GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs

3.5 MB·

by  S. Schulze

Astronomy & Astrophysics manuscript no. paper˙arxiv c ESO 2014
(cid:13)
January 17, 2014
GRB 120422A/SN 2012bz: Bridging the Gap between Low- And
High-Luminosity GRBs
S. Schulze1,2,3, D. Malesani4, A. Cucchiara5, N. R. Tanvir6, T. Kru¨hler4,20, A. de Ugarte Postigo7,4,
G. Leloudas8,4, J. Lyman9, D. Bersier9, K. Wiersema6, D. A. Perley10,11, P. Schady12, J. Gorosabel7,44,45,
J. P. Anderson13,20, A. J. Castro-Tirado7, S.B. Cenko14,15, A. De Cia16, L. E. Ellerbroek17, J. P. U. Fynbo4,
J. Greiner12, J. Hjorth4, D. A. Kann12,18, L. Kaper17, S. Klose18, A. J. Levan19, S. Mart´ın20, P. T. O’Brien6,
K. L. Page6, G. Pignata21, S. Rapaport22, R. S´anchez-Ram´ırez7, J. Sollerman23, I. A. Smith24, M. Sparre4,
C. C. Th¨one7, D. J. Watson4, D. Xu16,4, F. E. Bauer1,2,43, M. Bayliss25,26, G. Bj¨ornsson3, M. Bremer28,
Z. Cano3, S. Covino27, V. D’Elia29,46, D. A. Frail30, S. Geier4,31, P. Goldoni32, O. E. Hartoog17,
P. Jakobsson3, H. Korhonen33, K. Y. Lee23, B. Milvang-Jensen4, M. Nardini34, A. Nicuesa Guelbenzu18,
4 M. Oguri35,36, S. B. Pandey37, G. Petitpas25, A. Rossi18, A. Sandberg23, S. Schmidl18, G. Tagliaferri27,
1
0 R. P. J. Tilanus38,39, J. M. Winters28, D. Wright40, E.Wuyts41,42
2
(Affiliations can be found after the references)

n
a Received 8 January 2014; accepted XXXX
J

5
ABSTRACT
1

] At low redshift, a handful of gamma-ray bursts (GRBs) have been discovered with peak luminosities (Liso < 1048.5 ergs−1)
E substantially lower than the average of the more distant ones (Liso > 1049.5 ergs−1). The properties of several low-luminosity
(low-L) GRBs indicate that they can be due to shock break-out, as opposed to the emission from ultrarelativistic jets. Owing
H
to this, it is highly debated how both populations are connected, and whether there is a continuum between them. The burst
h. at redshift z = 0.283 from 2012 April 22 is one of the very few examples of intermediate-L GRBs with a γ-ray luminosity of
p L 1048.9 ergs−1 that have been detected up to now. Together with the robust detection of its accompanying supernova SN
∼
- 2012bz, it has the potential to answer important questions on the origin of low- and high-L GRBs and the GRB-SN connection.
o Wecarriedoutaspectroscopycampaignusingmedium-andlow-resolutionspectrographsat6–10-mclasstelescopes,coveringthe
r time span of 37.3 days, and a multi-wavelength imaging campaign from radio to X-ray energies over a duration of 270 days.
t ∼
s Furthermore, we used a tuneable filter centred at Hα to map star formation in the host galaxy and the surrounding galaxies. We
a usedthesedatatoextractandmodelthepropertiesofdifferentradiationcomponentsandincorporatespectral-energy-distribution
[ fittingtechniquestoextractthepropertiesofthehostgalaxy.Modellingthelightcurveandspectralenergydistributionfromthe

1 raaftdeirogtloowthheaXd-arnayesxrceevpetaiolendalthloewbplaesatk-wlauvmeintooseixtyp-adnednswitiythoafn(cid:46)in2itia1l0L30oreerngtsz−f1aHctzo−r1oifnΓt0h∼es6u0b,-lmowmf.oBreacahuigshe-oLfGthReBw,eaankdafttheartgltohwe
v ×
component, we were for the first time able to recover the signature of a shock break-out that was not a genuine low-L GRB. At
74 1.4 hours after the burst, the stellar envelope had a blackbody temperature of kBT ∼16 eV and a radius of ∼7×1013cm. The
37 ancicckoemlpmaansysinogf0S.N582M01(cid:12)2b,zejreecatcahmedaasspoefak5.l8u7mMin(cid:12)o,siatyndofkMineVti=ce−n1e9rg.7ymofag4,.100.3×m1a0g52meorrgewluemreinaomuosntghatnheSNhig1h9e9s8tbrwe.coTrhdeedsyvnatlhueessisfeodr
GRB-SNe,makingitthemostluminousspectroscopicallyconfirmedSNtodata.NebularemissionlinesattheGRBlocationwere
.
1 visible, extending from the galaxy nucleus to the explosion site. The host and the explosion site had close to solar metallicities.
0 Theburstoccurredinanisolatedstar-formingregionwithaSFRthatis1/10thofthatinthegalaxy’snucleus.Whiletheprompt
4 γ-ray emission points to a high-L GRB, the weak afterglow and the low Γ0 were very atypical for such a burst. Moreover the
1 detectionoftheshock-break-outsignatureisanewqualityforhigh-LGRBs.Sofar,shockbreak-outswereexclusivelydetectedfor
: low-L GRBs, while GRB 120422A had an intermediate Liso of 1048.9 ergs−1. Therefore, we conclude that GRB 120422A was a
v ∼
transitionobjectbetweenlow-andhigh-LGRBs,supportingthefailed-jetmodelthatconnectsshock-break-outdrivenlow-Land
i
X high-L GRBs that are powered by ultra-relativistic jets.
r Key words. dust, extinction gamma rays: bursts : individual: GRB 120422A - supernovae: individual: SN 2012bz
a
1. Introduction and brighter long-duration GRBs (for recent reviews see
Woosley&Bloom2006;Hjorth&Bloom2012),i.e.abright
The discovery of SN 1998bw in the error-box of GRB (M (cid:46) 19 mag), broad-lined (due to the expansion
bol,peak
980425byGalamaetal.(1998)gavethestudyoftheGRB- velocities of s−everal 104 kms−1) type Ic SN (i.e. lacking of
SN connection a flying start. This event remains unique in
hydrogen and helium). Interestingly, in only two out of 16
several ways among the many hundred GRBs that have
cases of nearby long-duration GRBs (z < 0.5) no SN was
been studied since. It is still the nearest GRB with a mea-
found to deep limits (Fynbo et al. 2006; Della Valle et al.
sured redshift and it is the least energetic GRB yet ob-
2006a; Gal-Yam et al. 2006; Ofek et al. 2007; Kann et al.
served. Nevertheless, SN 1998bw seems to be representa-
tive of the type of SNe that accompany the more typical
1
Schulze et al.: GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs
2011), though their classification is not free of ambiguity 2. Observations and data reduction
(e.g. Zhang et al. 2009; Kann et al. 2011).
On 2012 April 22 at 7:12:49 UTC (hereafter called T ;
0
So far, most GRBs with spectroscopically-confirmed MJD=56039.30057), the Burst Alert Telescope (BAT,
SN associations have had a much lower apparent lumi- Barthelmy et al. 2005) aboard Swift detected and localised
nosity than the bulk of the long-duration GRBs. GRB a faint burst (Troja et al. 2012). Its γ-ray light curve com-
030329 was the first example of a high-luminosity GRB prised a single peak with a duration of T = 5.4 1.4 s,
90
(log Liso/ ergs−1 = 50.9) that was accompanied by a followed by a fainter and lower-energetic emission±begin-
SN (Hjorth et al. 2003; Matheson et al. 2003; Stanek ning45safterthetriggerandlastingfor20s.Within86s,
(cid:0) (cid:1)
et al. 2003). However, there is a growing number of high- the Swift X-Ray Telescope XRT (Burrows et al. 2005) and
luminosity bursts, defined by log Liso/ ergs−1 > 49.5 the UV/Optical Telescope UVOT (Roming et al. 2005)
(Hjorth2013),withaspectroscopically-confirmedSN,such started to observe the field and detected an uncatalogued
(cid:0) (cid:1)
as GRBs 050525A (Della Valle et al. 2006b), 081007 (Della and rapidly decaying source at R.A., Dec.(J2000) =
Valleetal.2008;Jinetal.2013),091127(Cobbetal.2010; 09h07m38s42( 0.01),+14◦01(cid:48)07(cid:48).(cid:48)1( 0.2) (Beardmore
± ±
Berger et al. 2011), 101219B (Sparre et al. 2011), 130215A et al. 2012; Kuin & Troja 2012; Zauderer et al. 2012).
(de Ugarte Postigo et al. 2013), 130427A (Xu et al. 2013; Only 2(cid:48)(cid:48) NE of the explosion site there is a SDSS galaxy
Levan et al. 2013), and 130831A (Klose et al. 2013). (Cucchiara et al. 2012; Tanvir et al. 2012). Spectra of the
explosion site revealed several absorption and emission
Bromberg et al. (2011) suggested that low-luminosity
lines at a common redshift of z = 0.283, and a large
GRBs (log L / ergs−1 < 48.5; Hjorth 2013) are driven
iso number of emission lines at the location of the SDSS
by high-energy emission associated with the shock break-
out of their prog(cid:0)enitor s(cid:1)tars rather than an emerging jet galaxy at a redshift identical to that of the GRB (Schulze
et al. 2012b; Tanvir et al. 2012).
as in typical high-luminosity GRBs (Colgate & McKee
Thanks to its low redshift and its γ-ray luminosity
1969;Kulkarnietal.1998;Campanaetal.2006;Soderberg
(E 4.5 1049 erg and L 1049 ergs−1; Zhang et al.
et al. 2006a; Nakar & Sari 2012). A consequence of these iso ∼ × iso ∼
2012)beinginbetweenthatofhigh-andlow-LGRBs,itis
different energy sources is that low-L GRBs seem to be
anidealtargettosearchfortheaccompanyingGRB-SN,or
about 10-1000 times more common than high-L GRBs
placestringentconstraintsonitsabsence,ifitisanotherex-
(Pian et al. 2006; Guetta & Della Valle 2007; Virgili et al.
ampleofaSN-lesslongGRB.Wethereforetriggeredanex-
2009; Wanderman & Piran 2010), but because of their low
tensiveimagingcampaignwithseveraltelescopesfrommm
luminosities they are primarily found at low redshifts as
toopticalwavelengths,aswellasalargelow-andmedium-
rare events (one every 3 years). In contrast to high-L
∼ resolutionspectroscopycampaigncarriedoutat6-mto10-
GRBs, low-L GRBs typically have single-peak high-energy
m class telescopes. These campaigns began 31 min after
prompt light curves and can have soft high-energy spectra ∼
the trigger and ended 44.6 days later. Furthermore, we
with peak energies below 50 keV (Campana et al. 2006; ∼
∼ obtainedanX-rayspectrumwithXMM-Newton 12daysaf-
Starling et al. 2011, but see Kaneko et al. 2007). Their op-
ter the explosion. In addition to our own efforts, the GRB-
tical emission is dominated by the SN emission. Until now,
dedicated satellite Swift observed the GRB at UV/optical
radio and X-ray afterglows, but no optical afterglows have
andX-raywavelengthsfor54.3days.Weincorporatedthese
beendetectedforthem.TherecentGRB120422Aisapar-
data as well as radio data obtained with the Arcminute
ticularly interesting case. It has a γ-ray luminosity that is
Microkelvin Imager Large Array (AMI-LA; Staley et al.
intermediate between low- and high-luminosity GRBs and
2013) to present a comprehensive study of this event. In
hasarobustdetectionoftheassociatedSN(Malesanietal.
the following, we briefly summarise the observations and
2012a; S´anchez-Ram´ırez et al. 2012; Wiersema et al. 2012;
describe how the data were analysed. A log of our observa-
Melandri et al. 2012). A study of this event may thus an-
tions is presented in Tables 1, 2, A.1, and B.1.
swer important questions about the origin of both high-
and low-L GRBs.
Thepaperisstructuredasfollows.Wedescribethedata 2.1. Optical and NIR spectroscopy
gathering and outline the data analysis in Sect. 2, and
Ourspectroscopiccampaignbegan51minafterthetrigger
present the results on the transient following the GRB,
andcoveredatimespanof37.7days.Thespectralsequence
from radio to X-ray wavelengths, and the accompanying
comprised seven medium-resolution spectra obtained with
GRB-SN, SN 2012bz, in Sect. 3, and the properties of the
VLT/X-shooter (Vernet et al. 2011); the first three spec-
GRB environment and the host galaxy in 4. In Sect. 5 we
tra were obtained covering the full spectral bandwidth
compare our findings to other events and argue that GRB
from 3000 to 24800 ˚A, while for the remaining ones a K-
120422Arepresentsthemissing link between low- and high-
blockingfilter(cuttingthewavelengthcoverageat20700˚A;
L GRBs. Finally, we summarise our findings and present
Vernet et al. 2011) was adopted to increase the S/N in
our conclusions in Sect. 6.
the H band. These observations were complemented with
Throughout the paper we use the convention for the tenlow-resolutionspectraacquiredwiththeGeminiMulti-
fluxdensityFν(t) t−αν−β,whereαisthetemporalslope Object Spectrograph (GMOS, Hook et al. 2004), mounted
∝
andβ isthespectralslope.Werefertothesolarabundance onGemini-Northand-South,theGranTelescopioCanarias
compiled in Asplund et al. (2009) and adopt cm−2 as the (GTC) OSIRIS camera, the Keck Low Resolution Imaging
linear unit of column densities, N. Magnitudes reported in Spectrometer (LRIS; Oke et al. 1995) and the Magellan
the paper are given in the AB system and uncertainties Low Dispersion Survey Spectrograph 3 (LDSS3). Table 1
are given at 1σ confidence level (c.l.). We assume a ΛCDM summarises these observations.
cosmology with H = 71 kms−1Mpc−1, Ω = 0.27, and Observingconditionswerenotalwaysphotometric,and
0 m
Ω =0.73 (Larson et al. 2011). observations were performed irrespective of moon distance
Λ
2
Schulze et al.: GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs
Table 1. Summary of spectroscopic observations
MJD Epoch Spectral Resolving Exposure Slit Position
Telescope/Instrument Arm/Grating
(days) (days) range(˚A) power time(s) width angle
56039.345 0.0443 Gemini/GMOS-N R400+OG515 5942–10000 960 2×900 1(cid:48).(cid:48)0 180◦.0
56039.431 0.1301 Gemini/GMOS-N B600 3868–6632 844 2×400 1(cid:48).(cid:48)0 180◦.0
UVB 3000–5500 4350 4×1200 1(cid:48).(cid:48)0
56040.017 0.7160 VLT/X-shooter VIS 5500–10000 8800 4×1200 0(cid:48).(cid:48)9 41◦.0
NIR 10000–24800 5100 16×300 0(cid:48).(cid:48)9
56042.911 3.6112 GTC/Osiris R500R 4800–10000 500 4×1500 1(cid:48).(cid:48)2 100◦.0
UVB 3000–5500 4350 4×1200 1(cid:48).(cid:48)0
56044.014 4.7139 VLT/X-shooter VIS 5500–10000 8800 4×1200 0(cid:48).(cid:48)9 41◦.0
NIR 10000–24800 5100 16×300 0(cid:48).(cid:48)9
56044.257 4.9565 Keck/LRIS 400/3400 3000–5500 750 2×900 0(cid:48).(cid:48)7 50◦.0
400/8500 5500–10000 1700
UVB 3000–5500 4350 4×1200 1(cid:48).(cid:48)0
56048.061 8.7604 VLT/X-shooter VIS 5500–10000 8800 4×1200 0(cid:48).(cid:48)9 41◦.0
NIR 10000–24800 5100 16×300 0(cid:48).(cid:48)9
56048.304 9.0036 Gemini/GMOS-N R400 4442–8608 960 4×1200 1(cid:48).(cid:48)0 170◦.0
56052.978 13.6772 Gemini/GMOS-S R400+GG455 4892–9008 960 1×2400 1(cid:48).(cid:48)0 180◦.0
56053.930 14.6301 GTC/Osiris R500R 4800–10000 500 3×1200 1(cid:48).(cid:48)2 75◦.0
UVB 3000–5500 4350 4×1200 1(cid:48).(cid:48)0
56057.996 18.6962 VLT/X-shootera VIS 5500–10000 8800 4×1200 0(cid:48).(cid:48)9 52◦.0
NIR 10000–20700 5100 16×300 0(cid:48).(cid:48)9
56061.996 22.6953 Gemini/GMOS-S R400+GG455 4892–9108 960 2×2400 1(cid:48).(cid:48)0 -30◦.0
UVB 3000–5500 4350 4×1200 1(cid:48).(cid:48)0
56063.999 24.6992 VLT/X-shootera VIS 5500–10000 8800 4×1200 0(cid:48).(cid:48)9 52◦.0
NIR 10000–20700 5100 16×300 0(cid:48).(cid:48)9
56066.068 26.7680 Magellan/LDSS3 VPHALL 3700–9400 800 1×1400 1(cid:48).(cid:48)2 141◦.0
UVB 3000–5500 4350 4×1200 1(cid:48).(cid:48)0
56076.025 36.7250 VLT/X-shootera VIS 5500–10000 8800 4×1200 0(cid:48).(cid:48)9 -143◦.9
NIR 10000–20700 5100 16×300 0(cid:48).(cid:48)9
UVB 3000–5500 4350 4×1200 1(cid:48).(cid:48)0
56077.000 37.7001 VLT/X-shootera VIS 5500–10000 8800 4×1200 0(cid:48).(cid:48)9 151◦.1
NIR 10000–20700 5100 16×300 0(cid:48).(cid:48)9
Notes. Column ”Epoch” shows the logarithmic mean-time after the burst in the observer frame. Resolving powers and spectral
rangesarethenominalvaluesfrominstrumentmanuals.(a) TheK-bandblockingfilterwasusedtoincreasetheS/NinJH band.
and phase. For each epoch, we centred the slit on the ex- Table 2. Summary of mm and sub-mm observations
plosion site and in some cases varied the position angle to
probe different parts of the host galaxy, as illustrated in MJD Epoch Instrument Frequency Exposure Fν
Fig. 1. (days) (days) time(s) (mJy;3σ)
VLT/X-shooter data were reduced with the X-shooter 56039.3291 0.0537 SCUBA-2 350GHz 5639 <7.20
pipeline v2.0 (Goldoni et al. 2006).1 To extract the one- 56039.3291 0.0537 SCUBA-2 665GHz 5639 <225
56039.5676 0.2670 AMI-LAa 15GHz <0.62
dimensional spectra of the transient and the host galaxy, 56040.1923 0.8917 SMA 272GHz 3420 <3.60
we used a customised tool that adopts the optimal extrac- 56041.6806 2.3800 AMI-LAa 15GHz <0.47
56041.9422 2.6416 PdBI 86.7GHz 5040 <0.39
tion algorithm by Horne (1986). The Gemini, GTC, and
56041.9943 2.6937 CARMA 92.5GHz 3480 <1.15
Magellan spectra were reduced and calibrated using stan- 56043.6806 4.3800 AMI-LAa 15GHz <0.37
dard procedures in IRAF (Tody 1993). Keck data were re- 56046.7206 7.4200 AMI-LAa 15GHz <0.24
56048.8054 9.5048 PdBI 86.7GHz 5040 <0.24
duced with a custom pipeline that makes use of standard 56052.7506 13.450 AMI-LAa 15GHz <0.23
techniques of long-slit spectroscopy. In all cases we chose 56067.8906 28.590 AMI-LAa 15GHz <0.46
a small aperture for studying the optical transient. For
Notes.Column”Epoch”showsthelogarithmicmean-timeafter
studyingtheemissionlines,weextractedthespectralpoint
theburstintheobserverframe.(a)DatatakenfromStaleyetal.
spread function and extracted the spectrum of the nucleus
(2013).
andtheafterglowwithinanapertureof1 FWHMofeach
trace, e.g. the FWHMs were 1(cid:48).(cid:48)34 and 0(cid:48).(cid:48)×86 for the galaxy
nucleus and the explosion site, respectively, for the UVB
and VIS of the first X-shooter spectrum. shooterdatawerecorrectedforheliocentricmotion.Notel-
All spectra were flux-calibrated with corresponding luric correction was applied, as it has no implications for
spectrophotometric standard star observations and the ab- our analysis.
solute flux scale was adjusted by comparing to photome-
try. The data were corrected for the Galactic reddening of
2.2. Imaging
E(B V)=0.04mag(Schlegeletal.1998).Allwavelengths
−
were transformed to vacuum wavelengths. In addition, X- FollowingtheBATtrigger,Swift slewedimmediatelytothe
burstandUVOTtookav-bandsettlingexposure86safter
1 http://www.eso.org/sci/software/pipelines/ the BAT trigger. Science observations began at T0+104 s
3
Schulze et al.: GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs
andcycledthroughallfilters.Follow-upobservationsinthe Gemini−North/GMOS g’−band

v and b bands continued until T0+2.3 days, in the uvw1, T + 3.9384 d Host
uvm2 and uvw2 UV filters until T +9.7 days, and in the 0
0
u band until T +54.3 days, at which time a final set of
0
observations of the host galaxy was taken in all filters.2
G1
Ourground-basedimagingcampaignbegan31minafter Host Arm OT
theexplosionandspannedatimeintervalof 45days.Due
to the proximity of a R = 8.24 mag star (7∼9(cid:48)(cid:48) NW of the PA = 100(cid:31) T0 + 270.17 d
explosion site), we either moved the position of the optical Curved bridge
transient to the NW corner of the chip, or (most of the
time) obtained short dithered exposures to avoid excessive
saturation.
Observations were carried out with the 2.56-m
G1
Nordic Optical Telescope (NOT) equipped with ALFOSC,
MOSCA, and StanCAM in the u(cid:48)g(cid:48)Rr(cid:48)Ii(cid:48) bands (Malesani
OT P
et al. 2012b; Schulze et al. 2012a). These observations A
began at 14.29 hr post-burst and were stopped at 44.5 Tidal arm = 4
1
days because of the small Sun distance. Further imag- (cid:31)
ing data were acquired with GMOS-N and GMOS-S in NN
the u(cid:48)g(cid:48)r(cid:48)i(cid:48)z(cid:48) bands between 31 min and 40.7 days after
the explosion (Cucchiara et al. 2012; Perley et al. 2012a).
33 ""
TheGamma-rayOptical/Near-infraredDetector(GROND, EE
Greineretal.2007,2008)mountedattheMPG/ESO2.2m
telescopeonLaSillaimagedthefieldsimultaneouslyinfour
optical (g(cid:48)r(cid:48)i(cid:48)z(cid:48)) and three NIR (JHK ) bands starting at Fig.1. Field of GRB 120422A (12(cid:48)(cid:48) 12(cid:48)(cid:48)). The position
s ×
of the optical transient (OT) accompanying GRB 120422A
T +16.5 hr (Nardini et al. 2012). Additional epochs were
0
is marked, as well as of the host galaxy and the curved
obtained at nights 2, 9, 11, 20, 29, before the visibility of
bridge of emission connecting the explosion site with the
thefieldwascompromisedbyitssmallSundistanceonday
39. We monitored the optical transient in the g(cid:48)r(cid:48)i(cid:48) bands host’s nucleus. Galaxy G1 has the same redshift as the
GRB. The projected distance between the explosion site
with the 60-inch Palomar telescope for 37 days beginning
and the galaxy G1 is 28.7 kpc. The inset shows the field
at T +0.87 day and in the JHK bands with the Wide
0 observed in the g(cid:48)-band with GMOS-N at 270.2 days af-
FieldCamera(WFCAM)mountedattheUnitedKingdom
ter the burst. The image cuts were optimised to increase
InfraredTelescope(UKIRT)onMaunaKeaatsevenepochs
the visibility of the tidal arm that partly connects the host
between T +0.06 and 25.98 day.
0
galaxyandG1.Themostimportantslitorientationsofour
We complemented these optical observations with the
spectroscopic campaign (Table 1) are overlaid.
10.4-mGTCtelescopeequippedwithOSIRISintheg(cid:48)r(cid:48)i(cid:48)z(cid:48)
bands, the multi-filter imager BUSCA mounted at the 2.2-
mtelescopeofCalarAlto(CAHA)ing(cid:48) andther(cid:48) bands,3
TheCAHAobservationdidunfortunatelynotgoverydeep.
the 3.5-m CAHA telescope equipped with the Omega
2000 We will not discuss these data in the following.
camerainthez(cid:48) band,4 theLDSS3cameramountedatthe
In addition to these broad-band observations, we made
6-m Clay telescope telescope in the r(cid:48) and i(cid:48) bands, the
use of the tuneable filters at the 10.4-m GTC to trace the
Direct CCD Camera mounted on the Irenee du Pont 2.5-
Hα emission in the host galaxy on 2012 May 16, 25.5 days
m telescope at Las Campanas in the r(cid:48) and i(cid:48) bands, the
after the burst. Observations consisted of 5 600 s expo-
2.4-m Gao-Mei-Gu (GMG) telescope in i(cid:48), and the 1.04-m sures using a 15-˚A wide filter tuned to the w×avelength of
andthe2-moptical-infraredHimalayanChandraTelescope
Hα at the redshift of the burst (λ = 8420 ˚A), and a
in R and I . Additional NIR data were acquired with the obs
c c 3 100 s exposure with a 513-˚A-wide order-sorter filter
Omega intheYJHK bands,theNear-InfraredImager
2000 s ce×ntred at 8020 ˚A to probe the continuum emission (filter
(NIRI) mounted on Gemini-North in the J and K bands,
f802/51). The seeing was 1(cid:48)(cid:48), although the transparency
and the Wide-field Infrared Camera (WIRC) on the 200-
∼
was affected by extinction due to Saharan dust suspended
inch Hale telescope at Palomar Observatory in the J band
in the atmosphere (Calima).
(Perley et al. 2012b).
Ingeneral,observingconditionswerenotalwaysphoto-
Very late-time observations were secured with the 2.0-
metric;inparticular,partoftheNOTobservationssuffered
m Liverpool telescope, with BUSCA mounted at the 2.2-m
frompoortransparencyduetotheCalima.TableA.1sum-
CAHA,andGMOSmountedatGemini-North(TableB.1).
marises all observations with good data quality.
TheobservationwiththeLiverpooltelescopecomprises185
We obtained the UVOT data from the Swift Data
images. To minimise the data heterogeneity an observa-
Archive.5 Thesedatahadbadpixelsidentified,mod-8noise
tionalseeingconstraintof<1(cid:48).(cid:48)1wasimposedforallepochs.
corrected, and endowed with FK5 coordinates. We used
thestandardUVOTdataanalysissoftwaredistributedwith
2 Additional UVOT data were acquired in October 2012. HEASOFT 6.12 along with the standard calibration data.6
These data are not discussed in this paper. This has no im- Optical and NIR data were processed through standard
plications on our work.
3 http://www.caha.es/newsletter/news01a/busca/ 5 http://www.swift.ac.uk/swift portal/
4 http://www.mpia-hd.mpg.de/IRCAM/O2000/ 6 http://heasarc.nasa.gov/lheasoft/
4
Schulze et al.: GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs
procedures (bias subtraction and flat field normalisation) ibration observations spanned a larger range of weather
using IRAF or instrument specific software packages, i.e. conditions than during the GRB 120422A run, and were
the GEMINI IRAF software package for GMOS and NIRI, in general agreement with the standard values of the flux
for GROND data a customised pipeline (for details we re- conversion factors (Dempsey et al. 2013), which were then
fer to Yolda¸s et al. 2008 and Kru¨hler et al. 2008), a modi- used for the flux normalisation. We reduced CARMA and
fiedversionoftheWIRCSoftpackageforP200/WIRCdata,7 SMAdatawiththeMIRIADandMIR-IDLsoftwarepackages
andforWFCAMdatatheUKIRTpipeline.8 Someobserva- (Saultetal.1995).10 CARMAdatawereabsolutefluxcali-
tions suffered from variable conditions, and in those cases bratedwithobservationsof3C84andMars.Thecalibration
individual images were weighted according to their S/N. oftheSMAdataistwofold:firstweusedthenearbyquasars
The i(cid:48)- and the z(cid:48)-band images suffer from fringing, which J0854+201andJ0909+013asatmosphericgaincalibrators,
was corrected using a fringe pattern computed from the andthenJ0854+201forbandpasscalibration.Absoluteflux
science data themselves, although in some cases the pres- calibration was bootstrapped from previous measurements
ence of the halo from the nearby bright star hampered the of these quasars resulting in an absolute flux uncertainty
process. These data resulted in a lower S/N. Astrometric of 30%.PdBI datawere reducedwith thestandard CLIC
∼
calibration was computed against the USNO-B1 catalog andMAPPINGsoftwaredistributedbytheGrenobleGILDAS
(Monet et al. 2003), yielding an rms of 0(cid:48).(cid:48)4. All images group.11 The flux calibration was secured with the Be bi-
were then registered together, yielding a relative RMS of nary star system MWC349 (F =1.1 Jy at 86.7 GHz).
ν
less than 0(cid:48).(cid:48)08. We measure the afterglow location to be
R.A., Dec.(J2000)=09h07m38s42,+14◦01(cid:48)07(cid:48).(cid:48)5.
2.2.2. X-ray observations
Swift/XRT started to observe the BAT GRB error circle
2.2.1. Sub-mm/mm observations
roughly 90 s after the trigger, while it was still slewing.
Our sub-mm/mm observations comprised five epochs and Observations were first carried out in windowed timing
cover a time interval of 9.48 days. First, Smith et al. mode for 80 s. When the count rate was (cid:46) 1 cts−1, XRT
(2012) simultaneously obtained an early epoch at 450 µm switched to photon counting mode. Observations contin-
and 850 µm with the sub-millimetre continuum camera ueduntilT0+53.8days,whenthevisibilityofthefieldwas
SCUBA-2(Hollandetal.2013)ontheJamesClerkMaxwell compromised by its small Sun distance. We obtained the
Telescope (JCMT). The 1.6-hr observation began at T + temporalandspectroscopicdatafromtheSwift/XRTLight
0
41.5 min and was performed under moderate weather con- Curve and Spectrum Repository (Evans et al. 2007, 2009).
ditions. The CSO 225 GHz tau, which measures the zenith GRB 120422A was also observed by XMM-Newton with a
atmospheric attenuation, was 0.089 initially, but generally DDT, starting at 2012 May 3, 15:13 UT. At this epoch,
degraded through the run. The elevation of GRB 120422A exposures of 56841, 58421 and 58426 s were obtained with
fell from 54◦.6 to 30◦.4. In the consecutive night, Martin the PN, MOS1 and MOS2 detectors, respectively.
et al. (2012) triggered a short 45-min snapshot observation To analyse the spectroscopic data we used Xspec, ver-
attheSubmillimeterArray(SMA)atT +21.4hr.Receivers sion 12.7.1, as part of HeaSoft 6.12, XMM-Newton spe-
0
were tuned to the local oscillator (LO) centre frequency of cific calibration files and for the Swift/XRT pc mode data
271.8 GHz (λ=1.1 mm), with the correlator configured to the respective Swift calibration files version 13. The X-ray
covertwo4GHzbandscentredat 6GHzfromtheLOfre- emissionuptoT0+200swasdiscussedindetailinStarling
±
quency.All8SMAantennaswereusedinitsveryextended et al. (2012) and Zhang et al. (2012). Therefore, we focus
configuration under excellent weather conditions, with an on the analysis of the data after that epoch. In total, XRT
average zenith opacity of 0.03 (precipitable water vapour registered 270 background-subtracted photons between 0.3
ofPWV 0.5mm)at225GHz.Afurtherobservationwas and 10 keV; data that were flagged as bad were excluded
∼
carried out by Perley (2012) with the Combined Array for from analysis. We re-binned the spectrum to have at least
Research in Millimeter-Wave Astronomy (CARMA) in D- 20 count per bin and applied χ2 statistics.
configuration at 92.5 GHz (λ = 3 mm). This observation
was carried out between 23:13 UT on 24 April and 00:29
2.3. Photometry
UT on April 25. The total on-source integration time was
58 min. We finally obtained two epochs with the Plateau Measuring the brightness of the transient is complicated
de Bure Interferometer (PdBI) at a frequency of 86.7 GHz due to blending with its extended, offset host galaxy. To
(λ = 3.4 mm) in its 6 antenna compact D configuration. limitthehostcontributiontothetransientphotometry,we
These observations began at T0 +2.6416 and 9.5048 days used point-spread function (PSF) fitting techniques. Using
and lasted for 84 min each. AMI-LA obtained six epochs bright field stars, a model of the PSF was constructed for
between 0.27 and 28.59 days after the burst (Staley et al. each individual image and fitted to the optical transient.
2013). To provide reliable fit results, all images were registered
The SCUBA-2 data were reduced in the standard astrometrically to a precision of better than 0(cid:48).(cid:48)08, and the
manner (Chapin et al. 2013) using SMURF (Version 1.5.0) centroid of the fitted PSF was held fixed with a small mar-
and KAPPA (Version 2.1-4) from the Starlink Project.9 gin of re-centering corresponding to the uncertainty of the
Observations of the SCUBA-2 calibrator Mars bracketed astrometric alignment of the individual images. In addi-
theGRB120422Aobservation,andobservationsofthecal- tion, the PSF-fitting radius was adjusted to the specific
ibrator CRL2688 were taken several hours later. The cal- conditions of the observations and instrument, in particu-
7 http://humu.ipac.caltech.edu/˜jason/sci/wircsoft/index.html 10 http://www.atnf.csiro.au/computing/software/miriad/
8 http://casu.ast.cam.ac.uk/surveys-projects/wfcam https://www.cfa.harvard.edu/˜cqi/mircook.html
9 http://starlink.jach.hawaii.edu/starlink 11 http://www.iram.fr/IRAMFR/GILDAS
5
Schulze et al.: GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs
larseeingandpixelscale.Thefitradiusisdifferentforeach i.e. a host light contribution of 10%, 7% and 7% in g(cid:48)r(cid:48)i(cid:48)
observation,buttypicallyintherangebetween0(cid:48).(cid:48)5and0(cid:48).(cid:48)8. at maximum SN light. To estimate the fraction in differ-
Generally, the radius was smaller under unfavourable sky ent filters, we scaled the above numbers to the respective
conditions in an attempt to minimise the host’s effect on filters using the SED of the host. We assume that this fac-
the fit. Naturally, this leads to a lower S/N for these mea- tor is similar for all data from various telescopes. We note
surementsthanonewouldexpectforisolatedpointsources. that the values in Table A.1 are not corrected for this host
For images taken under adverse sky conditions (seeing contribution.
(cid:38) 1(cid:48).(cid:48)6), with imagers with large pixel scales (e.g. the NIR
channelsofGRONDwith0(cid:48).(cid:48)6perpixel),orinfilters/epochs
3. The transient accompanying GRB 120422A
with low S/N (e.g. most of the late NIR data), the indi-
vidual contributions of point-source and galaxy cannot be
Figure 2 displays the brightness evolution of the tran-
disentangled robustly. These measurements are ignored in
sientaccompanyingGRB120422AfromX-raystotheNIR.
the following analysis. For all observations the source was
During the first three days, its brightness in the UVOT fil-
close to the centre of the field of view, and differences in
ters gradually decreases with a decay slope of α=0.2 that
the PSF between observations were, therefore, negligible.
is followed by a rebrightening peaking at 20 days post-
TomeasurethebrightnessofthetransientintheUVOT burst. The time scale and the colour evol∼ution of the re-
images, we measured the host galaxy flux at the position brightening are comparable to those of GRB-SNe (e.g. Zeh
of the SN from the later UVOT observations, where there etal.2004).Theinitiallydecayingtransientcouldtherefore
was no longer a contribution from the GRB or SN. This be a superposition of the afterglow and the thermal emis-
additional flux was then subtracted from our photometric sion of the cooling photosphere after the SN emerged. Key
measurementsatthepositionoftheGRB.Incontrast,host- tounderstandingtheevolutionofthetransientaccompany-
galaxyphotometrywasperformedviaaperturetechniques. ing GRB 120422A is disentangling the different radiation
Here,weusedourPSF-modeltosubtractthetransientfrom components. In the following sections we will present our
the deepest images in each filter with the clearest separa- results on each component.
tionbetweengalaxyandpointsource,i.e.thoseimageswith
thesmallestFWHMofthestellarPSF.Acircularaperture
radius was chosen sufficiently large (2(cid:48).(cid:48)5, e.g. 10.7 kpc at 3.1. The stellar envelope cooling-phase
z = 0.2825), so that the missed emission from low sur-
Figure3displaysspectralenergydistributionsat0.054and
face brightness regions does not affect our photometry sig-
0.267 days after the GRB. While afterglows have spectra
nificantly. In addition, we also corroborated the galaxy
formed by piecewise-connected power laws from radio to
photometry using elliptical Kron apertures (Kron 1980)
X-rays (Sari et al. 1998), the cooling phase of the stellar
via their implementation in Source Extractor (Bertin &
envelope that was heated by the SN shock break-out is
Arnouts 1996).
characterised by thermal emission peaking in the UV.
Once a magnitude was established, it was calibrated
TheearlyUVemissionisindeedwellfittedwithablack-
photometrically against the brightness of a number of field
body (for details see Sect. 3.2.3). We measure a blackbody
stars measured in a similar manner. Photometry was tied
temperature of kT = 14 eV and a blackbody radius of
obs
to the SDSS DR8 (Aihara et al. 2011) in the optical fil- 9 1013 cm at T +0.054 days. These values are consis-
ters (u(cid:48)g(cid:48)r(cid:48)i(cid:48)z(cid:48)) and 2MASS (Skrutskie et al. 2006) in the × 0
tentwithexpectationfromtheshock-break-outmodel(e.g.
NIR (JHK ). For those filter bands not covered by our
s Ensman & Burrows 1992; Campana et al. 2006, and refer-
primary calibration systems (e.g. I or Y), we used the
C ences therein) and lie in the ballpark of observed values
instrument-specific band passes to transform magnitudes
of Ib/c SNe, such as 1993J (Richmond et al. 1994, 1996;
into the respective filter system via synthetic photometry
Blinnikov et al. 1998), 1999ex (Stritzinger et al. 2002),
similar to the procedure outlined in Kru¨hler et al. (2011b).
2008D(Soderbergetal.2008;Malesanietal.2009;Modjaz
UVOT images were calibrated using the method described
et al. 2009) and 2011dh (Arcavi et al. 2011; Soderberg
in Poole et al. (2008).
etal.2012;Ergonetal.2013),andoftheGRB-SNe2006aj
Thephotometricerrorwasthenestimatedbasedonthe
(Campana et al. 2006) and 2010bh (Cano et al. 2011a;
contributions from photon statistics and goodness of the
Olivares E. et al. 2012).
PSF fit (typically between 0.5 to 15 %), the absolute ac-
The observed decline in the u band between its first
curacy of the primary calibration system ( 2–3% ), the
detection and T +2.8 days and the local minimum in the
≈ 0
systematic scatter of different instrument/bandpasses with
light curve before the SN rise is 2 mag. It is comparable
respect to the primary calibrators ( 3–6%) or the uncer- ∼
to that observed in GRB 060218 (Campana et al. 2006).
≈
taintyinthecolourtransformation(ifapplicable, 6–9%).
However, for this event, these authors also reported a rise
≈
The photometry described in the earlier paragraph in-
inbrightnessupto0.57daysaftertheburst(shiftedtothe
evitably contains a seeing-dependent fraction of the host
observer frame of GRB 120422A). This initial rise is not
light directly at the position of the transient. This contri-
present in our data, although the first observation was at
bution is best removed via differential imaging with deep
86.4 s after the onset of the γ-ray emission.
reference frames from the same instrument/filter combina-
tion taken after the transient has faded completely. Given
the vast number of different observers taking part in our 3.2. The afterglow emission
photometrycampaign,however,thisprocedurewasnotfea-
3.2.1. X-rays
sible in our case for all images. We instead used reference
frames from a single telescope (Gemini-N, obtained 270 Zhang et al. (2012) reported that the early X-ray emission
days after the explosion) in three filters. We measure∼: g(cid:48) = (t < 200 s) is consistent with high-latitude emission from
24.62 0.10, r(cid:48) =24.09 0.09, and i(cid:48) =24.09 0.09 mag, the prompt emission phase (e.g. Fenimore & Sumner 1997;
± ± ±
6
Schulze et al.: GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs
Time since GRB trigger (days)
10 3 10 2 10 1 100 101 102
− − −
103
17
18
102 19
20
21
101
22
23
)
y
J )
µ g
( 100 24 ma
ν
F (
y ss
t 25 e
si n
n t
e h
d g
x 26 Bri
u10 1
Fl −
27
28
10−2 29
30
31
1.73 keV+2 b+2 z –2
10 3 0
− uvw2+6 g +1 J–3
0
32
uvm2+5 r H –4
0
uvw1+4 i –1 K–5
0
33
u+3
)
g
2.0 a
el 0.6 m
d
mo 0.4 (
ν, 0.2 ta
F 1.0 0.0 a
/bs 0.8 -0.2 -d
Fν,o 0.6 --00..64 del
o
102 103 104 105 106 107 M
Time since GRB trigger (s)
Fig.2. X-ray, optical and NIR light curves of the transient following GRB 120422A. Arrows indicate 3σ upper limits.
The UVOT v-band upper limits are very shallow and not displayed. Data in the g(cid:48)r(cid:48)i(cid:48)z(cid:48)J-bands were modelled with a
SN 1998bw template at z = 0.283 superposed on a power law (where the slope was identical in all bands) using the
formalisminZehetal.(2004).Thebest-fitmodelparametersareshowninTable3.Modellightcurvesinbluerorredder
filtersarenotshownsincetheywouldrequireextrapolationofthespectralrangeoftheSN1998bwtemplate.Fitresiduals
are displayed in the bottom panel. The XMM-Newton observation was carried out at 980 ks (open dot). The shifts (in
magnitude) of the different bands are given in the legend. To convert the X-ray light curve to flux density, we assumed a
spectral slope of β =0.9 and no spectral evolution (for details on the SED modelling see Sect. 3.2.3). Both assumptions
have no implications on our analysis. The XMM-Newton data point was discarded from the light curve fit because of
uncertainties in the cross-calibration between Swift/XRT and XMM-Newton. The vertical lines indicate the epochs of
7
the X-ray-to-NIR SEDs presented in Sect. 3.2.3.
Schulze et al.: GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs
102 110056 T0+0.0537days T0+0.267days T0+0.8917days
104
103
101
102
FluxdensityF(µJy)ν11001−−0210 FluxdensityF(µJy)ν111111111000000000112345656 TT00++29..65401468ddaayyss TT00++413.3.845ddayayss TT00++728.4.259ddayayss
Spherical,tb>270days
104 Collimated,tb=9.7days
10−3 TT00++00..025647ddaayyss×10−1 111000123
1014 1015 1016 1017 1018 1010 1011 1012 1013 10141010 1011 1012 1013 10141010 1011 1012 1013 1014
ObservedFrequency(Hz) ObservedFrequency(Hz)
Fig.3.Left:SpectralenergydistributionfromtheNIRtotheX-rayatearlyepochs.Theoptical-to-X-raySEDsarebest
described by absorbed broken power law (dashed lines) models modified by a blackbody (dotted lines). Data excluded
from fits are shown as empty symbols and upper limits by triangles. Right: Radio-to-sub-mm SEDs at the epochs listed
in Table 2. The NIR-to-X-ray SED from T +0.267 days was extrapolated to radio frequencies and evolved in time for a
0
collimatedandsphericalexpansionoftheblast-wave(fordetailsseetext).TheAMI-LTmeasurementfromT +2.38was
0
shifted to 2.6416 days, assuming the injection frequency to be blueward of the observed bandpass and using the scaling
relations in Sari et al. (1998). This has no implications on our analysis.
Kumar&Panaitescu2000;Dermer2004),withevidencefor 3.2.2. Optical/NIR
small-scaledeviationfrompower-lawmodels(Starlingetal.
2012),possiblyduetoathermalcomponentasseeninother As mentioned before, the thermal emission of the cooling
GRBs (e.g. Campana et al. 2006; Page et al. 2011; Starling photosphere has an intrinsically blue spectrum and does
et al. 2011, 2012; Sparre & Starling 2012; Friis & Watson not significantly contribute to the integrated emission in
2013).Friis&Watson(2013)suggestedthatsuchathermal the optical and NIR. Therefore, the optical/NIR emission
component is not produced by the stellar photosphere but can be decomposed into three distinct emission compo-
by the photosphere of the GRB jet. In the following, we nents: i) the afterglow, which can be modelled with simple
will focus on the emission at >200 s after the burst. and broken power-law models; ii) the supernova; and iii)
the host galaxy, which can be accounted for by a constant
AtthetimeofourXMM-Newton observationtheX-ray flux. To characterise the SN component, we follow the ap-
spectrum is adequately fit as an absorbed power-law with proachinZehetal.(2004).Theyusedthemulti-colorlight
a spectral slope of β = 94+−00..1121, with absorption entirely curves of the prototypical GRB-SN 1998bw (Galama et al.
consistent with the Galactic column (3.71 1020 cm−2). 1998; Patat et al. 2001) as templates. They derived the
×
The spectral slope is consistent with that derived from the SN 1998bw light curves at the given GRB redshift, and
latetimeXRTspectrum(β =0.98 0.13),andsuggestsno in the given observed band (including the cosmological k-
±
late time spectral changes (t>4600 s). The spectral slope correction), and additionally modified the template with
is typical for GRB afterglows at that phase. twoparameters.TheluminosityfactorkdeterminestheSN
peak luminosity in a given band in units of the SN 1998bw
The joint XRT and XMM-Newton light curve is shown
peak luminosity in that band. The stretch factor s deter-
in Fig. 2, where we converted the XRT observations to
mines if the light curve evolution is faster (s<1) or slower
flux based on the mean spectral index of the system (fol-
(s > 1) than that of SN 1998bw, whereby the actual evo-
lowing Evans et al. 2009), and then added the XMM-
lutionary shape remains the same, and the explosion time
Newton observations assuming their measured spectral pa-
is always identical to the GRB trigger time. However, we
rameters. The X-ray light curve is adequately fit by a mul-
limit the SN modelling to the g(cid:48)r(cid:48)i(cid:48)z(cid:48)J bands. Model light
tiply broken power-law with indices of α = 12.7 4.1,
1 ± curves in bluer or redder filters require extrapolating the
α = 6.09 0.16, α = 0.31 0.04, α = 1.48 0.40, and
2 ± 3 ± 4 ± spectral range of the SN1998bw template.
break times of t = 95.3 3.2 s, t = 394 19 s and
b,1 b,2
t =330.5 89.0ks,there±sultingχ2/d.o.f.=4±3.5/54.We The results of our fits are given in Table 3. In this sec-
b,3
± tion we report on the afterglow properties and on those of
note that an early break is needed to fit the WT settling
the SN in Sect. 3.3.2. The light curve fits reveal that there
mode exposures, which has a chance improvement proba-
bility of 6.6 10−5. isindeedapower-lawcomponent,andhenceprovidestrong
∼ × evidence for an optical/NIR afterglow accompanying GRB
The steep-to-shallow-to-normal decay-phase evolution 120422A. The fit with a simple power law makes the as-
is typical for X-ray afterglows of high-L GRBs (Nousek sumption that the afterglow light curve does not steepen
et al. 2006; Evans et al. 2010). In particular, the very untilT +270.2days.Foracollimatedoutflowtheobserver
0
rapid decay phase ( t−13) unambiguously points to high- sees the edge of the jet at a certain time, resulting in a sig-
∝
latitude emission, and has not been observed for low-L nificant steepening (Sari et al. 1999). A jet break after 270
GRBs so far. dayshasbeenobservedinGRB060729(Grupeetal.2010,
8
Schulze et al.: GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs
see also Perley et al. 2013a for a further example of a very were adjusted to the brightness of the X-ray afterglow at
latejetbreak),butatypicalvalueis 0.6day(rest-frame; the respective epochs.
∼
e.g. Zeh et al. 2006; Racusin et al. 2009). We refitted the The NIR-to-X-ray SEDs, shown in Fig. 3, have in com-
light curve with a smoothly broken power law (Beuermann mon that the UV emission is dominated by radiation from
et al. 1999), where the post-break decay slope was fixed to thecoolingstellarenvelopeaftertheshockbreak-out(Sect.
2. The pre-break slope is identical to the value from the 3.1). To account for this thermal emission, we fit the NIR-
simple power law fit. The jet-break time of 9.7 4.4 days to-X-ray SED with absorbed simple and broken power-law
±
(observer frame) is still large and very uncertain, but its modelsmodifiedbyablackbodymodelusingXspecv12.8.0.
value is more consistent with the observed distribution in The blackbody model is defined by:
Racusinetal.(2009).Areasonforthislargeuncertaintyin
the break time is the brightness of the SN. E2∆E
Both afterglow models over-predict the i(cid:48)-band bright- BB(E; C, T)=1.0344×10−3C exp(E/k T) 1
B
nessatT +1880sby0.9mag.Therequiredrisecouldbeei- −
0
therduetothecrossingoftheinjectionfrequencyνmordue where the numerical constant C is defined as R2 /D2 ,
km 10kpc
to the coasting phase before the afterglow blast-wave be-
where R is the blackbody radius in km, D the distance in
gundecelerating.Intheformercasetheriseslopeα is-0.5
r unitsof10kpc,k theBoltzmannconstant,T thetemper-
(withFν t−αr;Sarietal.1998),andinthelatterbetween atureinunitsofkBeV,E theenergyand∆E isthewidthof
∝
3 and 2 for constant-density medium and > 0.5 for a
the energy bin.
− −
free-stellar-wind density profile (Shen & Matzner 2012).
Both SEDs are best fitted by a broken power law with
The crossing of the injection frequency ν is by defi-
m β 0.5andβ 1andabreakenergyof 4eV.Thedif-
nition a chromatic feature. It evolves t−3/2 (Sari et al. o ∼ x ∼ ∼
ference in the slopes is consistent with the expected value
∝
1998). This means the ratio between break times in two
for synchrotron radiation, if the cooling break is between
−2/3
different bands has to obey t2/t1 = (ν2/ν1) . The J both bands (Sari et al. 1998). This is a further circum-
bandhastheearliestdetectionafterthefirsti(cid:48) observation stantial evidence that the optical and X-ray emission are
and is not affected by the thermal emission from the cool- produced by the afterglow. Given the sparse sampling of
ingstellarphotosphere.SincetheJ-bandlightcurveisonly the optical/NIR bands, we fit both epochs simultaneously
decaying,νm crossedthisbandatt<4550saftertheburst and fix the difference in the spectral slopes to 0.5 and set
and hence the i(cid:48) band at (cid:46)3260 s. Already in the limiting the break energies to identical values. The joint fit gives a
case, the expected i(cid:48) band magnitude is 0.24 mag brighter spectral slope of β 0.46 in the optical (i.e. β =0.96), a
o x
∼
thantheobservedvalue.Giventhesmallphotometricerror break energy of 4 eV, no evidence for a significant host
∼
of0.04magmakesthedeviationstatisticallysignificantand absorptionatX-rayenergies,andablackbodytemperature
hence this scenario unlikely. The blast-wave’s coasting into of 16 eV and radius of 7 1013 cm at 1.4 hours after
∼ ∼ ×
afree-stellar-windambientdensityprofileisalsoinconflict the burst. The blackbody component in the second epoch
withourdata,sincewedetectaclearriseandnotashallow is barely constrained because of the limited amount of UV
decay. data. The combined fit statistics is 114.7/74 d.o.f.
A steep rise of αr = 2 to 3 is fully consistent with The peak of an afterglow spectrum is typically at
− −
our data. In both cases, the break time is 2550 s (ob- cm/sub-mm wavelengths, and usually crosses this band
∼
server frame). We hence identify the coasting phase into within the first week. We therefore extrapolate the after-
a constant-density circumburst medium as the most likely glowSEDfromT +0.267daystoradiowavelengths(Fig.3)
0
scenario. Since the break time determines the transition and evolve the SED to all epochs of the radio and sub-mm
from the coasting to the deceleration phase, it can be used observation listed in Table 2, using the scaling relations
to measure the initial Lorentz factor Γ0 of the decelerat- for the injection frequency and the peak flux density for
ing blast-wave (Sari & Piran 1999; Panaitescu & Kumar a spherical expansion and a post-jet beak evolution from
2000; M´esz´aros 2006). Following Molinari et al. (2007), we Sari et al. (1998, 1999). In both dynamical scenarios, the
measure Γ0 60 using the observed break time and the peak flux density is (cid:46)800 µJy, corresponding to a specific
measurement∼of the energy Eiso = 4.5 1049 erg released luminosity of (cid:46) 2 1030 ergs−1Hz−1 before the jet break
during the prompt γ-ray emission. × occurred. ×
3.2.3. The SED from the radio to the X-rays 3.3. Supernova properties
To characterise the afterglow properties in more detail, we 3.3.1. Supernova spectrum
model the joint NIR-to-X-ray spectral energy distribution
(SED). We limit this analysis to < T +1 day, since SN Our spectra of SN2012bz are displayed inFig. 4.The very
0
2012bzstartedcontributinganon-negligibleamountofflux early spectra are dominated by a smooth power-law con-
totheintegratedlightatlatertimes.Wechoosetheepochs tinuum, characteristic of GRB afterglows. At around 4.7
T +0.054 days and T +0.267 days to match the dates of days, after the transient started re-brightening (Fig. 2),
0 0
the sub-mm/mm observations. The optical and NIR fluxes the shape of the spectrum changes and it becomes red-
wereobtainedthroughinterpolationbetweenadjacentdata der. By May 1 (8.8 days after the GRB), the spectrum has
points.12 Errors were estimated by interpolation. The flux clearly started to resemble that of a supernova with broad
scales of the XRT and XMM (MOS1, MOS2, PN) data lines (Sect. 5.1.1; Malesani et al. 2012a; S´anchez-Ram´ırez
et al. 2012; Wiersema et al. 2012). By May 10 (18.7 days
12 In the UV, there are cases where one of the adjacent data
points is an upper limit but the epoch of the SED is very close treatedtheinterpolateddatapointasdetectionbutnotasupper
to the time of the detection (∆t < 0.1 dex). In these cases we limit.
9
Schulze et al.: GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs
Rest-frame wavelength (˚A)
3000 4000 5000 6000 7000
0.04 days
0.72 days
3.61 days
4.71 days
t 4.96 days
n
a
t
s
n
o
c
8.76 days
+
λ
F
g
o
l 14.63 days
18.70 days
24.70 days
26.79 days
37.70 days
3000 4000 5000 6000 7000 8000 9000
Observed wavelength (˚A)
Fig.4. Spectral evolution of the optical transient accompanying GRB 120422A. The first two epochs show a smooth
power-law-shaped continuum, characteristic of GRB afterglows. After the transient started re-brightening, the shape
of the spectrum becomes redder. At 8.8 days after the GRB, the spectrum has clearly started to resemble that of a
broad-linedSN.At18.7days,thetransformationiscompleteandthespectralooksimilartootherGRB-SNe.Allspectra
were shifted vertically by an arbitrary constant. They were rebinned (18 ˚A) to increase S/N for presentation purposes.
We only display spectra with a large spectral range. Strong telluric lines (transparency < 20%) are highlighted by the
grey-shaded areas.
10