The Astrophysical Journal, 750:128 (8pp), 2012 May 10
C 2012.
doi:10.1088/0004-637X/750/2/128
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
EARLY ULTRAVIOLET OBSERVATIONS OF A TYPE IIn SUPERNOVA (2007pk)
1
T. A. Pritchard1,2 , P. W. A. Roming1,2 , P. J. Brown3 , N. P. M. Kuin4 , Amanda J. Bayless2 ,
S. T. Holland5,6 , S. Immler7,8,9 , P. Milne10 , and S. R. Oates4
Department of Astronomy & Astrophysics, Penn State University, 525 Davey Lab, University Park, PA 16802, USA; [email protected]
2 Southwest Research Institute, Department of Space Science, 6220 Culebra Rd, San Antonio, TX 78238, USA
3 Department of Physics & Astronomy, University of Utah, 115 South 1400 East 201, Salt Lake City, UT, USA
4 Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
5 Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD 20771, USA
6 Code 660.1, NASA/GSFC, Greenbelt, MD 20771, USA
7 Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
8 Center for Research and Exploration in Space Science and Technology, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
9 Department of Astronomy, University of Maryland, College Park, MD 20742, USA
10 Steward Observatory, 933 North Cherry Avenue, RM N204, Tucson, AZ 85721, USA
Received 2011 June 16; accepted 2012 March 1; published 2012 April 24
ABSTRACT
We present some of the earliest UV observations of a Type IIn supernova (SN)—SN 2007pk, where UV and
optical observations using Swift’s Ultra-Violet/Optical Telescope began 3 days after discovery or ∼5 days after
shock breakout. The SN observations commence at approximately maximum light in the UV and u-band filters,
suggesting that the UV light curve peaks begin very rapidly after the initial explosion, and subsequently exhibit
a linear decay of 0.20, 0.21, 0.16 mag day−1 in the UVOT uvw2, uvm2, uvw1 (λc = 1928, 2246, 2600 Å) filters.
Meanwhile the b- and v-band light curves begin approximately seven days before v-band peak and exhibit a shallow
rise followed by a subsequent decay. A series of optical/near-IR spectra taken with the Hobby–Eberly Telescope at
days 3–26 after discovery show spectra similar to that of the peculiar Type IIn 1998S. The emission from 2007pk
falls below detection ∼20 days after discovery in the UV and 50 days in the optical, showing no sign of the long
duration emission seen in other Type IIn SNe. We examine the physical and spectral characteristics of 2007pk and
compare its UV light curve and decay rate with other Type II SNe.
Key words: supernovae: general – supernovae: individual (SN 2007pk) – ultraviolet: general
Online-only material: color figures
the IIn SNe’s hydrogen-rich spectra and exceptional brightness
(Gal-Yam et al. 2007).
The Swift UV/Optical observations of 2007pk which we
present here are some of the earliest phase UV observations
of a IIn SN. Follow-up optical spectroscopic observations are
obtained to allow us to examine the physical characteristics of
the system in concert with the photometric data. We go on to
offer comparison with other UV-bright Type II SNe observed in
the UV in order to both give context to how this observed IIn
light curve fits in with the taxonomy of UV–SNe.
1. INTRODUCTION
Supernovae (SNe) are observationally classified by their
spectra and may be broadly placed into two categories: Type I
SNe which have no hydrogen lines in their spectra, and Type II
SNe which do (Filippenko 1997).
Type II subclasses include the Type IIP (“plateau”),
IIL (“linear”), and IIn (“narrow”) SNe. Type IIP SNe are distinguished by the long plateau phase in their optical light curves
which have been observed out to hundreds of days. These are
arguably the most common subtype of all SNe, accounting for
∼40% of all SNe observed in a recent volume limited sample
(Li et al. 2011). Type IILs are distinguished by the linear decline
in their optical light curves and are some of the most rare SNe.
They have been seen both with and without broad P-Cygni profiles in their spectra. Current theory suggests that IIPs originate
from the core collapse of red supergiants, while the progenitors
for IILs are more uncertain; perhaps blue supergiants or binary
systems (Smartt 2009).
The Type IIn SNe subclass (Schlegel 1990) are those corecollapse SNe whose expanding ejecta exhibits interaction with
a strong circumstellar wind given off by the SNe progenitor.
These objects are characterized by strong emission lines, most
prominently Hα, and a lack of broad absorption lines. Typically
only narrow P-Cygni profiles are visible in the hydrogen lines
that are superimposed on top of a broader emission component,
and the continuum is well characterized by a blackbody. Type IIn
SN progenitors are also uncertain, but observations of the
brightest objects favor luminous blue variables (LBVs) due to
2. OBSERVATIONS
SN 2007pk was discovered on 2007 November 10.31 UT
(JD 2454414.81; Parisky & Li 2007). Previous observations of
this field on November 5.33 (JD 2454409.83) by the discovery
team yield only upper limits. We estimate the shock breakout
to have occurred between these dates and for the purposes
of this paper define it as the midpoint of these observations
(JD 2454412.2 ± 2.5).
Observations with the Swift (Gehrels et al. 2004) UltraViolet/Optical Telescope (UVOT; Roming et al. 2000, 2004,
2005) began on November 13.16 (2.85 days after discovery)
using three optical (u, b, v) and three UV filters (uvw2, uvm2,
uvw1: λc = 1928, 2246, 2600 Å, respectively; Poole et al.
2008). SN 2007pk fell below the detection limit in the UV after
day 23 of the campaign, and the u band was visible until day 47.
Optical detections in the b and v bands were observed until day
56 where we had our first u-band upper limit which then caused
us to terminate the campaign since we had lost detection of any
1
The Astrophysical Journal, 750:128 (8pp), 2012 May 10
Pritchard et al.
13
uvw2
uvm2
uvw1
u
b
v
14
mVega
15
16
Observed
17
18
19
Δ mvega
1.5
1.0
0.5
uvw2−uvm2
uvm2−uvw1
uvw1−u
u−b
b−v
0.0
−0.5
−1.0
−1.5
0
10
20
30
Days Since Discovery
40
50
60
Figure 1. Left top: observed UVOT light curves of SN 2007pk (galaxy subtracted, though no extinction or k-corrections have been applied). Left bottom: UV and
optical colors—the dashed line corresponds to the best-fit line.
(A color version of this figure is available in the online journal.)
Table 1
Observed UVOT Photometry
Time
(JD 2450000+)
4417.66
4418.64
4420.21
4420.68
4421.68
4425.07
4429.65
4433.90
4436.95
4440.80
4451.68
4462.02
4474.10
Vega Mag
uvw2
uvm2
uvw1
14.54 ± 0.05
14.91 ± 0.06
15.14 ± 0.05
15.44 ± 0.06
16.03 ± 0.06
16.91 ± 0.08
17.85 ± 0.14
18.42 ± 0.21
19.00 ± 0.34
14.47 ± 0.05
14.70 ± 0.05
14.83 ± 0.05
15.11 ± 0.05
15.72 ± 0.05
16.71 ± 0.08
17.57 ± 0.13
18.51 ± 0.27
14.50 ± 0.06
14.65 ± 0.05
14.78 ± 0.05
14.87 ± 0.05
15.07 ± 0.05
15.48 ± 0.06
16.32 ± 0.07
17.01 ± 0.09
17.64 ± 0.14
18.07 ± 0.19
u
b
v
14.90 ± 0.05
14.94 ± 0.05
15.06 ± 0.05
15.07 ± 0.06
16.27 ± 0.07
16.22 ± 0.06
16.21 ± 0.06
16.25 ± 0.06
16.27 ± 0.05
16.20 ± 0.05
16.21 ± 0.05
16.14 ± 0.05
15.56 ± 0.05
16.00 ± 0.06
16.32 ± 0.07
16.77 ± 0.08
17.65 ± 0.15
18.66 ± 0.30
16.39 ± 0.06
16.53 ± 0.06
16.69 ± 0.06
16.81 ± 0.07
17.39 ± 0.09
17.89 ± 0.12
18.32 ± 0.17
16.17 ± 0.08
16.39 ± 0.06
16.39 ± 0.27
16.44 ± 0.08
16.81 ± 0.10
16.95 ± 0.11
17.18 ± 0.11
mpeak (mag)
tpeak (JD 2450000 + )
trise (days)
<14.54
<4418 ± 2.5
<5.8 ± 2.5
<14.47
<4418 ± 2.5
<5.8 ± 2.5
<14.50
<4418 ± 2.5
<5.8 ± 2.5
<14.82
<4418 ± 2.5
<5.8 ± 2.5
16.15 ± 0.07
4420.5 ± 1
8.3 ± 3.7
15.95 ± 0.164
4424.5 ± 1
12.3 ± 3.6
k-correction
Extinction
Host (LMC) + MW
0.04
1.54
0.03
1.54
0.02
1.26
−0.03
0.93
−0.03
0.75
0.01
0.58
(2009). The data reduction pipeline used the HEASOFT 6.6.3
and Swift Release 3.3 analysis tools with UVOT zero points from
Poole et al. (2008) and updated calibrations from Breeveld et al.
(2010). The corresponding light curve is shown in Figure 1.
To determine the peak magnitude (mPeak ) and time of peak
(tPeak ) in each filter, 3 × 105 Monte Carlo simulations fitting a
cubic spline to the data points in each filter were performed.
UV emission. A median cadence of four days was used with
time between observations increasing after the non-detection of
UV emission (Table 1). A later observation on 2009 November
24 was made after the SN had faded and was no longer detected
for use as a galaxy subtraction template. Photometry using a
3 source aperture, including template galaxy flux subtraction,
was performed following the method outlined in Brown et al.
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The Astrophysical Journal, 750:128 (8pp), 2012 May 10
Pritchard et al.
The mean of the distributions was taken as the peak times
and magnitudes, with the standard deviations as the errors
(Table 1). Rise times to peak (tRise ) were calculated using the
time of shock breakout as the temporal zero point. Errors in
the explosion date and peak date are the standard deviation of
the Monte Carlo peaks and the error in explosion date added in
quadrature (Table 1). None of the magnitudes include extinction
or k-corrections since these values calculated depend upon the
template spectrum and are therefore only valid right around
the optical light curve peak. Since the peak was not detected
in the UV+u we calculate only upper limits (trise < 5.8 ± 2.5)
using the explosion date. A more conservative upper limit may
be placed using the date of the pre-discovery image upper limit
resulting in a trise < 8.3.
To determine the host reddening at maximum light in our
optical bands, we use an archival spectrum of SN 1998S
(Fransson et al. 2005; Lentz et al. 2001) at a similar phase
(∼20 days after explosion), de-reddened and redshifted to the
observer rest frame, for use as a template. Line-of-sight Milky
Way (MW) extinction was then computed using the E(B −V )
value from Schlegel et al. (1998) and applied to our spectral
template using the Cardelli et al. (1989) MW law. Host extinction was calculated by first “mangling” (Hsiao et al. 2007) our
observed MW-corrected photometry to match spectrophotometry from our template spectra, and then computing the correction factors between our mangled photometry and our observed
spectrophotometric magnitudes. The values found have a good
match to a host extinction of E(B −V )host ∼ 0.13 with a Large
Magellanic Cloud (LMC) extinction law applied to our template. We also fit for both MW and Small Magellanic Cloud
(SMC) extinction, but these models resulted in a poorer fit. Extinction and k-corrections (Table 1) were computed using spectrophotometric methods; extinction corrections were computed
via the subtraction of synthetic magnitudes of an unreddened
and reddened template spectrum in the observed frame, while
k-corrections were computed via the subtraction of synthetic
magnitudes from the unreddened template spectrum in the rest
and observed frames.
The small offset of the SN from the X-ray bright and variable
nucleus of the host galaxy (7 E, 2 S), well within the pointspread function of the X-Ray Telescope (90% encircled energy
radius 24 FWHM; Burrows et al. 2005), prevents us from using
the X-ray data that were obtained simultaneous to the UVOT
data.
Optical through near-IR spectra were obtained on the
Hobby–Eberly Telescope (HET; Ramsey et al. 1998) beginning on 2007 November 13 UT (three days after discovery),
and seven epochs were obtained between the start of observations and 2007 December 6 (Figure 2, left). The Low Resolution Spectrograph (LRS; Hill et al. 1998) was used with a 2
slit (R ≈ 300; Δλ ≈ 4500–10000 Å). Standard IRAF11 reduction techniques of bias subtraction, flat fielding, and wavelength
calibration were used. Relative flux calibration was performed
using several flux standards (BD262606, HD84937, and HZ44;
Fukugita et al. 1996; Massey & Gronwall 1990) observed during 2007 November. We use the HET standard Schott Glass
blocking filter GG385 to filter out second-order contamination.
Observing conditions were poor on 2007 November 17 and led
to poor flux calibration blueward of 6000 Å; these data have
been cut in Figure 2.
10−14
11−13−07 (+3d)
Flux Density Fλ [erg cm−2 s−1 Å−1]
11−17−07 (+7d) −1 dex
10−16
11−20−07 (+10d) −2 dex
11−22−07 (+12d) −3 dex
10−18
11−24−07 (+14d) −4 dex
10−20
11−27−07 (+17d) −5 dex
12−06−07 (+24d) −6 dex
1998S Template
(+20d)
10−22
2000
4000
6000
Wavelength (Å)
8000
10000
Figure 2. Observed HET spectra of SN 2007pk, no removal of telluric lines or
reddening has been applied. Each epoch has been sequentially shifted down by
∼1 dex, and the time in parenthesis is the time since discovery. Below we plot
the Hubble Space Telescope archival spectrum of 1998S we use as a template
as well as the UVOT filter curves (uvw2, uvm2, uvw1, u, b, v in the same color
scheme as in Figure 1).
(A color version of this figure is available in the online journal.)
3. RESULTS
The Swift observations of SN 2007pk are some of the earliest
observations of a Type IIn SN in the UV (Figure 1, top left). No
rise in the light curve is detected in the UV or u filters; however,
the shallow decay of the light curve in the first several epochs of
the uvw1 and u bands suggests that our observations began at
or very shortly after maximum in these filters. Meanwhile, the
b- and v-band light curves rise to a maximum at 5.75 ± 1.0 and
9.7 ± 0.5 days after discovery. Over the course of observations
the UV-light curve decays linearly until detections are lost
at ∼20 days after they began. The optical, post-maximum
decays more slowly than the UV and with more evidence for
nonlinearity (as is typical of Type II SNe; Brown et al. 2009)
until the observations are terminated at 60 days. The light curve
of 2007pk is UV-bright, with an observed peak at 14.5 mag in
uvw1 while the optical is fainter, peaking at ∼15.95 ± 0.164 in
the v band a week later. We see a systematic linear reddening
in all UV and optical colors except for uvw2–uvm2 which
initially rises and plateaus after day four of our observations.
This behavior is consistent with a monotonically decreasing
temperature as the ejecta expands and cools (see Figure 1,
bottom left).
The optical spectra of SN 2007pk (Figure 2) are similar to
that of the IIn-pec SN 1998S at early times. SN 2007pk exhibits
the same relatively featureless continuum as SN 1998S with
the exception of a much stronger Hα component around the
same time as the optical light curve maximum. SN 1998S had
a similarly strong Hα narrow profile at both early (∼7 days)
and late (∼27 days—where it developed a broad Hα profile
as well) times, but it featured a weaker Hα profile with a broad
P-Cygni component in between the two (Fassia et al. 2001). The
spectrum of SN 2007pk appears to be consistent with a thermal
blackbody with Hα emission lines from CSM interaction at all
epochs. The spectra in Figure 2 are in the observer frame with
no dust corrections applied or telluric lines removed. While
evolution in both the underlying continuum and spectral lines
(predominantly Hα) occur in the first 26 days, see Figure 3,
continuum emission is seen to dominate over the line emission
11
IRAF is distributed by the National Optical Astronomy Observatory, which
is operated by the Association of Universities for Research in Astronomy
(AURA) under cooperative agreement with the National Science Foundation.
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The Astrophysical Journal, 750:128 (8pp), 2012 May 10
Pritchard et al.
Table 2
Physical Parameters
Epoch
TBB
(k)
RBB
(au)
LBB
(109 L )
Lpseudo
(109 L )
Lbol
(109 L )
+4.64
+6.20
+6.68
+7.68
+15.65
+19.90
+22.95
19800 ± 1150
16500 ± 650
15400 ± 600
13800 ± 450
8800 ± 200
7300 ± 400
6500 ± 450
32.9
40.6
44.0
50.5
93.3
125.1
154.1
6.8
5.0
4.5
3.8
2.1
1.8
1.7
4.3
3.6
3.4
3.0
1.4
1.0
0.8
6.8
5.0
4.5
3.8
2.2
1.8
1.7
6•10−15
Hβ
Hα
He I
5•10−15
11−13−07 (+3d)
T=19800 K
4•10−15
Flux Density Above Continuum
Flux Density Fλ [erg cm−2 s−1]
1•10−15
3•10−15
2•10−15
12−06−07 (+24d)
T=7300 K
1•10−15
2000
4000
6000
Wavelength (Å)
8000
10000
Figure 3. Here we re-plot the first and final spectrum in linear space to emphasize
the change in continuum which is not as apparent in Figure 2 due to the
logarithmic scaling. The dotted red lines are reddened blackbody SEDs from
the temporally closest fit epochs. Prominent observed and telluric lines have
been indicated as well. Fits at late times appear to diverge from our observed
spectra due to increasing line features and the reddening of the spectrum away
from our UV-focused Swift observations.
(A color version of this figure is available in the online journal.)
11−13−07
11−17−07
11−20−07
11−22−07
11−24−07
11−27−07
12−06−07
8•10−16
6•10−16
4•10−16
2•10−16
0
−5000
in all epochs. This is typical behavior of an early IIn SN, where
the ejecta is still optically thick, and is in contrast to observations
of late time IIn emission where the much of the observed flux
can be due to emission lines caused by circumstellar material
(CSM) interaction (Smith et al. 2009).
The optical spectra exhibit a bright Hα line composed of two
components (Figure 4). A commonly used approximation for
Type IIn SNe emission is that of an expanding and cooling
optically thick ejecta shell emitting as a thermal blackbody
which is colliding with the CSM of the progenitor star (e.g.,
Fassia et al. 2001; Smith et al. 2008; Chatzopoulos et al. 2011).
In this situation the optically thick shell gives rise to the thermal
continuum, the ionized CSM causes the Hα narrow component,
and the interaction of ejecta with the CSM causes the broad
component (Smith et al. 2009). In our observations of SN
2007pk, the narrow Hα component is unresolved due to the
resolution of the LRS (R ≈ 300). This allows us to put an upper
limit on the velocity of the CSM of VCSM 1000 km s−1 .
We may also fit the larger component to determine an ejecta
expansion velocity. We subtract off the continuum around the
Hα line and fit a Gaussian profile to each component of the Hα
emission and the two lines (seen at 1000 and 7000 km s−1 in
Figure 4) at each epoch. We note the presence of a flat top on
the broad component which is evidence that the emission arises
in a shell around the star. We find that the broad component
fit velocity remains constant within errors across epochs and
0
Velocity [km/s]
5000
Figure 4. Hα lines with continuum subtracted off. We see a broad asymmetric
component that develops with time in addition to the increasing brightness of
the primary peak.
(A color version of this figure is available in the online journal.)
we therefore combine our measurements from the individual
spectral observations to calculate that the average ejecta velocity,
Vejecta , is 8000 ± 400 km s−1 where the error is our fit velocity
error and standard velocity deviation between epochs added in
quadrature. As is typical in IIn SNe, we also see Hβ and He i
emission without broad P-Cygni profiles (Figures 2 and 3).
If we assume that the UV and optical emission is dominated
by a thermal blackbody component as suggested, then we may
model the temperature of our idealized SN photosphere. Using
a χ 2 -minimization technique, we fit our observed photometry at
those epochs with six filter observations to synthetic magnitudes
generated from a blackbody spectrum. This gives us the best-fit
blackbody temperature of the SN at various epochs (Figure 5,
left top and Table 2). We note the presence of a bias in the residuals of our photometry (Figure 5, left bottom)—this is expected
to occur due to deviations in the spectra from a blackbody; in
particular iron line blanketing in the UV and emission lines in
the optical. With a blackbody temperature, observed fluxes and
a known distance (μ = 70.1 Mpc, corrected for Virgo infall and
calculated from a host redshift of z = 0.016655; Falco et al.
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The Astrophysical Journal, 750:128 (8pp), 2012 May 10
Pritchard et al.
15
10
16
17
uvw2
uvm2
uvw1
u
b
v
18
Fit Residual Magnitude
19
0.2
0.1
Blackbody Temperature
Blackbody Luminosity
Blackbody Radius
Swift Pseudo-Bolometric
Bolometric
0.0
1
-0.1
Temperature [103K] / Luminosity [109LO•] / Radius [10 AU]
Model Fit vs. Observed Light curve
14
-0.2
0
5
15
10
Days Since Discovery
20
25
5
10
15
20
25
Figure 5. Left top: calculated synthetic magnitudes in the UVOT filters from our best-fit blackbody model at each epoch (filled circles) over plotted on top of our
observed UVOT photometry error bars for comparison. Left bottom: residuals from the blackbody synthetic magnitudes for our best-fit temperature and observed
magnitudes. Right: blackbody temperature, luminosity, radius as well as Swift pseudo-bolometric and bolometric light curves from the fits to our photometry at
different epochs. The lines are best-fit cubic polynomials to guide the eye and do not represent a physical model.
(A color version of this figure is available in the online journal.)
tra. The first spectra appear to be well fit by our TBB = 19,800 K
blackbody, while looking at the TBB = 7300 K fit it appears that
at late times we may be underestimating the temperature slightly
as our model curve begins to diverge from our continuum at the
blue end of our spectra.
We also calculate a bolometric and pseudo-bolometric light
curve using the Swift observations at these epochs. For the
pseudo-bolometric we take observed counts, convert them to
fluxes using count to flux rate conversions found in Brown et al.
(2010, Appendix A), and integrate them over the filter passband.
We calculate a UV and IR correction for the unobserved portions
of the spectral energy distribution (SED) by integrating the
model flux of our blackbody model at that epoch from 0–1600 Å
and 6000−∞ for the UV and IR, respectively. We then add these
to our pseudo-bolometric light curve to arrive at a bolometric
light curve. UV flux corrections range between 25% of the total
flux at the first epoch to less than 1% by epoch 5. IR flux
corrections range between 7% of the total flux at the first epoch
to 48% at the final fit epoch.
1998) we may integrate under the blackbody curve to compute a
bolometric blackbody luminosity, and from this a blackbody radius corresponding to an idealized uniform spherical shell. Since
we have calculated the change in blackbody radius with time,
we can also make a quick calculation of the average velocity by
using the measurements at each epoch. The photospheric radius
over the first seven epochs increases linearly with an average
velocity of 1100 ± 1500 km s−1 . The velocity of the photosphere as calculated here is consistent with the line velocity of
∼8000 km s−1 which was calculated earlier. The data appear to
fit the blackbody data well up to about 10 days after discovery,
after which time the fitting errors increase significantly and we
appear to be underestimating the temperature. We attribute this
to a combination of increasing photometric errors as SN 2007pk
fades, and as much of the emitted flux moves redward of our
Swift UV-focused observations. Another contributing factor is
the presence of increasing line features which become visible
in the later spectral epochs, as well as likely line blanketing in
the UV filters which would suppress the UV flux. The residuals
are still similar to the photometric error bars and seem to provide
a sufficient fit as may be seen in Figure 5. To examine this we
take our first and last blackbody fit model and overlay them on
our first and last observed spectra in Figure 3. There is a time
difference of one to two days between the Swift observations
used to construct the blackbody models and the observed spec-
4. DISCUSSION
4.1. Lack of Late Time Emission
Type IIn SNe have been seen months or even years after
the SN explosion in the UV, optical, and infrared as the
5
The Astrophysical Journal, 750:128 (8pp), 2012 May 10
Pritchard et al.
SN-shock/CSM interaction drives emission at constant or very
slowly decaying levels. Swift in particular has observed this
behavior in the UV/Optical bands in other Type IIn SNe; notably
2005ip (UV-bright) and the ultra-luminous IIn 2006gy (UV
faint). We returned to the 2007pk field on 2008 July 9 for a
follow-up observation of the SNe and host galaxy. This field
was also fortuitously observed by Swift at two later epochs,
2009 November 24 and 2010 March 17. The photometry of the
host galaxy at the SN position at all late time epochs is consistent
within 1σ errors and small displacements of the aperture from
the SN position result in non-significant changes in photometry,
suggesting that the SN is below our detection limit.
While we are unable to ascertain when optical emission
from the SNe fell below our detection limit due to the gap
in observations, we may say that emission from 2007pk was
unobserved as of 2008 July 9. This allows us to determine a
lower limit of 2007pk’s decay rate of 0.01 mag day−1 . If we
assume that any residual luminosity at this date is caused purely
by SN–CSM interaction, we can calculate the progenitor mass
loss rate necessary to sustain this (Chugai & Danziger 1994):
2L vCSM
Ṁ =
,
3
vejecta
1998S
1994W
2005cl
2005cp
2005db
2007pk
2010jl
−19
Mv
−18
−17
−16
(1)
where is the kinetic energy conversion efficiency. Unfortunately as discussed previously, we only have an upper limit
on the CSM velocity for 2007pk of ∼1000 km s−1 . Using
this, a luminosity of 5 × 1041 erg s−1 calculated from the upper limits of our template images, assumed the maximum efficiency of 0.5, and our measured ejecta velocity of 8000 km s−1
we may calculate the upper limit on the mass loss rate of
Ṁ 1.6 × 10−3 M yr−1 at the current radius of the shell’s
expansion.
−15
−20
0
20
40
Time Since Peak (days)
60
Figure 6. Comparison of 2007pk with published light curves of SNe 1994W
(Sollerman et al. 1998), 1998S (Liu et al. 2000), 2005cl/2005cp/2005db (Kiewe
et al. 2012), and 2010jl (Stoll et al. 2011). All objects have been shifted so that
the peak observed maximum is at zero.
(A color version of this figure is available in the online journal.)
4.2. Comparison with Other Published
Core-collapse Supernovae
also exclude Type Ia, Ibc, and IIb SNe as these systems are
fundamentally different; the progenitors possess a negligible or
nonexistent hydrogen envelope and their light curves in the
optical and UV are driven by radioactive decay rather than
energy deposition into a hydrogen envelope. As a result they
have substantively different light curves which are UV faint with
longer rise times than we observe here (Brown et al. 2009). The
remaining SNe, the UV-bright/hydrogen-rich Type II subclasses
(II, IIP, IIn, and IIL), are differentiated through their optical
light curves or spectra as these properties are well known and
commonly observed. Members of this group all have substantial
hydrogen envelopes which leads to their SNe having higher
temperatures and less line blanketing at early times, which leads
in turn to a greater UV luminosity. However, the UV properties
of this class and how they relate to the SNe progenitors is
considerably less constrained than their optical counterparts.
To put our early time observations of 2007pk in context,
we show in Figure 7 the five published UV light curves of
UV-bright SNe observed by Swift plus 2007pk. We stated previously that the 1998S archival spectra showed great similarity
to that of SN 2007pk, and for fiducial purposes we create synthetic magnitudes in the Swift bands using the archival spectra
from days 20 and 34 after the SN explosion and calculate decay
rates for it as well. This brings our sample to seven UV-bright
SNe. We find that while the optical properties between UVbright subclasses may be significantly different their UV properties are highly similar; consisting of an apparent linear decay
in all filters over a period of weeks to months. Our two IIn
shown here appear brighter than the sample’s IIP SNe which is
The v-band light curve of 2007pk has a modest absolute
observed peak magnitude of Mv = −18.28. Super-luminous IIn
SNe such as 2006gy or 2008am may show peak brightnesses of
up to −21 to −22 mag (Smith et al. 2007). Previous work in
Smith et al. (2011, Figure 9) suggests that SNe imposters (which
are thought to be LBV eruption events) may have a similarly
shaped light curve and Hα profile but tend to stay fainter than
−16th magnitude. This places its optical observations in the
typical brightness range for a IIn SN (Figure 6; see also Smith
et al. 2011, Figure 9). As the previous upper limit for 2007pk is
eight days before the first Swift observation, this suggests that
it rose to peak magnitude quicker than any other IIn SNe in
our limited sample, as shown in Figure 6, except for possibly
1994W. Its decay rate at early times post-maximum is typical
of this sample if slightly on the steeper end compared to the
others in the sample at similar phases. Between maximum and
maximum + 60 days it has an average decay of 0.02 mag day−1 ;
at similar times we have decay rates of 0.03, 0.03, 0.01, 0.01,
0.005, and 0.008 mag day−1 for the SNe 1994W, 1998S, 2005cl,
2005cp, 2005db, and 2010jl, respectively.
UV observations of IIn SNe at early times are rare—while
we would like to perform a similar comparison as in Figure 6,
we do not have enough of a sample in the literature to do
so. We will instead do our best and compare SN 2007pk with
early observations of other UV-bright SNe. Pulling our sample
from Brown et al. (2009) and Gezari et al. (2009), we select
only those SNe with multiple detections in all UV filters. We
6
Absolute Magnitude
The Astrophysical Journal, 750:128 (8pp), 2012 May 10
Pritchard et al.
−20
2008es IIL +3
−18
1998S IIn
is typical of the IIn subtype. Physical characteristics of the
explosion and progenitor were then calculated. From the optical
photometry and spectra we calculate a decreasing blackbody
temperature of ∼20,000–7000 K, bolometric luminosity of
(1–7) × 109 L , radius of ∼30–150 au, and an ejecta velocity
of ∼8000 km s−1 .
We also compare the UV light curve of 2007pk with a sample
of other Swift observed SNe that have been published in the
literature. The UV light curve is broadly similar to that of the
other Type II UV-bright subclasses; linear and with a decay
rate that falls within the observed values for other SNe in our
sample. Although our sample is small, this suggests that the
driver of the UV light curve could be similar across these
subtypes. Future work examining the UV light curves of the
UV-bright subclasses with an increased sample is necessary,
and underway by the authors, in order to look for differences
between the various subtypes, correlations with optical or
X-ray properties, and absolute magnitude, with an eye toward
constraining the progenitor systems and possible differences in
explosion physics between the subtypes.
2007pk IIn
−16
2006at IIP
−14
2006bp IIP
−12
2006bc IIP
The HET is a joint project of UT-Austin, PSU, Stanford, Ludwig-Maximilians-Universität München, and GeorgeAugust-Universität Göttingen, and is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly.
This research has made use of the CfA Supernova Archive,
which is funded in part by the National Science Foundation
through grant AST 0606772.
This work was in part supported by the UK STFC and the
UK Space Agency.
uvw2
uvm2
uvw1
−10
2005cs IIP
−8
0
10
20
30
40
Approximate Days Since Explosion
50
Figure 7. Sample of published UV-bright SNe light curves that have been
observed with Swift (Brown et al. 2009; Gezari et al. 2009). SN 2008es has been
shifted down by 3 mag so that while substantially brighter it fits on the same
scale as the other SNe.
(A color version of this figure is available in the online journal.)
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7
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8