARRS and LSST

Extremely wide, deep future optical surveys

Pan-STARRS

PS1

1.8m telescope, FOV of 7 sq. deg.

Entire visible sky to R~23 each week

OA rate ~ 100 / year

Completion 2007

PS4

2.2m telescope, FOV of 28 sq. deg?

1 OA per day?

LSST

Large Synoptic Survey Telescope

8.4m telescope, FOV 3.5 sq. deg

Entire visible sky to R~24 in 3 nights

OA rate ~ many / night

Completion 2012?

Future Surveys


ATA and SKA

Upcoming radio instruments

ATA

Allen Telescope Array

4 sq. deg FOV

Visible sky to 1 mJy each week

OA rate ~ 1 / night

Future Surveys

SKA

Square Kilometer Array

Large telescope to be completed in effectively indefinite future

Many times ATA transient rate


The Future

The future for these orphans may not be so grim after all!

Future Surveys

orphan afterglows daniel perley astro 250 3 april 2007

Documents

afterglow grb review

orphan afterglows grb

jet jet

grb emission

orphan afterglows afterglow

orphan afterglows grbs

jet geometry grb review

months grb review slide