Discovery of radio afterglow from
the most distant cosmic explosion
Poonam Chandra
Royal Military College of Canada
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
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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 (< 2 sec) and long duration (> 2sec)
AVERAGE REDSHIFT = 1.5
SWIFT
AVERAGE REDSHIFT = 2.7
Prospects for future observations
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GRB are bright. Detectable at high redshift
GRB 080319B, Bloom et al. (2009)
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Hubble Space Telescope Realm
of the Galaxies / ~ 1e5
The First Stars (aka Population III)
Turk, Abel, O”Shea (2009)
• 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
• H2 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~105 L, T~105
K, Lifetime~2-3 Myrs
• Single or double stars form
• Dominant mode of SF below 10-3.5
Z
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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<M*<140 M, M*>260 M
Direct collapse to BH
BHs may be seeds for first quasars
Ideal properties to produce long duration gamma-ray bursts
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)
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Open questions in the high z universe
Barkana and Loeb (2007)
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• How did the first stars, black holes and
galaxies form?
• How and when did the metals get added?
• How was the universe re-ionized?
Negative k-correction at radio wavelengths
• Effect was first noted
by Ciardi & Loeb
(2000)
• Steep synchrotron
self-absorption (ν2)
partially counteracts
dL2 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%
µ dL2
Frail et al. (2006)
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Only 3 GRBs with z > 6
• GRB 050904
• GRB 080913
• GRB 090423
Discovery of a GRB Afterglow at redshift 8.26
• Swift. April 23, 2009
• T+73 s. X-ray begins
• T+109 s. Optical begins
• No optical transient
Tanvir et al. 2009
• T+20 min. UKIRT begins. Faint K band
transient discovered
ISAAC Spectroscopy
• 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
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Re-cap. Redshifts of important objects
Object Name
Milky Way
Redshift
z=0.0
Virgo Cluster
z=0.004
Quasar 3C273
z=0.158
“Era of Galaxy formation”
z=1-2
Most distant quasar
z=6.43
Most distant galaxy (firm)
z=6.96
GRB 090423
First Stars appear
Cosmic Microwave Background (CMB)
Big Bang
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z=8.2
z=20-30
z=1089
z
Chris Carilli (NRAO)
Berlin June 29, 2005
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
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Radio observations of GRB 090423
Detections VLA: 8.5 GHz on Apr 25-Jun 27.
– 74 +/- 22 uJy at Δt~8 d
– 2-hr integrations every 2 days
– Data sets averaged (in UV
plane) to improve detection
sensitivity
– Undetectable after Δt~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)
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Non-Detections WSRT: 4.9 GHz on May 22-23
CARMA: 95 GHz on Apr. 25
IRAM 30-m: 250 GHz on Apr 25
Last Chandra
measurement
Chandra et al. (2009)
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Peak flux
Peak flux
-1.10
-1.35
Last Chandra
measurement
Chandra et al. (2009)
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Broadband modeling
• What can be learned from simple afterglow theory?
Peak flux NIR ~ Peak flux radio (Fmax=const.): ISM
Time to peak NIR=0.08 d, Radio=50 d. (νm α t-3/2)
No obvious jet break until later than day 50
αNIR-αX~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.)
–
–
–
–
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Broadband modeling
• Now fit broadband data to a detailed afterglow model
– Model fitting is from Yost et al.
– Constant density circumburst medium
– Fit for Ek, no, θj,εB and εe (keep p fixed)
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Best Fit Parameters for p=2.46
Parameters
Isotropic
Jet (t>45 d)
E(Gamma) ergs 1x1053
>2.2x1051
E(Kinetic) ergs 3.8x1053
>8.4x1051
Density cm-3
0.9
---
ee
0.02
---
eB
0.28
---
Previous high redshift GRB 050904 z=6.26
Haislip et al. 2006; Kawai et al. 2006; Gou, Fox & Meszaros (2006)
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Previous high redshift GRB 050904 z=6.26
DENSITY> 100-680 cm-3
Haislip et al. 2006; Kawai et al. 2006; Gou, Fox & Meszaros (2006)
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Comparisons with other afterglows
Chandra et al. (2010)
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Comparisons with other afterglows
Afterglow Properties –
– GRB 050904 (z=6.26). Both
are hyper-energetic (>1051
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 θj, εB, εe
&p
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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 Zo not
robust
Neither the high energy nor the afterglow properties are sufficiently
unique to distinguish GRB 090423 from other GRBs
A rumination on the early radio
emission
• Flux on first VLA detection
(trest~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 CastroTirado et al. is dominated by a RS
• mm emission from RS is bright,
redshift-independent (no
extinction or scintillation). ALMA
will be ideal.
•
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Chandra et al. (2009)
A rumination on the early radio
emission
Inoue, Omukai, Ciardi (2007)
• Flux on first VLA detection
(trest~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 CastroTirado et al. is dominated by a RS
• mm emission from RS is bright,
redshift-independent (no
extinction or scintillation). ALMA
will be ideal.
•
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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=8.5, EVLA 3σ, Δt=1 hr
z=2.5, EVLA 3σ, Δt=1 hr
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
(θj>12o), hyper-energetic (1052 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
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