---
author:
- |
    \
    George P. and Cynthia Woods Mitchell Institute for Fundamental Physics & Astronomy, Texas A. & M. University, Department of Physics and Astronomy, 4242 TAMU, College Station, TX 77843, USA E-mail:
bibliography:
- '../bibtex.bib'
title: 'SOUSA’s Swift Supernova Siblings'
---

Swift Supernova Observations
============================

In the first ten years of Swift operations [@Gehrels_etal_2004], Swift has observed over three hundred supernovae. All young supernovae (i.e. excluding supernova remnants and others which exploded before the launch of Swift) known by the author to have been observed by Swift are listed on the Swift Supernova website[^1]. The observed supernovae cover a wide variety of explosion and environment characteristics. The UV data from the Ultra-Violet/Optical Telescope (UVOT; [@Roming_etal_2005]) are being uniformly reduced and publicly released as the Swift Optical/Ultraviolet Supernova Archive (SOUSA). The details of the photometric reduction are available in @Brown_etal_2014_SOUSA.

As we look ahead to the next years of Swift supernova observations, results from the first ten years can guide the choice of future targets and observing strategies. Most of the Swift supernova observations in the first ten years have focused on nearby supernovae with redshifts z$< $0.02. Figure 1 shows the wide variety of absolute magnitudes in the mid-ultraviolet uvm2 filter. Distance limits are estimated based on a limiting magnitude of 20 in the UVOT Vega system [@Poole_etal_2008; @Breeveld_etal_2011]. Fainter supernovae such as type Ib/c and subluminous type Ia supernovae can only be well observed within a redshift of z$\sim$0.005. UV-bright type IIP and Ibn supernovae can be observed to z$\sim0.05$. Some type Ia supernovae are also this bright, including so-called “Super-Chandrasekhar” type Ia supernovae [@Brown_etal_2014] and the peculiar SN 2011de [@Brown_2014]. Very UV-bright type IIn can be observed to z$\sim0.2$ and the type II superluminous supernovae to z$\sim0.5$. Expanding the redshift range will increase the volume and numbers of some supernova types which can be observed with Swift. In particular measuring the peak magnitudes of type Ia supernovae in the Hubble flow will reduce the uncertainties in the distances and absolute magnitudes, a problem with some of the nearby Swift supernova samples [@Brown_etal_2010].

![ UV absolute magnitudes and UV-optical colors are plotted for prototypes of most supernova types to understand the detectability of supernovae based on the type and age and/or optical magnitude. The redshift limits are based on a uvm2 limiting Vega magnitude of 20. []{data-label="fig1"}](m2plots.eps){width="100.00000%"}

Swift Supernova Siblings
========================

As the Swift supernova sample grows, so does the chance that some of the host galaxies have or will host another supernova. We call these pairings of supernovae from the same hosts “siblings.” We reserve the term “twins” for supernovae which are nearly identical in many properties. Supernova twins and siblings are more useful than just the sum of their individual utility.

Supernova twins are useful because their similar spectroscopic properties allow color differences to be traced to line of sight reddening, and their extinction corrected magnitudes can be used to infer distance differences [@Krisciunas_etal_2007; @Krisciunas_etal_2009]. The optical similarities of SNe 2011by and 2011fe led @Foley_Kirshner_2013 to conclude that their UV differences were from metallicity. @Brown_etal_2015 find other effects such as density gradients also cause large UV differences while leaving the optical unscathed. Since “twin-ness” can be very subjective, below we just focus on siblings, since host identification for nearby supernovae is a much more straightforward process.

Follow up observations of a supernova often result in a higher cadence of galaxy observations, so sometimes an additional supernova in the galaxy is discovered at a younger age than otherwise would have happened. Observations of SN 1999ee in IC5179 led to the very early discovery of SN 1999ex and the first observations of the fading shock breakout of a Ib/c supernova [@Stritzinger_etal_2002]. The best example is the discovery in Swift observations of SN 2007uy of the X-ray shock breakout of SN 2008D [@Soderberg_etal_2008].

Supernova siblings are important objects in the study of supernova rates as a function of supernova type and host galaxy morphology [@Anderson_Soto_2013]. They are also important tests of the utility of global (rather than local) environment measurements and their correlation with rates or supernova properties. Sibling supernovae provide independent and joint constraints on the distance to the host galaxy [@Stritzinger_etal_2010] and a direct comparison of the absolute brightness of the siblings without the uncertainties on the distance. In Table 1 we list the 16 host galaxies and the 34 supernovae they have hosted that were observed by Swift. Other multiples have occurred during this period of time, but we are only listing those supernovae observed by Swift.

  [**Host Galaxy**]{}   [**Redshift**]{}   [**Supernova**]{}   [**Type**]{}   [**Supernova**]{}   [**Type**]{}   [**Supernova**]{}   [**Type** ]{}
  --------------------- ------------------ ------------------- -------------- ------------------- -------------- ------------------- ---------------
  M51                   0.002              SN2005cs            IIP            SN2011dh            IIb            . . .               . . .
  NGC1316               0.006              SN2006dd            Ia             SN2006mr            Ia             . . .               . . .
  NGC7364               0.016              SN2006lc            Ib/c           SN2011im            Ia             . . .               . . .
  MCG+05-43-16          0.017              SN2007ck            IIP            SN2007co            Ia             . . .               . . .
  NGC4039               0.005              SN2007sr            Ia             SN2013dk            Ic             . . .               . . .
  NGC2770               0.006              SN2007uy            Ib             SN2008D             Ib             . . .               . . .
  ESO121-26             0.008              SN2008M             IIP            SN2009mg            IIb            SN2012hr            Ia
  NGC2765               0.013              SN2008hv            Ia             ASASSN-13dd         Ia             . . .               . . .
  M61                   0.005              SN2008in            IIP            SN2014dt            Ia-02cx        . . .               . . .
  NGC1954               0.010              SN2010ko            Ia             SN2011fi            II             SN2013ex            Ia
  Arp299                0.010              SN2010O             Ib             SN2010P             IIb            . . .               . . .
  NGC1566               0.005              SN2010el            Ia-02cx        ASASSN-14ha         II             . . .               . . .
  NGC6240               0.024              SN2010gp            Ia             PS1-14xw            Ia             . . .               . . .
  NGC2207               0.009              SN2010jp            IIn            SN2013ai            IIP            . . .               . . .
  NGC5806               0.005              SN2012P             Ibc            iPTF13bvn           Ic             . . .               . . .
  NGC7331               0.003              SN2013bu            II             SN2014C             Ib             . . .               . . .
  \[1ex\]                                                                                                                            

  : Swift Supernova Siblings

[^1]: http://swift.gsfc.nasa.gov/docs/swift/sne/swift\_sn.html.
