arXiv:1111.4527v1 [astro-ph.HE] 19 Nov 2011
The Fast and Faint SN2010bh Associated with GRB100316D
Filomena Bufano1, Elena Pian2,3,4, Jesper Sollerman5, Stefano Benetti6, Giuliano Pignata7,
Stefano Valenti6, Stefano Covino8, Paolo D’Avanzo8, Daniele Malesani9, Enrico
Cappellaro6, Massimo Della Valle10,11, Johan Fynbo9, Jens Hjorth9, Paolo A. Mazzali6,12,
Daniel E. Reichart13, Rhaana L. C. Starling14, Massimo Turatto2, Susanna D. Vergani8,
Klass Wiersema14, Lorenzo Amati15, David Bersier16, Sergio Campana8, Zach Cano16,
Alberto J. Castro-Tirado17, Guido Chincarini18, Valerio D’Elia19,20, Antonio de Ugarte
Postigo9, Jinsong Deng21, Patrizia Ferrero22, Alexei V. Filippenko23, Paolo Goldoni24,25,
Javier Gorosabel17, Jochen Greiner26, Francois Hammer27, Pall Jakobsson28, Lex Kaper29,
Koji S Kawabata30, Sylvio Klose31, Andrew J. Levan32, Keiichi Maeda33, Nicola Masetti34,
Bo Milvang-Jensen9, Felix I. Mirabel25,35, Palle Møller36, Ken’ichi Nomoto33, Eliana
Palazzi34, Silvia Piranomonte19, Ruben Salvaterra37, Giulia Stratta19, Gianpiero
Tagliaferri8, Masaomi Tanaka33, and Ralph A.M.J. Wijers29
– 2 –
1INAF Post-Doc Fellow; INAF - Osservatorio Astronomico di Catania, Catania, Italy, 95123
2INAF - Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34143, Trieste, Italy
3Scuola Normale Superiore di Pisa, Piazza dei Cavalieri 7, 56126 Pisa, Italy
4INFN - Sezione di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
5The Oskar Klein Centre, Department of Astronomy, AlbaNova, SE-106 91, Stockholm, Sweden
6INAF - Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122, Padova, Italy
7Departamento de Ciencias Fisicas, Universidad Andres Bello, Av. Republica 252, Santiago, Chile
8INAF - Osservatorio Astronomico di Brera, Via Emilio Bianchi 46, Merate, I-23807, Italy
9Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-
2100, Copenhagen, Denmark
10INAF - Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, I-8013, Napoli, Italy
11International Center for Relativistic Astrophysics Network, Pescara, Italy
12Max-Planck Institut fur Astrophysik, Karl-Schwarzschildstr. 1, D-85748 Garching, Germany
13University of North Carolina at Chapel Hill, Campus Box 3255, Chapel Hill, NC 27599-3255, USA
14Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH,
15INAF- Istituto di Astrofisica Spaziale e Fisica cosmica, Via Gobetti 101, I-40129 Bologna, Italy
16Astrophysics Research Institute, Liverpool John Moores University, 2 Rodney St, Liverpool, L3 5UX,
17Instituto de Astrofisica de Andalucia (IAA-CSIC), Glorieta de la Astronomia s/n, 18008 Granada, Spain
18Univerisit Milano Bicocca, Dip. Fisica G. Occhialini, P.zza della Scienza 3, Milano 20126, Italy
19INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Rome, Italy
20ASI-Science Data Center, Via Galileo Galilei, I-00044, Frascati, Italy
21National Astronomical Observatories, CAS, 20A Datun Road, Chaoyang District, Beijing 100012, China
22Instituto de Astrofsica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
23Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
24Laboratoire Astroparticule et Cosmologie, 10 rue A. Domon et L. Duquet, 75205 Paris Cedex 13, France
25Service d’Astrophysique, DSM/IRFU/SAp, CEA-Saclay, 91191 Gif-sur-Yvette, France
26Max-Planck Institut f¨ ur extraterrestrische Physik, Giessenbachstrasse 1, D-85740 Garching, Germany
27GEPI-Observatoire de Paris Meudon. 5 Place Jules Jannsen, F-92195, Meudon, France
28Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reyk-
– 3 –
We present the spectroscopic and photometric evolution of the nearby (red-
shift 0.059) spectroscopically confirmed Type Ic supernova, SN2010bh, associ-
ated with a soft, long-duration gamma-ray burst (X-ray flash) GRB100316D.
Intensive follow-up observations of SN2010bh were performed at the ESO Very
Large Telescope (VLT), using the X-shooter and FORS2 instruments. Owing
to the detailed temporal coverage and the extended wavelength range (300–2500
nm), we obtained an unprecedentedly rich spectral sequence among the hyper-
novae, making SN2010bh one of the best studied representatives of this SN class.
We find that SN2010bh has a more rapid rise to maximum brightness (8.0 ± 1.0
days) and a fainter absolute peak luminosity (Lbol ≈ 3 × 1042ergs) than pre-
viously observed SN events associated with GRBs. Our estimate of the ejected
56Ni mass is 0.12± 0.02 M⊙. From the broad spectral features we measure large
expansion velocities, higher than those of SNe 1998bw (GRB980425) and 2006aj
(GRB060218). The light-curve shape and photospheric expansion velocities of
SN2010bh suggest that we witnessed a relatively high-energy explosion with a
small ejected mass (Ek≈ 1052erg and Mej≈ 3 M⊙). The observed properties of
SN2010bh further extend the heterogeneity of the class of GRB supernovae.
Subject headings: supernovae: general — supernovae: individual SN2010bh,
jav´ ık, Iceland
29Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Ams-
terdam, The Netherlands
30Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima,
Hiroshima 739-8526, Japan
31Thuringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany
32Department of Physics, University of Warwick, Coventry CV4 7AL, UK
33Institute for the Physics and Mathematics of the Universe, University of Tokyo, 5-1-5 Kashiwanoha,
Kashiwa, Chiba 277-8583, Japan
34INAF - Istituto di Astrofisica Spaziale e Fisica cosmica, Via Gobetti 101, I-40129 Bologna, Italy
35IAFE-CONICET-UBA. cc67, suc 28, Buenos Aires, Argentina
36European Organization for Astronomical Research in the Southern Hemisphere (ESO), Karl-
Schwarzschild-Str. 2, 85748 Garching, Germany
37Dipartimento di Fisica e Matematica, Universit` a dell’Insubria, via Valleggio 7, 22100 Como, Italy
– 4 –
During the past decade, the link between long-duration gamma-ray bursts (GRBs)
and Type Ic core-collapse supernovae (SNe; e.g., Filippenko 1997) has been firmly estab-
lished; see Woosley & Bloom (2006) and Hjorth & Bloom (2011) for reviews. The first
clear case occurred in 1998, when the luminous SN1998bw was found spatially and tempo-
rally coincident with GRB980425 (Galama et al. 1998). The GRB-SN connection was sup-
ported in 2003 by two further associations between nearby GRBs and spectroscopically con-
firmed SNe: GRB030329/SN2003dh at redshift z = 0.17 (Hjorth et al. 2003; Stanek et al.
2003; Matheson et al. 2003) and GRB031203/ SN2003lw at z = 0.10 (Malesani et al. 2004;
Thomsen et al. 2004; Gal-Yam et al. 2004; Cobb et al. 2004). The most recent case of a
spectroscopic connection is GRB060218/SN2006aj (Campana et al. 2006; Pian et al. 2006;
Mirabal et al. 2006; Modjaz et al. 2006; Cobb et al. 2006; Ferrero et al. 2006). On average,
SNe associated with classical GRBs appear to be more luminous at peak than SNe Ic not
accompanied by GRBs, while SNe associated with X-ray flashes have maximum luminosi-
ties more similar to those of normal SNe Ic (see, e.g., Pian et al. 2006; Pignata et al. 2011;
Drout et al. 2010). However, SNe associated with both GRBs and X-ray flashes exhibit
broader features in their spectra, indicating unusually large expansion velocities. From the
modeling of their light curves and spectra, very high explosion energies are inferred (∼ 1052
erg, about 10 times higher than typical SNe), earning them the name of hypernovae (HNe;
Paczy´ nski 1998; Iwamoto et al. 1998).
The GRB-SN connection has been best studied at low redshift (z < 0.2), where the
clear, spectroscopically confirmed cases have been detected. In spite GRBs at these low
redshifts are rarely observed, we can confidently extend the association between GRBs and
SNe up to z ∼ 1, (corresponding to a look back time of about 60% the age of the Uni-
verse) thanks to a number of GRB-SNe which have been identified through single epoch
spectra characterized by the presence of prominent SN features (Della Valle et al. 2003,
2006, 2008; Soderberg et al. 2005; Bersier et al. 2006; Cobb et al. 2010; Cano et al. 2011a;
Sparre et al. 2011; Berger et al. 2011). The investigation of all confirmed GRB-SN associ-
ations is critical to understanding the nature of their progenitors and the mechanism by
which powerful stripped-envelope SNe produce ultra-relativistic jets (e.g., Zhang et al. 2003;
Uzdensky & MacFadyen 2006; Mazzali et al. 2008; Fryer et al. 2009; Lyutikov 2011).
GRB100316D (Stamatikos et al. 2010) is a low-redshift event (z = 0.059; Vergani et
al. 2010; Chornock et al. 2011; Starling et al. 2011; see also §3.1) whose prompt emis-
sion is characterized by a very soft spectral peak, similar to that of X-ray flashes (XRFs;
GRBs with energy peak at low frequency; Heise et al. 2001) and an extended and slowly
decaying flux. A few days after its detection, an associated Type Ic supernova was iden-
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tified through the spectral features of the early optical counterpart, SN2010bh (Chornock
et al. 2010a,b; Wiersema et al. 2010; Bufano et al. 2010). Similar to the low-redshift X-
ray flash GRB060218, GRB100316D had an unusually long duration (T90 > 1300s), and
a spectral hardness evolution with a stable and soft spectral shape throughout the prompt
and late-time emission (Starling et al. 2011). GRB100316D also resembles GRB060218 in
its prompt gamma-ray spectral properties, as evidenced in Figure 1, where the intrinsic peak
energy is plotted as a function of the isotropic emitted energy (see, e.g., Amati et al. 2009),
based on the spectral analysis reported by Starling et al. (2011). The early X-ray spectrum
of GRB100316D, like that of GRB060218 (Campana et al. 2006), is best described by a
power law and a thermal component (Starling et al. 2011; see also Fan et al. 2011). The
latter may be either the signature of shock breakout following core collapse (Campana et al.
2006; Waxman et al. 2007), or it could represent radiation from the central engine (e.g.,
Ghisellini et al. 2007; Li 2007; Chevalier & Fransson 2008).
We intensively monitored the optical and near-infrared (NIR) spectrophotometric evo-
lution of SN2010bh, starting 12hr after the GRB trigger of Swift/BAT (on 2010 March 16
at 12:44:50 [UT dates are used throughout this paper]; Stamatikos et al. 2010) until about
2 months past the discovery, when the SN could no longer be observed because of solar
constraints. To enable accurate host-galaxy subtraction, we reobserved the field 0.5–1 yr
after the explosion. This campaign was the outcome of the coordination of various observing
programs at the European Southern Observatory (ESO, Chile) Very Large Telescope (VLT)1
and at the Cerro Tololo Interamerican Observatory (CTIO, Chile). The spectral behavior of
SN2010bh from day 1.39 to day 21.2 has also been discussed by Chornock et al. (2010c), and
photometry during the first 3 months after after explosion has been presented by Cano et al.
(2011b) and Olivares et al. (2011). The larger wavelength range and improved phase cover-
age of our observations allow us to analyze the early phases in more detail and to push the
investigation into the late evolutionary stages.
In §2, we present the dataset and describe the data reduction methods, while §3 shows
the spectrophotometric evolution of SN2010bh and compares it to that of the previously
well-studied GRB-SNe. In §4, we discuss the derived properties of the progenitor star and
the explosion parameters, and we summarize our conclusions.
1VLT observations were taken within the GTO programs 084.D-0265 and 085.D-0701 (P.I. S. Benetti)
and 084.A-0260 and 085.A-0009 (P.I. J. Fynbo) at UT2/X-shooter (XS), and GO program 085.D-0243 (P.I.
E. Pian) at UT1/FORS2. SN2010bh photometry on March 23 and 28 was obtained with GO program
084.D-0939 with UT1/FORS2 (P.I. K. Wiersema). Observations were performed in ToO mode.
– 6 –
with a spectroscopically confirmed SN are shown with red dots. Similar to GRB060218,
GRB100316D is consistent with the correlation Ep,i−Eiso(solid line) derived by Amati et al.
(2002). The two parallel dotted lines delimit the 2.5σ confidence region (Amati et al. 2009).
Location of GRB100316D in the Ep,i− Eiso plane. GRBs/XRFs connected
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2. Data Acquisition and Reduction
UBV RI photometry of SN2010bh was obtained with the FOcal Reducer and low disper-
sion Spectrograph (FORS2; field of view [FOV] 6.8′×6.8′; scale 0.25′′pixel−1; Appenzeller et al.
1998) at the ESO VLT UT1 and with the 0.41m Panchromatic Robotic Optical Monitoring
and Polarimetry Telescopes (PROMPTs) 1 and 5 located at CTIO (FOV 10′× 10′, 0.6′′
pixel−1; Reichart et al. 2005). R-band magnitudes from the X-shooter Acquisition Camera
(FOV 1.47′×1.47′; 0.173′′pixel−1; D’Odorico et al. 2006) images were also used to cover the
early evolution. The data were reduced with standard techniques using IRAF2tasks. Since
the SN exploded in a region with a complex background, a template subtraction method
was applied, based on the ISIS package (Alard & Lupton 1998, Alard 2000), to remove con-
tamination from the host-galaxy light. As no pre-explosion observations were available of
SN2010bh, we used images obtained with VLT/FORS2 on September 17 (about 185 days
past explosion; see Table 1), assuming that the SN flux contribution was negligible at this
epoch (see §3). For PROMPTs observations, template images were acquired on 2011 Febru-
ary 2, with the exception of the I band at PROMPT5, for which the template image was
acquired on 2011 January 26 (see Table 1).
A point-spread-function (PSF) fitting method was applied to measure the SN magni-
tudes in the difference images. The uncertainties were estimated by means of artificial stars,
placing simulated stars close to the SN and with the same magnitude as the SN. By observ-
ing several photometric standard fields, Landolt (1992) for the U band and Stetson (2000)
for the BV RI filters, we obtained the color equations for each night and instrument, and
used them to transform instrumental magnitudes to the standard photometric system. In
particular, we calibrated the magnitudes of a local star sequence in the SN2010bh field over
four photometric nights (April 3, 5, 8 and 11) and used them to obtain the photometric
zero-points for the nonphotometric nights (Fig. 2 and Table 2). Because of the significant
color term, following Pignata et al. (2008) we applied to the PROMPTs instrumental SN
magnitudes a calibration path to transform them into a standard photometric system (S-
correction). Finally, a K-correction was applied to the observed BV RI magnitudes, based
on the nearly simultaneous spectra. For the U-band magnitudes, the noise contamination
was too high to measure a reliable K-correction from the spectra. UBV RI magnitudes of
the supernova are reported in Table 3 and plotted in Figure 3.
2IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Asso-
ciation of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.
– 8 –
Fig. 2.— SN2010bh and the sequence of local reference stars (Table 2). The R-band image
was taken with VLT/FORS2 on April 3, 2010. North is up, east is to the left.
– 9 –
Fig. 3.— UBV RI light curves of SN2010bh. The abscissa shows rest-frame epochs after
the explosion, which is assumed to be coincident with the burst trigger (2010 Mar. 16.53;
Stamatikos et al. 2010). K-corrections have been applied to the BV RI magnitudes. FORS2
and X-shooter R-band photometry is reported with black and red solid symbols, respectively.
Empty symbols are used for V RI magnitudes obtained with PROMPT. For clarity, the light
curves are vertically displaced by the amount reported in the legend for each filter.
– 10 –
We followed the spectroscopic evolution of SN2010bh with VLT/X-shooter (12 epochs)
and VLT/FORS2 (8 epochs), as reported in Table 4. When possible, the spectra were ac-
quired with the slit positioned along the North-South direction, to minimize the host-galaxy
contamination. For both instruments, the effects of atmospheric dispersion (Filippenko
1982) at high airmass have been reduced by using an atmospheric dispersion corrector. Si-
multaneous UV, VIS, and NIR spectra (∼ 300–2480 nm) were taken with X-shooter, using
slit widths of 1.0′′, 0.9′′, and 0.9′′, for each arm, respectively (D’Odorico et al. 2006). We used
a nodding throw along the slit (2′′, 4′′nodding lengths) to obtain better sky subtraction. The
data were reduced using version 0.9.4 of the ESO X-shooter pipeline (Goldoni et al. 2006),
with the calibration frames (biases, darks, arc lamps, and flatfields) taken during daytime.
After reducing the data with more advanced versions, no relevant changes were found. With
FORS2 we used the 300V grism (3300–9000˚ A) and a slitwidth of 1′′.
Both X-shooter and FORS2 spectra were extracted using standard IRAF tasks. Spec-
trophotometric and telluric standard-star exposures, taken on the same night as the SN2010bh
observations, were used to flux-calibrate the extracted spectra and to remove telluric absorp-
tion features. We checked the absolute flux calibration of the spectra by using the nearly
simultaneous R-band magnitudes. Figure 6 shows the spectral sequence after correcting for
reddening; both the wavelength scale and epochs are reported in the host-galaxy rest frame
(see §3.1 and §3.3). The most prominent emission lines of the host galaxy have been re-
moved. Figures 7 and 8 display the spectra in the optical and NIR ranges separately. The
three spectra taken in the nebular phase (2010 September 28 to October 1; see Table 4)
have been combined to improve the signal-to-noise ratio. The coadded spectrum is shown in
Figure 11 (see §3.4).
SN2010bh exploded at α = 07h10m30.53sand δ = −56◦15′19.78′′(J2000; Starling et al.
2011), in a bright anonymous galaxy. We measured the host-galaxy redshift, calculating
the average shift of the central wavelengths of its strongest emission lines ([OII] λ3727,
NeIII λ3869, [OIII] λλ4959, 5007, HI Balmer lines, HeI λ5876, [OI] λ6300, [NII] λ6584,
[SiII] λλ6716, 6731) in each of the 12 X-shooter spectra and correcting it for the radial
component of the Earth’s motion. The weighted mean of the resulting heliocentric redshifts
is z = 0.0592 ± 0.0001. The high precision of the redshift estimate is made possible by
– 11 –
the accuracy of the wavelength solution over the whole wavelength range of the X-shooter
spectra (2 km s−1for the UV and VIS arms). This value is in excellent agreement with
the values presented in previous works (z ≈ 0.059, Vergani et al. 2010; z = 0.0591 ± 0.0001,
Starling et al. 2011; z = 0.0593, Chornock et al. 2010c).
For a concordance cosmology (with Hubble constant H0= 73
and Ωm= 0.27), we obtained a luminosity distance of about 254 Mpc (i.e., distance modulus
µ= 37.02 mag).
kms−1Mpc−1, ΩΛ= 0.73,
We estimated the host-galaxy extinction by measuring the total equivalent width (EW)
of the interstellar NaID absorption doublet (λλ5890.0, 5895.9) with the assumption of a
gas-to-dust ratio similar to the average ratio in our Galaxy. Using the spectrum obtained
by combining the early-epoch X-shooter spectra, we found EW(λ5890.0)host= 0.59 ± 0.05
˚ A and EW(λ5895.9)host = 0.30 ± 0.02˚ A, giving a total EW(NaID)host = 0.89 ± 0.07˚ A
(Fig. 4). Applying the relation by Turatto et al. (2003), E(B − V ) = 0.16 × EW(NaID),
we obtain E(B − V )host= 0.14 ± 0.01 mag, which is the value we adopt throughout this
work. For the Milky Way extinction, we measured EW(λ5890.0)MW= 0.40 ± 0.10˚ A, while
the second doublet component was not detectable (Fig. 4). Assuming a flux ratio 2:1
between the two absorption lines, we obtained a total EW(NaID)MW = 0.60 ± 0.15˚ A,
implying E(B − V )MW= 0.10 ± 0.03 mag. This value is in agreement with that found by
Schlegel et al. (1998), E(B − V )MW= 0.12 mag. We decided to adopt the latter because of
the large uncertainty in our estimate of EW(NaID)MW.
Recently, Poznanski et al. (2011) claimed that NaID absorption can be a bad proxy
for the extinction; thus, we alternatively estimated the total reddening from the Balmer-
line intensity ratios of the HII region underlying the SN. Assuming Case B recombination
(T = 104K; Osterbrock 1989), we measured the Hα/Hβ ratios from both X-shooter and
FORS2 spectra, obtaining an average value of E(B −V )tot= 0.30±0.06 mag, in agreement
with that used in this work (E(B−V )tot= 0.26 mag). From the Balmer decrement measured
in the spectrum (taken at 0.5 days after the GRB trigger) of a bright HII region spatially
separated from the SN region, Starling et al. (2011) found E(B − V )tot= 0.178 mag. An
independent estimate of the reddening has been given by Cano et al. (2011b), who found a
total color excess E(B − V ) = 0.18 mag, comparing the SN2010bh colors with those of the
Type Ibc SN sample studied by Drout et al. (2010).
We also used the 12 X-shooter spectra to estimate the metallicity of the bright region
underlying SN2010bh. We measured both the N2 and O3N2 diagnostic ratios (Pettini et al.
2004), obtaining an average oxygen abundance 12 + log(O/H) = 8.20 ± 0.24, where the
error is dominated by the uncertainties associated with the adopted linear relationships. The
values at the SN location reported by Chornock et al. (2010c), ∼8.2, based on a spectrum at
– 12 –
+3.3 days after the explosion taken with the Low Dispersion Survey Spectrograph (LDSS3)
at the Magellan Telescope, and 8.2 ± 0.1 by Levesque et al. (2011), based on an LDSS3
spectrum at +52 days, are in excellent agreement with ours. From the spectrum of the HII
region located close to the SN, Starling et al. (2011) found an oxygen abundance of 8.23 ±
3.2.SN2010bh Light Curves
Figure 3 illustrates the SN2010bh light curves. Our R-band light curve traces well the
early evolutionary stages; the SN reaches maximum light (MR≈ −18.5 mag) at 8.0 ± 1.0
days past the explosion. This is the steepest rise to maximum brightness ever found among
both SNe associated with GRBs and broad-lined (BL) SNe for which explosion dates have
been well constrained (e.g., SN2002ap: Rmaxat 12 days; Mazzali et al. 2007b; Foley et al.
2003; SN2003jd: Rmaxat ∼ 16 days; Valenti et al. 2008b; SN2005nc: Rmaxat ∼ 12 days;
Della Valle et al. 2006).
A decline of 0.056 ± 0.015 mag day−1is measured between 0 and 15 days after R-band
maximum. Good sampling of the post-maximum phases was also obtained in the UBV I
bands (Fig. 3). In Figure 5, we compare the R-band light curve of SN2010bh with those
of two previous well-sampled GRB-SNe: SN1998bw (Galama et al. 1998; Patat et al. 2001)
and SN2006aj (Sollerman et al. 2006; Pian et al. 2006; Ferrero et al. 2006). The light curve
of a more typical Type Ic SN (SN1994I; Richmond et al. 1996) is also shown.
After maximum brightness, SN2010bh and SN1998bw have a similar behavior, but the
decay rate of SN2010bh between the last two R-band points corresponds to ∼ 0.03 mag
day−1in the rest frame, to be compared with that of SN1998bw (0.013 mag day−1). Using
the decline rates found by Patat et al. (2001) for SN1998bw, we estimate the magnitudes of
SN2010bh at the epoch when the VLT subtraction images were acquired (+166 rest-frame
days from maximum; see Table 1): V ≈ 25.0, R ≈ 23.8, and I ≈ 23.6 mag. If this was
indeed the flux level of SN2010bh, it would cause an oversubtraction and an underestimate
of the SN fluxes in the V , R, and I bands by less than 0.03 mag around maximum and 0.15
mag in the latest epochs. For each epoch, the corresponding inferred possible contamination
by a residual SN flux has been included in the error estimate (Table 3).
SN2010bh is less luminous than the other two GRB-SNe (Fig.5), which suggests a
smaller amount of ejected56Ni mass. Moreover, since the width of a light curve scales with
the ratio between the total explosion kinetic energy Ekand the total ejected mass Mej(Arnett
1982, 1996), the fast evolution of SN2010bh likely reveals a relatively high-energy explosion
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RestFrame Wavelength (Å)
Flux Density (10−17 erg cm−2 s−1 Å−1)
Na I D Host
Observed Wavelength (Å)
Na I D MilkyWay
Fig. 4.— NaID doublet absorption lines in the early-time X-shooter SN2010bh spectrum.
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and a small Mej. On the other hand, a highly asymmetric explosion may also result in a
faster expansion in the polar direction, leading to a short diffusion time and a fast rise time
(Maeda et al. 2006).
We note that our R-band photometry at 0.5 days differs from that of Cano et al. (2011b)
by about 1 mag and a ∼0.6 mag from that of Olivares et al. (2011). This implies that we
observe in our data the smooth rise in flux typical of a SN, and no evidence of the extra
early component that Cano et al. (2011b) interpret as shock breakout. While we processed
all raw data in a homogeneous way and consider this first flux point and its uncertainty
formally correct, we caution that at these low flux levels the X-Shooter acquisition camera
imaging data may depend very critically on the assumed background.
3.3. Spectrum in the Photospheric Phase
The spectral sequence of SN2010bh (Figures 5, 6, and 7) is unique among BL-SNe
and HNe for its detailed temporal coverage and extended wavelength range (300–2500 nm).
At early epochs, the spectral energy distribution can be fit with a blackbody spectrum with
Tbb≈ 8500K. Thereafter, the continuum, shaped by broad bumps and absorptions, becomes
redder with time because of expansion and cooling. The two main minima at ∼ 5500˚ A and
∼ 7500˚ A (Fig.7) can be identified with the blueshifted SiII (λ6355) and CaII NIR triplet
(gf-weighted line centroid λ8579) absorption lines, respectively. The expansion velocities
measured from these lines and their evolution with time are shown in Figure 93. The velocity
of SiII λ6355 ranges from ∼36,000 kms−1at about 7 days from the explosion to ∼25,500
kms−1at the last epoch. CaII velocities are systematically about 20% higher than those of
Considering its importance in constraining the nature and the evolutionary state of the
progenitor star, we have searched for the spectroscopic signature of helium in our spectra.
We observe weak absorption features in the optical and NIR that may be compatible with
HeI λ5876 and HeI 1.083 µm blueshifted by ∼20,000–30,000 kms−1and ∼28,000–38,000
kms−1, respectively. The latter is in agreement with the velocities measured for SiII and
CaII. However, HeI features may be blended with other species, like NaI in the optical
and CI or SiI in the NIR (Mazzali & Lucy 1998; Millard et al. 1999; Sauer et al. 2006;
Taubenberger et al. 2006). In particular, the contribution of CI cannot be ruled out. Indeed,
3We caution the reader that the velocities of SN2006aj have been derived with the aid of a model
(Mazzali et al. 2006a), while those of the other SNe have been measured directly from the spectra. However
for SN1998bw and SN2003dh, the results of the 2 methods do not differ by more than 20%.
– 15 –
0 20 406080
Fig. 5.— Light curve of SN2010bh compared to those of SN1998bw (Galama et al. 1998;
Patat et al. 2001) and SN2006aj (Sollerman et al. 2006; Pian et al. 2006; Ferrero et al.
2006), both of which were associated with a GRB/XRF, and to that of the Type Ic SN1994I
(Richmond et al. 1996), not accompanied by a high-energy event. All SN light curves have
been corrected for reddening and are reported in the host-galaxy rest frame. The epochs are
given with respect to the burst detection. For SN1994I, the explosion date was obtained
from the light-curve models of Iwamoto et al. (1994).