Draft version June 7, 2018 A Preprint typeset using LTEX style emulateapj v. 11/10/09 DETAILED RADIO VIEW ON TWO STELLAR EXPLOSIONS AND THEIR HOST GALAXY: XRF080109/SN2008D AND SN2007UY IN NGC2770 A. J. van der Horst1, A. P. Kamble2, Z. Paragi3,4, L. J. Sage5, S. Pal6, G. B. Taylor7, C. Kouveliotou8, J. Granot9, E. Ramirez-Ruiz10, C. H. Ishwara-Chandra11, T. A. Oosterloo12,13, R. A. M. J. Wijers2, K. Wiersema14, R. G. Strom12,2, D. Bhattacharya15, E. Rol2, R. L. C. Starling14, P. A. Curran16, M. A. Garrett12,17,18 Draft version June 7, 2018 ABSTRACT The galaxy NGC 2770 hosted two core-collapse supernova explosions, SN2008D and SN2007uy, within 10 days of each other and 9 years after the first supernova of the same type, SN1999eh, was found in that galaxy. In particular SN 2008D attracted a lot of attention due to the detection of an X-ray outburst, which has been hypothesized to be caused by either a (mildly) relativistic jet or the supernova shock breakout. We present an extensive study of the radio emission from SN 2008D and SN 2007uy: flux measurements with the Westerbork Synthesis Radio Telescope and the Giant Metrewave Radio Telescope, covering ∼600 days with observing frequencies ranging from 325 MHz to 8.4 GHz. The results of two epochs of global Very Long Baseline Interferometry observations are also discussed. We have examined the molecular gas in the host galaxy NGC 2770 with the Arizona Radio Observatory 12-m telescope, and present the implications of our observations for the star formation and seemingly high SN rate in this galaxy. Furthermore, we discuss the near-future observing possibilities of the two SNe and their host galaxy at low radio frequencies with the Low Frequency Array. Subject headings: Gamma rays: bursts 1. INTRODUCTION According to the prevailing model, gamma-ray bursts (GRBs) of the long duration class (LGRBs; Kouveliotou et al. 1993) and the spectrally soft and less- 1 energetic X-ray flashes (XRFs; e.g. Heise et al. 2001) are NASA Postdoctoral Program Fellow, Space Science Office, caused by a relativistic jet emerging from the collapse of a NASA/Marshall Space Flight Center, Huntsville, AL 35812; [email protected]. massive star, e.g. the collapsar model (Woosley 1993) or 2 Astronomical Institute, University of Amsterdam, Science through the production of a millisecond magnetar (Usov Park 904, 1098 XH Amsterdam, The Netherlands. 1992; Bucciantini et al. 2009). This drives a long-lived 3 Joint Institute for VLBI in Europe (JIVE), Postbus 2, 7990 AA Dwingeloo, The Netherlands. relativistic blast wave into the ambient medium, which 4 MTA Research Group for Physical Geodesy and Geodynam- accelerates relativistic electrons that emit synchrotron ics, H-1585 Budapest, P. O. Box 585, Hungary. radiation observed from radio to X-ray frequencies − the 5 University of Maryland, Department of Astronomy, College afterglow (Meszaros & Rees 1997; Wijers et al. 1997). Park, MD 20742. 6 International Centre for Radio Astronomical Research, Uni- Since 1998 a firm association has been estab- versity of Western Australia, 7 Fairway, Crawley, 6009, Aus- lished between LGRBs/XRFs and core-collapse super- tralia. novae (SNe; Galama et al. 1998; Hjorth et al. 2003; 7 University of New Mexico, Department of Physics and As- Stanek et al. 2003; Malesani et al. 2004; Pian et al. 2006; tronomy, MSC07 4220, 800 Yale Blvd NE Albuquerque, New Mexico 87131-0001. G.B. Taylor is also an Adjunct Astronomer Starling et al. 2010; Chornock et al. 2010). Up to now, at the National Radio Astronomy Observatory. five LGRBs (of which two are XRFs) have been associ- 8 Space Science Office, NASA/Marshall Space Flight Center, ated with spectroscopically identified type Ic SNe. It arXiv:1011.2521v1 [astro-ph.HE] 10 Nov 2010 Huntsville, AL 38512, USA. has, however, been shown that not all LGRBs/XRFs 9 Centre for Astrophysics Research, University of Hertford- shire, College Lane, Hatfield, Herts, AL10 9AB, UK. have an associated SN (Gehrels et al. 2006; Fynbo et al. 10 Department of Astronomy and Astrophysics, University of 2006). Their relative event rates suggest that only a California, Santa Cruz, CA 95064. small fraction (∼ 10−3) of Ic SNe produce highly lumi- 11 National Centre for Radio Astrophysics, Post Bag 3, Ganeshkind, Pune 411007, India. nous GRBs (Podsiadlowski et al. 2004; Soderberg et al. 12 Netherlands Institute for Radio Astronomy (ASTRON), 2006b). A detailed study of the first four GRB-SN as- Postbus 2, 7990 AA Dwingeloo, The Netherlands. sociations (Kaneko et al. 2007) has demonstrated that 13 Kapteyn Astronomical Institute, University of Groningen, the total energy budget of the SN explosion only moder- Postbus 800, 9700 AV Groningen, the Netherlands. 14 Department of Physics & Astronomy, University of Leices- ately varies between different events, while the fraction ter, Leicester, LE1 7RH, UK. of that energy that ends up in highly relativistic ejecta 15 Inter-University Center for Astronomy and Astrophysics, (GRB) can vary much more dramatically between differ- Pune, Ganeshkhind, Post-bag No. 4, India. ent events. In fact, only one GRB out of these five GRB- 16 Mullard Space Science Laboratory, University College of London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK. SN associations, GRB030329, can be considered a mem- 17 Leiden Observatory, University of Leiden, P.B. 9513, Leiden ber of the high-luminosity long GRB class, both in terms 2300 RA, the Netherlands. 18 of its prompt and afterglow emission, albeit at the lower Centre for Astrophysics and Supercomputing, Swinburne end of the distributions in terms of its isotropic equiv- University of Technology, Hawthorn, Victoria 3122, Australia. 2 van der Horst et al. alent energy output in gamma rays (e.g. Kaneko et al. The nature of the X-ray outburst observed in SN 2008D 2007). Therefore, a much larger fraction (≤ 5 − 10%) has been extensively discussed in the literature. It has of type Ic SNe may produce less relativistic collimated been claimed that the outburst is a weak XRF caused outflows with Lorentz factors of 2 < Γ ≪ 100 that by a mildly relativistic outflow (Li 2008; Mazzali et al. can power XRFs, compared to Γ ≥ 100 jets that pro- 2008). However, there are counter-claims that we have duce bright GRBs (e.g. Granot & Ramirez-Ruiz 2004; witnessed the X-ray emission from a supernova shock Soderberg et al. 2006b). breakout (Soderberg et al. 2008; Chevalier & Fransson Paragi et al. (2010) argue that it is very well possi- 2008; Wang et al. 2008), caused by the transition ble that most or even all Ib/c SNe produce (mildly) from a radiation dominated to a collisionless shock. relativistic ejecta, but in most cases these ejecta carry Soderberg et al. (2008) have collected 2 months of (high- a much smaller fraction of the explosion energy than frequency) radio observations from this source and con- in GRBs/XRFs, making them detectable in the ra- cluded that the outflow is freely expanding at non- dio only for very nearby events. Current observational relativistic velocities. They also argued that the flux constraints (e.g. Soderberg et al. 2006c) on these radio of the X-ray outburst in combination with the non- sources could be satisfied by low-energy mildly relativis- detections at UV/optical wavelengths is inconsistent tic jets and/or relatively low values for external density with a (mildly) relativistic outflow. On the other hand, or shock microphysics parameters. It has also been pro- the measured variable optical polarization suggests an posed (Paczynski 2001; Granot & Loeb 2003) that some axisymmetric aspherical expansion with variable eccen- Ib/c SNe are producing relativistic GRB/XRF jets that tricity (Gorosabel et al. 2008), as expected in the collap- point away from our line of sight and are thus not de- sar model (Woosley 1993). This asphericity has also been tected at early times in optical and X-rays, but could be inferred from optical spectra of SN2008D (Modjaz et al. detected in radio bands after a few months to years (or- 2008). We note, however, that the optical and radio phan afterglows). After initially unsuccessful efforts to emission are not unambiguously coming from the same find this latter type of afterglow (e.g. Soderberg et al. emission region. 2006c), there is currently evidence that two of these In this paper we present the results from our ex- events, SN2007gr (Paragi et al. 2010) and SN2009bb tensive radio follow-up campaigns of SN 2008D with (Soderberg et al. 2010) have been observed, although the WSRT and the Giant Metrewave Radio Telescope their radio emission was already detected within days (GMRT). Combined with VLA and CARMA data from of the initial explosions. In the case of the very nearby Soderberg et al. (2008), we study these well-sampled SN2007gr (d ≈ 11 Mpc), mildly-relativistic expansion of light curves up to 17 months after the initial explo- the radio source was measured using Very Long Base- sion, across a broad frequency range, from 325 MHz to line Interferometry (VLBI) observations, while for the 95 GHz. We also discuss here the Type Ib SN2007uy radio-luminous SN 2009bb (at d ≈ 40 Mpc) a similar ex- (Nakano et al. 2008; Blondin & Calkins 2008) that went pansion speed of the ejecta was inferred from modeling off in the same galaxy ten days before the discovery of the broadband radio data. We note that the inferred en- SN 2008D. Besides radio photometry measurements, we ergy in (mildly) relativistic ejecta was significantly larger present the results of two epochs of global VLBI obser- in SN 2009bb than in SN 2007gr, ∼ 1049 and ∼ 1046 erg, vations of SN 2008D and discuss their implications for respectively.