Over the past few years, long-duration gamma-ray bursts (GRBs), including the
subclass of X-ray flashes (XRFs), have been revealed to be a rare variety of
Type Ibc supernova (SN Ibc). While all these events result from the death of
massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those
of ordinary Type Ibc SNe by many orders of magnitude. The observed diversity of
stellar death corresponds to large variations in the energy, velocity, and
geometry of the explosion ejecta. Using multi-wavelength (radio, optical,
X-ray) observations of the nearest GRBs, XRFs, and SNe Ibc, I show that while
GRBs and XRFs couple at least 10^48 erg to relativistic material, SNe Ibc
typically couple less than 10^48 erg to their fastest (albeit non-relativistic)
outflows. Specifically, I find that less than 3% of local SNe Ibc show any
evidence for association with a GRB or XRF. Recently, a new class of GRBs and
XRFs has been revealed which are under-luminous in comparison with the
statistical sample of GRBs. Owing to their faint high-energy emission, these
sub-energetic bursts are only detectable nearby (z < 0.1) and are likely 10
times more common than cosmological GRBs. In comparison with local SNe Ibc and
typical GRBs/XRFs, these explosions are intermediate in terms of both
volumetric rate and energetics. Yet the essential physical process that causes
a dying star to produce a GRB, XRF, or sub-energetic burst, and not just a SN,
remains a crucial open question. Progress requires a detailed understanding of
ordinary SNe Ibc which will be facilitated with the launch of wide-field
optical surveys in the near future.Comment: 8 pages, Proceedings for "Supernova 1987A: 20 Years After: Supernovae
  and Gamma-Ray Bursters" AIP, New York, eds. S. Immler, K.W. Weiler, and R.
  McCra

Alicia M. Soderberg

Kurt Weiler

Stefan Immler

English

arXiv

Over the past few years, long-duration gamma-ray bursts (GRBs), including the subclass of X-ray flashes (XRFs), have been revealed to be a rare variety of Type Ibc supernova (SN Ibc). While all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary Type Ibc SNe by many orders of magnitude. The observed diversity of stellar death corresponds to large variations in the energy, velocity, and geometry of the explosion ejecta. Using multi-wavelength (radio, optical, X-ray) observations of the nearest GRBs, XRFs, and SNe Ibc, I show that while GRBs and XRFs couple at least ~10^48 erg to relativistic material, SNe Ibc typically couple <~10^48 erg to their fastest (albeit non-relativistic) outflows. Specifically, I find that less than 3% of local SNe Ibc show any evidence for association with a GRB or XRF Recently, a new class of GRBs and XRFs has been revealed which are under-luminous in comparison with the statistical sample of GRBs. Owing to their faint high-energy emission, these sub-energetic bursts are only detectable nearby (z<~0.1) and are likely 10 times more common than cosmological GRBs. In comparison with local SNe Ibc and typical GRBs/XRFs, these explosions are intermediate in terms of both volumetric rate and energetics. Yet the essential physical process that causes a dying star to produce a GRB, XRF, or sub-energetic burst, and not just a SN, remains a crucial open question. Progress requires a detailed understanding of ordinary SNe Ibc which will be facilitated with the launch of wide-field optical surveys in the near future

Soderberg, Alicia M.

Caltech Authors - Main

The Radio Properties of Type Ibc Supernovae 
Alicia M. Soderberg 
Caltech Astronomy, MC 105-24, Pasadena, CA 91106, USA 
Abstract. Over the past few years, long-duration 7-ray bursts (GRBs), including the subclass of X-
ray flashes (XRFs), have been revealed to be a rare variety of Type Ibc supernova (SN Ibc). While 
all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and 
XRFs exceed those of ordinary Type Ibc SNe by many orders of magnitude. The observed diversity 
of stellar death corresponds to large variations in the energy, velocity, and geometry of the explosion 
ejecta. Using multi-wavelength (radio, optical. X-ray) observations of the nearest GRBs, XRFs, and 
SNe Ibc, I show that while GRBs and XRFs couple at least -^ 10^^ erg to relativistic material, SNe 
Ibc typically couple < 10^^ erg to their fastest (albeit non-relativistic) outflows. Specifically, I find 
that less than 3% of local SNe Ibc show any evidence for association with a GRB or XRF Recently, 
a new class of GRB s and XRFs has been revealed which are under-luminous in comparison with the 
statistical sample of GRBs. Owing to their faint high-energy emission, these sub-energetic bursts are 
only detectable nearby (z < 0.1) and are likely 10 times more common than cosmological GRBs. In 
comparison with local SNe Ibc and typical GRBs/XRFs, these explosions are intermediate in terms 
of both volumetric rate and energetics. Yet the essential physical process that causes a dying star to 
produce a GRB, XRF, or sub-energetic burst, and not just a SN, remains a crucial open question. 
Progress requires a detailed understanding of ordinary SNe Ibc which will be faciUtated with the 
launch of wide-field optical surveys in the near future. 
Keywords: Stars: Supemovae 
PACS: 97.60.Bw 
INTRODUCTION 
The discovery of several gamma-ray bursts (GRBs) and X-ray flashes (XRFs) at z < 0.3 
in the last few years has firmly established that GRBs and XRFs are accompanied by 
supemovae of Type Ibc (SNe Ibc) (see Woosley and Bloom 1 for a review). While the 
distribution of GRB/XRF-SN optical luminosities appears indistinguishable from that of 
local SNe Ibc [2], spectroscopy reveals unusually broad absorption lines ("broad-lined", 
BL) in every case [3]. At other wavelengths, these explosions are easily distinguished 
since GRBs and XRFs produce mildly-relativistic ejecta, which gives rise to strong non-
thermal "afterglow" emission. Radio observations are critical in this analysis since they 
provide the best calorimetry of the fastest ejecta. 
Recenfly, we have identified a population of GRBs/XRFs that are sub-energetic by a 
factor of'~ 10^ and about 10 times more common than typical bursts [4, 5]. Given their 
under-luminous prompt energy release, current satellites can only detect them nearby 
(z < 0.1). These sub-energetic explosions are intermediate between GRBs/XRFs and 
local SNe Ibc with respect to gamma-ray energy, jet collimation, and volumetric rate, 
and thus hint at an overall continuum. 
Motivated by the GRB/XRF-SN connection and the discovery of sub-energetic GRBs, 
I embarked on a survey of optically-selected local {d < 200 Mpc) SNe Ibc with the 
CP937, Supernova 1987A: 20 Years After, edited by S. Immler, K. Weiler, and R. McCray 
© 2007 American Institute of Physics 978-0-7354-0448-9/07/$23.00 
492 
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30 -
S 20 
10 -
I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 
• 
• 
SNe Ibc d i s c o v e r e d 
SNe Ibc wi th r a d i o 
d a t a a t t < 150 d a y s 
VLA 
Survey 
Begins 
J 
ffl 
VLA Ded ica t i on 
I 
I. n n TTTTT HI 
I 
1960 1970 1980 
Year 
1990 2000 
FIGURE 1. The discovery rate of SNe Ibc each year (yellow) is compared with the fraction observed 
in the radio on timescales less than 150 days (red). Early radio observations are crucial for constraining 
underluminous and/or off-axis GRBs since they trace the fastest ejecta in the explosion. Since the launch 
of our VLA survey nearly every SN Ibc has been observed on this timescale. 
Very Large Array (VLA). The goal is to determine the fraction of SNe Ibc that produce 
mildly-relativistic ejecta. This survey is well-suited to recognizing bursts for which no 
gamma-ray emission is detected, either because the jets are pointed away from our 
line-of-sight ("off-axis") or the emission is below the detection thresholds of current 
satellites. 
Since then I have observed '~ 200 SNe Ibc with the VLA, on timescales of days to 
years after the explosion. This dedicated effort has served to characterize in a systematic 
way the environments and ejecta properties of SNe Ibc for the first time. Early radio 
observations are crucial for identifying sub-energetic and/or uncollimated explosions. 
On the other hand, late-time radio observations are sensitive to jets initially beamed 
away from our line-of-sight. Through this intense VLA program, I have established that 
(i) roughly 15% of SNe Ibc show detectable radio emission, (ii) less than '~ 3% show 
radio luminosities comparable to those observed for GRB/XRF afterglows [6, 5], (iii) 
less than 10% of SNe Ibc harbor typical GRBs pointed away from our line-of-sight 
[7, 8], and (iv) basic optical properties (peak luminosity, photospheric velocities) are not 
reliable indicators of strong radio emission and/or relativistic ejecta. 
493 
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A LARGE RADIO SURVEY OF SNE IBC: THE SAMPLE 
Beginning in 2002,1 have obtained radio observations for every newly discovered SN 
Ibc within a maximum distance of '~ 200 Mpc and accessible to the VLA. All targets 
are drawn from astronomical circulars which typically report'~ 20 new SNe Ibc each 
year, of which'~ 90% are accessible to the VLA. Almost all of the reported SNe Ibc are 
found through "targeted" SN search campaigns which monitor only the most luminous 
local galaxies (e.g., RC3 catalog). 
Upon spectral classification, I immediately trigger a VLA Target-of-Opportunity ob-
servation resulting in first epoch radio observations within a few days of discovery. Since 
most SNe Ibc are discovered near maximum light, the first epoch VLA observations cor-
respond to days to weeks after the explosion. Follow-up observations for each SN are 
scheduled logarithmically in time since the explosion. 
I supplement this sample with radio data of SNe Ibc taken before 2002. The majority 
of these data were extracted from the VLA archive and were primarily taken for radio 
studies of nearby galaxies, while some were taken specifically for SN follow-up. As a 
result, the archival data generally probe significantly later timescales than my VLA sur-
vey. This is highlighted in Figure 1 where the discovery rate of SNe Ibc is compared with 
the fraction observed with the VLA; early observations (/ < 150 days) were uncommon 
in the years preceding my VLA survey. 
The combined dataset includes all VLA observations of local SNe Ibc to date. 
THE RADIO PROPERTIES OF SNE IBC 
This large sample of radio observations are presented in detail in Soderberg [9]. The 
large majority of these observations were conducted at 4.86 and/or 8.46 GHz. As shown 
in Figure 2, the fraction of SNe Ibc with detectable radio emission is small, '~ 15%, 
corresponding to a total of 30 SNe Ibc detected to date. 
Of those with positive detections, the majority ('~ 70%) are discovered before radio 
maximum with well-constrained spectral peaks. As shown in Figures 2 and 3, the peak 
radio luminosities span four orders of magnitude, iv,radio ~ 10^^ — 10^" erg s^^ Hz^^ on 
timescales of 1 to 1000 days. Figure 2 shows that GRB/XRF-SNe are distinguished from 
optically-selected SNe Ibc by their strong, early-peaking radio emission. As discussed 
in the next section, these properties can be used as a proxy for the presence of relativistic 
ejecta. 
Normalizing by the distance to which each SN could be detected and the effective 
monitoring time of the survey, we produce the radio luminosity function shown in 
Figure 3. Clearly, sub-luminous radio SNe Ibc are the most common, though least often 
detected. 
As discussed in detail in Soderberg [9], I find no evidence for any correlation between 
basic optical properties (peak luminosity, photospheric velocities) and radio luminosity. 
This holds, in particular, for the fraction of broad-lined SNe Ibc for which the radio 
detection rate is no higher than that observed for ordinary SNe Ibc. I therefore emphasize 
that neither optical luminosity nor BL spectral features are reliable proxies for strong 
radio emission. 
494 
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1024 
10 100 1000 
Time Since Explosion (days) 
10000 
FIGURE 2. To date, 30 local (d < 200 Mpc) SNe Ibc have been detected at radio wavelengths, the 
majority of which were found through my dedicated VLA survey. Detections are shown as colored light-
curves and 3(7 upper limits as inverted grey triangles. GRB-SN 1998bw and XRF-SN 2006aj, also within 
the maximum distance of this sample, are distinguished by their bright early peaking radio emission 
(black). Both of these events were sub-energetic in comparison with typical GRBs (e.g., GRB 030329; 
grey) andXRFs (e.g., XRF020903; grey). 
THE VELOCITY AND ENERGY OF THE FASTEST EJECTA 
As discussed by Chevalier [10], the radio emission from SNe Ibc is produced as the 
fastest ejecta shock-accelerate particles in the circumstellar medium (CSM). Turbulence 
amplifies the magnetic field and the accelerated (relativistic) electrons produce syn-
chrotron radiation which peaks in the radio band just after the explosion. The spectral 
peak, defined by a low-frequency turn-over, is dominated by synchrotron self-absorption 
(SSA) as shown by our detailed studies of several SNe Ibc [11, 12]. This is different than 
the case for most Type II SNe which are dominated by external free-free absorption due 
to a dense CSM [13]. 
As shown by Readhead [14], for radio sources dominated by SSA the brightness 
temperature, Zg = c^Fy/{2nk0^v^), is constrained to 5 x 10^" K, assuming the post-
shock energy density is in equipartition between magnetic fields, eg, and relativistic 
electrons, e .^ With this assumption, robust constraints on the radius, r, and internal 
energy of the shocked CSM, it,, are derived [10,15,16]. These quantities scale as simple 
495 
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10 
3 
Z 
10^ 
o 
I 10-= 
06aj 9Bbw 
++++t^—I—P I I n i l 
* 
H—M I n i l H—|l I I MM H—M I MM —I—I I I MM 
IQZ5 IQZe lO^'' 1028 1028 103° 
Radio Luminosity (erg s~' Hz~') 
FIGURE 3. Top: The peak radio luminosity distribution for local SNe Ibc peaks near L v ~ 
IQp-^ erg s^i Hz^i. The peak radio luminosities of GRB-SN 1998bw and XRF-SN2006aj are shown for 
comparison (arrows) since they lie within a comparable distance as the optically-selected sample. Bottom: 
The radio luminosity function of SNe Ibc. Low luminosity SNe clearly dominate the intrinsic sample, 
though they are rarely observed since they fall below the current VLA detection limits. We emphasize 
that this analysis includes several biases, including effective monitoring time pre-2002 and efficiency of 
optical spectroscopic classification. 
observables including the peak spectral frequency, Vp, the flux at the spectral peak, fp, 
and the luminosity distance, d (see Soderberg et al. 11, 12 for a detailed discussion). 
Here we adopt the simple model of Chevalier and Fransson 16, assuming e^  = e^ = 0.1 
which provides the following relations: 
: 4 X \0^^{fplxniyfl^\dlMpc)^'^l^\vpl5 GHz) cm (1) 
E, = 1.5 X 10'*^((//Mpc)'^(/p/mJy)'*(Vp/5 GHz)"'(r/10^^ cm)"^ erg. (2) 
We note that the radius of the emitting material is only weakly dependent on the energy 
partition fractions, while departures from equipartition produce a strong increase the 
496 
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3 
J 
X) 
K 
,^  
(0 
0! 
[1, 
1030 
1039 
1028 
102'^ 
1026 
1025 
1 1 •< 1—1 1 1 1 1 1 1—1—1—1 1 ^ 1 1 1—1—1 1 1 yi 1 1 : 
^ \ ^ * 
-. \ ^ * 
^ \ * * 
^^Ni) / "^  
\ . • r \ \ ^^^fc / / ^ ' 
*%:vr ^3f.M */ • 
/ / > • m/ / • 
f f^ ^' i' 1 
® / # # 
' / ' 
• . / ' •' 
1 10 100 1000 
(tp/days)( i /p/5 GHz) 
FIGURE 4. The peak radio luminosities of all the SNe Ibc detected to date are plotted against the 
timescale of peak. SNe with well-constrained peaks are shown as black encircled dots while those where 
the peak is not well constrained are shown as black dots with Unes indicating the extrapolation of the radio 
Ught-curves between the range of typically observed peak times. GRB-SN 1998bw and XRF-SN 2006aj 
were also discovered within this volume and are shown as grey stars. Dashed lines indicate how the 
velocity of the fastest ejecta scales with the observed spectral parameters. 
energy. Taken together with the observed peak time, tp, the velocity of the shock is 
easily estimated from Equation 1 and the resulting values are shown for these SNe Ibc 
in Figure 4. SNe with early, bright radio emission have the fastest ejecta. As shown 
in the Figure, of the thirty radio bright SNe Ibc in this sample, the inferred velocities 
range from 0.01 to 0.5c, and none show the mildly-relativistic ejecta inferred for GRB-
SN 1998bw [15] or XRF-SN 2006aj [5]. 
Next is a discussion of the explosion energetics. As derived by Chevalier [17], it, is 
just r^ 20% of the total energy (kinetic and internal) of the shocked CSM, which in turn 
is equal to the kinetic energy of the fastest ejecta. Including this conversion factor, we 
find that the kinetic energy of the fastest ejecta, E^:^ ^^ 5Ei, spans 10^^ — 10^^ erg for SNe 
Ibc, a factor of 10^ to 10^ times less than that traced by the optical emission. 
Finally, we compare these ejecta parameters (velocity, kinetic energy) with those of 
GRB and XRFs, including the class of sub-energetic bursts (Figure 5). We find evidence 
of a clear dichotomy between ordinary SNe Ibc and typical GRBs/XRFs: ordinary 
SNe couple roughly 10^^ erg to their fastest (but non-relativistic) ejecta at v '~ 0.15c. 
497 
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1052 -
S 10^° - Ordinary 
SNe Ibc M 
CD 
W 
S 
104B 
1046 -
o o 
o 
o 
Non-relativistic 
I 1 . . . 
Sub-energetic 
GRB-SNe 
Relativistic 
I L. 
0.01 0.1 1 10 
Velocity of the Fastest Ejecta (T/S) 
FIGURE 5. The kinetic energy and velocity of the fastest ejecta for radio bright SNe Ibc are compared 
withthoseof GRBs,XRFs, and sub-energetic bursts. Ordinary SNe Ibc (red) are distinguished in that they 
couple 10^^ erg to non-relativistic ejecta at v a; 0.15c while GRBs and XRFs (blue) couple at 10^^ erg 
to relativistic material (T a; 10). Sub-energetic bursts (black) are intermediate between the two classes, 
coupling at least 10^^ erg to mildly-relativistic (T a; 3). 
Meanwhile, typical GRBs/XRFs couple 10^ ^ erg to relativistic jets with F '~ 10. Sub-
energetic explosions bridge these two classes, coupling at least 10^^ erg to mildly-
relativistic outflows with r '~ 3. 
A related question is whether the observed radio emission for some SNe Ibc may 
be suppressed due to viewing angle effects, specifically important in the case of an 
associated GRB jet directed away from our line-of-sight [18]. In this scenario a rapid 
increase in radio emission is expected as the jet decelerates and spreads sideways, 
eventually crossing our line-of-sight on a timescale of '~ 1 yr after the explosion. As 
shown in Figure 2 and discussed in Soderberg et al. [8], radio observations of '~ 70 
SNe Ibc talcen between months to decades after the explosion reveal no evidence for 
associated GRB jets. Statistically we constrain the fraction of SNe Ibc harboring typical 
GRB jets to be less than 10%. 
498 
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CONCLUSIONS 
In conclusion, based on our large survey of optically selected local SNe Ibc, I find that (i) 
roughly 15% show detectable radio emission, (ii) the peak radio luminosities of SNe Ibc 
span four orders of magnitude with peak times between 1 and 1000 days, (iii) the kinetic 
energy and fastest velocity of these explosions are significantly different from those of 
typical GRBs and XRFs, (iv) compared with the sample of sub-energetic GRBs, <^ 3% 
of SNe Ibc show ejecta properties indicative of an engine-driven explosion, and (v) the 
fraction of SNe Ibc harboring classical GRB jets viewed off-axis is constrained to less 
than 10%. 
Despite this progress, it remains an open question why just'~ 1% of SNe Ibc give rise 
to GRBs or XRFs. A deeper understanding of the progenitors of ordinary SNe Ibc will 
shed light on this issue. With the recent advent of wide-field optical surveys, SNe Ibc 
from "blind" search campaigns will soon dominate new discoveries. Any investigation 
of the relation between the large-scale environment (host galaxy characteristics) and 
ejecta properties relies on SNe discovered through these unbiased surveys. In particular, 
host galaxy metallicity is argued to be a critical parameter as a proxy for the progenitor 
metallicity. Numerical models suggest that only low metallicity progenitors are able to 
produce the accretion-disk powered outflows inferred for GRBs [19]. Therefore, these 
models predict that engine-driven explosions are unlikely to be hosted in the luminous, 
high-metallicity galaxies monitored by targeted SN searches [20]. Looking forward, a 
radio survey focused exclusively on the SNe Ibc discovered through blind surveys will 
directly address and answer these crucial questions. 
REFERENCES 
1. S. E. Woosley, and J. S. Bloom, ARA&A 44, 507-556 (2006). 
2. A. M. Soderberg, et al., ApJ 636, 391-399 (2006). 
3. E. Plan, etal. Nature 442,1011-1013 (2006). 
4. A. M. Soderberg, et al., Nature 430, 648-650 (2004). 
5. A. M. Soderberg, etal.,Nature 442, 1014-1017 (2006), 
6. E. Berger, S. R. Kulkami, D. A. Frail, and A. M. Soderberg,^;j/ 599, 408^18 (2003). 
7. A. M. Soderberg, D. A. Frail, and M. H. Wieringa, ApJL 607, LI 3-L16 (2004). 
8. A. M. Soderberg, E. Nakar, E. Berger, and S. R. Kulkami, ApJ 638, 930-937 (2006). 
9. A. M. Soderberg, ^ ^ (2007), in preparation. 
10. R. A. Chevalier, ^ ; j / 499, 810 (1998). 
11. A. M. Soderberg, et al, ApJ 621, 908-920 (2005). 
12. A. M. Soderberg, R. A. ChevaUer, S. R. Kulkami, and D. A. Frail, ^ p J 651,1005-1018 (2006). 
13. K. W. Weiler, N. Panagia, M. J. Monies, and R. A. Siamek, ARA&A 40, 387^38 (2002). 
14. A. C. S. Readhead,^;^/ 426, 51-59 (1994). 
15. S. R. Kulkami, et al. Nature 39?,, 663-669 (1998). 
16. R. A. Chevalier, and C. Fransson, ^ ; j / 651, 381-391 (2006). 
17. R. A. Chevalier, ApJ 111, 165-112 (1983). 
18. B. Vaczyn&ki, Acta Astronomica 51, 1 ^ (2001). 
19. S. E. Woosley, A. Heger, and T A. Weaver, Reviews of Modem Physics 74, 1015-1071 (2002). 
20. K. Z. Stanek, et al., Acta Astronomica 56, 333-345 (2006). 
499 
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The Radio Properties of Type Ibc Supernovae

Crossref

ar
X
iv
:0
70
6.
30
47
v1
  [
as
tr
o-
ph
]  
20
 J
un
 2
00
7
The Radio Properties of Type Ibc Supernovae
Alicia M. Soderberg
Caltech Astronomy, MC 105-24, Pasadena, CA 91106, USA
Abstract. Over the past few years, long-durationγ-ray bursts (GRBs), including the subclass of X-
ray flashes (XRFs), have been revealed to be a rare variety of Tpe Ibc supernova (SN Ibc). While
all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and
XRFs exceed those of ordinary Type Ibc SNe by many orders of magnitude. The observed diversity
of stellar death corresponds to large variations in the energy, velocity, and geometry of the explosion
ejecta. Using multi-wavelength (radio, optical, X-ray) observations of the nearest GRBs, XRFs, and
SNe Ibc, I show that while GRBs and XRFs couple at least∼ 1048 erg to relativistic material, SNe
Ibc typically couple∼< 10
48 erg to their fastest (albeit non-relativistic) outflows. Specifically, I find
that less than 3% of local SNe Ibc show any evidence for associati n with a GRB or XRF. Recently,
a new class of GRBs and XRFs has been revealed which are under-luminous in comparison with the
statistical sample of GRBs. Owing to their faint high-energy emission, these sub-energetic bursts are
only detectable nearby (z
∼
< 0.1) and are likely 10 times more common than cosmological GRBs. In
comparison with local SNe Ibc and typical GRBs/XRFs, these explosions are intermediate in terms
of both volumetric rate and energetics. Yet the essential physical process that causes a dying star to
produce a GRB, XRF, or sub-energetic burst, and not just a SN,remains a crucial open question.
Progress requires a detailed understanding of ordinary SNeIbc which will be facilitated with the
launch of wide-field optical surveys in the near future.
Keywords: Stars: Supernovae
PACS: 97.60.Bw
INTRODUCTION
The discovery of several gamma-ray bursts (GRBs) and X-ray flashes (XRFs) atz
∼
< 0.3
in the last few years has firmly established that GRBs and XRFsare accompanied by
supernovae of Type Ibc (SNe Ibc) (see Woosley and Bloom 1 for areview). While the
distribution of GRB/XRF-SN optical luminosities appears indistinguishable from that of
local SNe Ibc [2], spectroscopy reveals unusually broad absorption lines (“broad-lined”,
BL) in every case [3]. At other wavelengths, these explosions are easily distinguished
since GRBs and XRFs produce mildly-relativistic ejecta, which gives rise to strong non-
thermal “afterglow” emission. Radio observations are critical in this analysis since they
provide the best calorimetry of the fastest ejecta.
Recently, we have identified a population of GRBs/XRFs that are sub-energetic by a
factor of∼ 102 and about 10 times more common than typical bursts [4, 5]. Given their
under-luminous prompt energy release, current satellitescan only detect them nearby
(z
∼
< 0.1). These sub-energetic explosions are intermediate between GRBs/XRFs and
local SNe Ibc with respect to gamma-ray energy, jet collimaton, and volumetric rate,
and thus hint at an overall continuum.
Motivated by the GRB/XRF-SN connection and the discovery ofsub-energetic GRBs,
I embarked on a survey of optically-selected local (d
∼
< 200 Mpc) SNe Ibc with the
FIGURE 1. The discovery rate of SNe Ibc each year (yellow) is compared with the fraction observed
in the radio on timescales less than 150 days (red). Early radio observations are crucial for constraining
underluminous and/or off-axis GRBs since they trace the fast st ejecta in the explosion. Since the launch
of our VLA survey, nearly every SN Ibc has been observed on this timescale.
Very Large Array (VLA). The goal is to determine the fractionf SNe Ibc that produce
mildly-relativistic ejecta. This survey is well-suited torecognizing bursts for which no
gamma-ray emission is detected, either because the jets arepointed away from our
line-of-sight (“off-axis”) or the emission is below the detec ion thresholds of current
satellites.
Since then I have observed∼ 200 SNe Ibc with the VLA, on timescales of days to
years after the explosion. This dedicated effort has servedto characterize in a systematic
way the environments and ejecta properties of SNe Ibc for thefirst time. Early radio
observations are crucial for identifying sub-energetic and/or uncollimated explosions.
On the other hand, late-time radio observations are sensitive to jets initially beamed
away from our line-of-sight. Through this intense VLA program, I have established that
(i) roughly 15% of SNe Ibc show detectable radio emission, (ii) less than∼ 3% show
radio luminosities comparable to those observed for GRB/XRF afterglows [6, 5], (iii)
less than 10% of SNe Ibc harbor typical GRBs pointed away fromour line-of-sight
[7, 8], and (iv) basic optical properties (peak luminosity,photospheric velocities) are not
reliable indicators of strong radio emission and/or relativis ic ejecta.
A LARGE RADIO SURVEY OF SNE IBC: THE SAMPLE
Beginning in 2002, I have obtained radio observations for every newly discovered SN
Ibc within a maximum distance of∼ 200 Mpc and accessible to the VLA. All targets
are drawn from astronomical circulars which typically report ∼ 20 new SNe Ibc each
year, of which∼ 90% are accessible to the VLA. Almost all of the reported SNe Ibc are
found through “targeted” SN search campaigns which monitoronly the most luminous
local galaxies (e.g., RC3 catalog).
Upon spectral classification, I immediately trigger a VLA Target-of-Opportunity ob-
servation resulting in first epoch radio observations within a few days of discovery. Since
most SNe Ibc are discovered near maximum light, the first epoch VLA observations cor-
respond to days to weeks after the explosion. Follow-up observations for each SN are
scheduled logarithmically in time since the explosion.
I supplement this sample with radio data of SNe Ibc taken before 2002. The majority
of these data were extracted from the VLA archive and were primarily taken for radio
studies of nearby galaxies, while some were taken specifically for SN follow-up. As a
result, the archival data generally probe significantly later timescales than my VLA sur-
vey. This is highlighted in Figure 1 where the discovery rateof SNe Ibc is compared with
the fraction observed with the VLA; early observations (t ∼< 150 days) were uncommon
in the years preceding my VLA survey.
The combined dataset includesall VLA observations of local SNe Ibc to date.
THE RADIO PROPERTIES OF SNE IBC
This large sample of radio observations are presented in detail in Soderberg [9]. The
large majority of these observations were conducted at 4.86and/or 8.46 GHz. As shown
in Figure 2, the fraction of SNe Ibc with detectable radio emission is small,∼ 15%,
corresponding to a total of 30 SNe Ibc detected to date.
Of those with positive detections, the majority (∼ 70%) are discovered before radio
maximum with well-constrained spectral peaks. As shown in Figures 2 and 3, the peak
radio luminosities span four orders of magnitude,Lν,radio≈ 1026−1030 erg s−1 Hz−1 on
timescales of 1 to 1000 days. Figure 2 shows that GRB/XRF-SNeare distinguished from
optically-selected SNe Ibc by their strong, early-peakingradio emission. As discussed
in the next section, these properties can be used as a proxy for the presence of relativistic
ejecta.
Normalizing by the distance to which each SN could be detected and the effective
monitoring time of the survey, we produce the radio luminosity function shown in
Figure 3. Clearly, sub-luminous radio SNe Ibc are the most comm n, though least often
detected.
As discussed in detail in Soderberg [9], I find no evidence forany correlation between
basic optical properties (peak luminosity, photospheric velocities) and radio luminosity.
This holds, in particular, for the fraction of broad-lined SNe Ibc for which the radio
detection rate is no higher than that observed for ordinary SNe Ibc. I therefore emphasize
that neither optical luminosity nor BL spectral features are reliable proxies for strong
radio emission.
FIGURE 2. To date, 30 local (d
∼
< 200 Mpc) SNe Ibc have been detected at radio wavelengths, the
majority of which were found through my dedicated VLA survey. Detections are shown as colored light-
curves and 3σ upper limits as inverted grey triangles. GRB-SN 1998bw and XRF-SN 2006aj, also within
the maximum distance of this sample, are distinguished by their bright early peaking radio emission
(black). Both of these events were sub-energetic in comparison with typical GRBs (e.g., GRB 030329;
grey) and XRFs (e.g., XRF 020903; grey).
THE VELOCITY AND ENERGY OF THE FASTEST EJECTA
As discussed by Chevalier [10], the radio emission from SNe Ibc is produced as the
fastest ejecta shock-accelerate particles in the circumstellar medium (CSM). Turbulence
amplifies the magnetic field and the accelerated (relativistic) electrons produce syn-
chrotron radiation which peaks in the radio band just after th explosion. The spectral
peak, defined by a low-frequency turn-over, is dominated by synchrotron self-absorption
(SSA) as shown by our detailed studies of several SNe Ibc [11,12]. This is different than
the case for most Type II SNe which are dominated by external fee-free absorption due
to a dense CSM [13].
As shown by Readhead [14], for radio sources dominated by SSAthe brightness
temperature,TB = c2Fν/(2πkθ2ν2), is constrained to 5× 1010 K, assuming the post-
shock energy density is in equipartition between magnetic fields, εB, and relativistic
electrons,εe. With this assumption, robust constraints on the radius,r, and internal
energy of the shocked CSM,Ei, are derived [10, 15, 16]. These quantities scale as simple
FIGURE 3. Top: The peak radio luminosity distribution for local SNe Ibc peaks nearLν ≈
1028 erg s−1 Hz−1. The peak radio luminosities of GRB-SN 1998bw and XRF-SN 2006aj are shown for
comparison (arrows) since they lie within a comparable distance as the optically-selected sample. Bottom:
The radio luminosity function of SNe Ibc. Low luminosity SNeclearly dominate the intrinsic sample,
though they are rarely observed since they fall below the current VLA detection limits. We emphasize
that this analysis includes several biases, including effectiv monitoring time pre-2002 and efficiency of
optical spectroscopic classification.
observables including the peak spectral frequency,νp, the flux at the spectral peak,fp,
and the luminosity distance,d (see Soderberg et al. 11, 12 for a detailed discussion).
Here we adopt the simple model of Chevalier and Fransson 16, assumingεe = εB = 0.1
which provides the following relations:
r = 4×1014( fp/mJy)
9/19(d/Mpc)18/19(νp/5 GHz)−1 cm (1)
Ei = 1.5×10
41(d/Mpc)8( fp/mJy)
4(νp/5 GHz)−7(r/1015 cm)−6 erg. (2)
We note that the radius of the emitting material is only weakly dependent on the energy
partition fractions, while departures from equipartitionproduce a strong increase the
FIGURE 4. The peak radio luminosities of all the SNe Ibc detected to date are plotted against the
timescale of peak. SNe with well-constrained peaks are shown as black encircled dots while those where
the peak is not well constrained are shown as black dots with lines indicating the extrapolation of the radio
light-curves between the range of typically observed peak times. GRB-SN 1998bw and XRF-SN 2006aj
were also discovered within this volume and are shown as greystars. Dashed lines indicate how the
velocity of the fastest ejecta scales with the observed spectral parameters.
energy. Taken together with the observed peak time,tp, the velocity of the shock is
easily estimated from Equation 1 and the resulting values arshown for these SNe Ibc
in Figure 4. SNe with early, bright radio emission have the fastest ejecta. As shown
in the Figure, of the thirty radio bright SNe Ibc in this sample, the inferred velocities
range from 0.01 to 0.5c, and none show the mildly-relativistic ejecta inferred for GRB-
SN 1998bw [15] or XRF-SN 2006aj [5].
Next is a discussion of the explosion energetics. As derivedby Chevalier [17],Ei is
just∼ 20% of the total energy (kinetic and internal) of the shockedCSM, which in turn
is equal to the kinetic energy of the fastest ejecta. Including this conversion factor, we
find that the kinetic energy of the fastest ejecta,EK ∼ 5Ei, spans 1046−1049 erg for SNe
Ibc, a factor of 102 to 105 times less than that traced by the optical emission.
Finally, we compare these ejecta parameters (velocity, kinetic energy) with those of
GRB and XRFs, including the class of sub-energetic bursts (Figure 5). We find evidence
of a clear dichotomy between ordinary SNe Ibc and typical GRBs/XRFs: ordinary
SNe couple roughly 1048 erg to their fastest (but non-relativistic) ejecta atv ∼ 0.15c.
FIGURE 5. The kinetic energy and velocity of the fastest ejecta for radio bright SNe Ibc are compared
with those of GRBs, XRFs, and sub-energetic bursts. Ordinary SNe Ibc (red) are distinguished in that they
couple 1048 erg to non-relativistic ejecta atv ≈ 0.15c while GRBs and XRFs (blue) couple at 1051 erg
to relativistic material (Γ ≈ 10). Sub-energetic bursts (black) are intermediate between th two classes,
coupling at least 1048 erg to mildly-relativistic (Γ ≈ 3).
Meanwhile, typical GRBs/XRFs couple 1051 erg to relativistic jets withΓ ∼ 10. Sub-
energetic explosions bridge these two classes, coupling atleas 1048 erg to mildly-
relativistic outflows withΓ ∼ 3.
A related question is whether the observed radio emission for some SNe Ibc may
be suppressed due to viewing angle effects, specifically important in the case of an
associated GRB jet directed away from our line-of-sight [18]. In this scenario a rapid
increase in radio emission is expected as the jet decelerates and spreads sideways,
eventually crossing our line-of-sight on a timescale of∼ 1 yr after the explosion. As
shown in Figure 2 and discussed in Soderberg et al. [8], radioobservations of∼ 70
SNe Ibc taken between months to decades after the explosion reveal no evidence for
associated GRB jets. Statistically we constrain the fraction of SNe Ibc harboring typical
GRB jets to be less than 10%.
CONCLUSIONS
In conclusion, based on our large survey of optically selectd local SNe Ibc, I find that (i)
roughly 15% show detectable radio emission, (ii) the peak radio luminosities of SNe Ibc
span four orders of magnitude with peak times between 1 and 1000 days, (iii) the kinetic
energy and fastest velocity of these explosions are significa tly different from those of
typical GRBs and XRFs, (iv) compared with the sample of sub-energetic GRBs,∼< 3%
of SNe Ibc show ejecta properties indicative of an engine-driven explosion, and (v) the
fraction of SNe Ibc harboring classical GRB jets viewed off-axis is constrained to less
than 10%.
Despite this progress, it remains an open question why just∼ 1% of SNe Ibc give rise
to GRBs or XRFs. A deeper understanding of the progenitors ofordinary SNe Ibc will
shed light on this issue. With the recent advent of wide-fieldoptical surveys, SNe Ibc
from “blind” search campaigns will soon dominate new discoveries. Any investigation
of the relation between the large-scale environment (host galaxy characteristics) and
ejecta properties relies on SNe discovered through these unbiased surveys. In particular,
host galaxy metallicity is argued to be a critical parameteras a proxy for the progenitor
metallicity. Numerical models suggest that only low metallicity progenitors are able to
produce the accretion-disk powered outflows inferred for GRBs [19]. Therefore, these
models predict that engine-driven explosions are unlikelyto be hosted in the luminous,
high-metallicity galaxies monitored by targeted SN searches [20]. Looking forward, a
radio survey focused exclusively on the SNe Ibc discovered through blind surveys will
directly address and answer these crucial questions.
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The Radio Properties of Type Ibc Supernovae

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