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Late Emission from the Type Ib/c SN 2001em: Overtaking the Hydrogen Envelope
The Type Ib/c supernova SN 2001em was observed to have strong radio,X-ray, and Hα emission at an age of ~2.5 yr. Although the radioand X-ray emission have been attributed to an off-axis gamma-ray burst,we model the emission as the interaction of normal SN Ib/c ejecta with adense, massive (~3 Msolar) circumstellar shell at a distanceof ~7×1016 cm. We investigate two models, in which thecircumstellar shell has or has not been overtaken by the forward shockat the time of the X-ray observation. The circumstellar shell waspresumably formed by vigorous mass loss with a rate of~(2-10)×10-3 Msolar yr-1 at~(1-2)×103 yr prior to the supernova explosion. Thehydrogen envelope was completely lost and subsequently was swept up andaccelerated by the fast wind of the presupernova star up to a velocityof 30-50 km s-1. Although interaction with the shell canexplain most of the late emission properties of SN 2001em, we need toinvoke clumping of the gas to explain the low absorption at X-ray andradio wavelengths.
| e-VLBI observations of SN2001em - an off-axis GRB candidate .
Studying transient phenomena with the Very Long Baseline Interferometry(VLBI) technique faces severe difficulties because the turnaround timeof the experiments from the observations to the scientific result israther long. The e-VLBI technique has made it possible to transfer thedata from a number of European VLBI Network (EVN) telescopes to thecentral data processor at JIVE through optical fibres, and correlatethem in real time. The main goal of this paper is to introduce thisrapidly developing new technique, by presenting observational resultsfrom a recent experiment. We observed SN2001em, a Type Ib/c supernovawith an e-VLBI array and the Multi-Element Radio Linked InterferometerNetwork (MERLIN) in the UK. The source is marginally detected in ourobservations. We cannot make definite conclusions whether it is resolvedat 1.6 GHz or not. Our data show that SN2001em either started fading inthe last couple of months, or its radio spectrum is inverted at lowfrequencies, indicating free-free or synchrotron self-absorption. Thisis quite unusual, but not unprecedented in radio SNe.
| Supernova 2001em in UGC 11794
IAUC 8472 available at Central Bureau for Astronomical Telegrams.
| An Upper Limit on the Expansion Velocity of Gamma-Ray Burst Candidate SN 2001em
We report on VLBI observations of the Type I b/c supernova SN 2001em, 3years after the explosion. It has been suggested that SN 2001em might bea jet-driven gamma-ray burst (GRB), with the jet oriented far from theline of sight so that the GRB would not be visible from Earth. To testthis conjecture, we determined the size of SN 2001em. It is onlymarginally resolved at our resolution of ~0.9 mas. The 3 σ upperlimit on the major axis angular size of the radio source was 0.59 mas(FWHM of an elliptical Gaussian), corresponding to a one-sided apparentexpansion velocity of 70,000 km s-1 for a distance of 80 Mpc.No low-brightness jet was seen in our image to a level of 4% of the peakbrightness. If we assume instead a spherical shell geometry typical of asupernova, we find the angular radius of SN 2001em was0.17+0.06-0.10 mas, implying an isotropicexpansion velocity of 20,000+7000-12,000 kms-1, which is comparable to the expansion velocities ofsupernova shells. Our observations are therefore not consistent with arelativistically expanding radio source in SN 2001em but are insteadconsistent with a supernova shell origin of SN 2001's radio emission.
| Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II Regions
We present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1.
| Kinematics of the local universe . XII. 21-cm line measurements of 586 galaxies with the new Nançay receiver
This paper presents 586 new 21-cm neutral hydrogen line measurementscarried out with the FORT receiver of the meridian transit Nançayradiotelescope in the period July 2000-March 2003. This observationalprogramme is part of a larger project aiming at collecting an exhaustiveand magnitude-complete HI extragalactic catalogue for Tully-Fisherapplications. It is associated with the building of the MIGALEspectroscopic archive and database.Tables 2, 3 and HI-profiles and corresponding comments are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/373, or directly atour web site http://klun.obs-nancay.fr
| Supernova 2001em in UGC 11794
IAUC 8323 available at Central Bureau for Astronomical Telegrams.
| Supernova 2001em in UGC 11794
IAUC 8282 available at Central Bureau for Astronomical Telegrams.
| Type Ic SN2001em (off-axis GRB jet?), optical spectrum.
Not Available
| The Case for a Misaligned Relativistic Jet from SN 2001em
SN 2001em, identified as a Type Ic supernova (SN Ic), has recently beendetected in the radio and X-rays, >~2 yr after the explosion. Thehigh luminosities at such late times might arise from a relativistic jetviewed substantially off-axis that becomes visible only when it turnsmildly relativistic and its emission is no longer strongly beamed awayfrom us. Alternatively, the emission might originate from theinteraction of the SN shell with the circumstellar medium. We find thatthe latter scenario is hard to reconcile with the observed rapid rise inthe radio flux and optically thin spectrum,Fν~ν-0.36+/-0.16t1.9+/-0.4, whilethese features arise naturally from a misaligned relativistic jet. Thehigh X-ray luminosity provides an independent and more robustconstraint; it requires ~1051 ergs in mildly relativisticejecta. The source should therefore currently have a large angular size(~2 mas), which could be resolved in the radio with the Very LongBaseline Array. It is also expected to be bipolar and is thus likely toexhibit a large degree of linear polarization (~10%-20%). The presenceof a relativistic outflow in SN 2001em would have interestingimplications. It would suggest that several percent of SNe Ib/c producemildly relativistic jets, with an initial Lorentz factorΓ0>~2, while the fraction that produces gamma-rayburst (GRB) jets (with Γ0>~100) is ~100 timessmaller. This could considerably increase the expected number oftransients similar to orphan GRB afterglows in the radio and to a lesserextent in the optical and X-rays, if there is a continuous distributionin Γ0. Furthermore, this may give further credence tothe idea that core-collapse SNe, and in particular SNe Ib/c, aretriggered by bipolar jets.
| Classifications of the Host Galaxies of Supernovae
Classifications on the DDO system are given for the host galaxies of 177supernovae (SNe) that have been discovered since 1997 during the courseof the Lick Observatory Supernova Search with the Katzman AutomaticImaging Telescope. Whereas SNe Ia occur in all galaxy types, it isfound, at a high level of statistical confidence, that SNe Ib, Ic, andII are strongly concentrated in late-type galaxies. However, attentionis drawn to a possible exception provided by SN 2001I. This SN IInoccurred in the E2 galaxy UGC 2836, which was not expected to harbor amassive young supernova progenitor.
| Supernova 2001em in UGC 11794
IAUC 7722 available at Central Bureau for Astronomical Telegrams.
| An Imaging and Spectroscopic Survey of Galaxies within Prominent Nearby Voids. II. Morphologies, Star Formation, and Faint Companions
We analyze the optical properties of ~300 galaxies within and aroundthree prominent voids of the Center for Astrophysics Redshift Survey. Wedetermine CCD morphologies and Hα equivalent widths from ourimaging and spectroscopic survey. We also describe a redshift survey of250 neighboring galaxies in the imaging survey fields. We assess themorphology-density relation, EW(Hα)-density relation, and theeffects of nearby companions for galaxies in low-density environmentsselected with a smoothed large-scale (5 h-1 Mpc) galaxynumber density n. Both the morphological mix and the Hα line widthdistribution of galaxies at modest underdensities, 0.5R=16.13, demonstrates that the incidence ofa close companion in redshift space is insensitive to global densityover the range we investigate (0.163 σ) fromΔcz>~200 km s-1 at 0.5-1 at n<=0.5n. In the lowest densityenvironments, galaxies with companions clearly (~4 σ) havestronger star formation than comparable galaxies at larger globaldensity (0.5-1 kpc and 1000 km s-1) varies little over theentire density range. These results, combined with the luminosity- andcolor-density relations of this sample (Paper I), suggest that theformation and evolution of field galaxies are insensitive to large-scaleunderdensity down to a threshold of roughly half the mean density. Thedifferences in galaxy properties at the lowest global densities we canexplore (n<=0.5n) may be explained by (1) a relative scarcity of thesmall-scale primordial density enhancements needed to form massiveearly-type/absorption-line galaxies and (2) present-day galaxyencounters that are relatively more effective because of the lowervelocity dispersion on small scales (<~200 h-1 kpc) weobserve in these regions. In the voids, where the luminous galaxiespresumably formed more recently, there should be more gas and dustpresent for active star formation triggered by nearby companions.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| An Imaging and Spectroscopic Survey of Galaxies within Prominent Nearby Voids. I. The Sample and Luminosity Distribution
We study the optical properties of a large sample of galaxies inlow-density regions of the nearby universe. We make a 5 h-1Mpc smoothed map of the galaxy density throughout the Center forAstrophysics Redshift Survey (CfA2) to identify galaxies within threeprominent nearby ``voids'' with diameter >~30 h-1 Mpc. Weaugment the CfA2 void galaxy sample with fainter galaxies found in thesame regions from the more recent and deeper Century and Redshiftsurveys . We obtain B and R CCD images and high signal-to-noiselong-slit spectra for the resulting sample of 149 void galaxies, as wellas for an additional 131 galaxies on the periphery of these voids. Herewe describe the photometry for the sample, including B isophotalmagnitudes and B-R colors. For the 149 galaxies that lie in regionsbelow the mean survey density, the luminosity functions in B and R arewell fit by Schechter functions with respective parameters(αB=-0.5+/-0.3, B*=-18.9+/-0.2) and(αR=-0.9+/-0.3, R*=-20.4+/-0.3). The Bluminosity function (LF) is consistent with typical survey LFs (e.g.,the Southern Sky Redshift Survey), and the R LF is consistent with theCentury Survey. The B and R LFs of 131 galaxies in the ``voidperiphery''-regions between the mean density and twice the mean-havesimilar Schechter parameters. The CfA2 LF is inconsistent with bothsamples at the 3.5 σ level. When we narrow our analysis to the 46galaxies in regions below half the mean density, the LF is significantlysteeper: α~-1.4+/-0.5. The typical survey LFs are inconsistentwith this subsample at the ~2 σ level. The B-R color distributionof galaxies in the lowest density regions is also shifted significantly(~3 σ) blueward of the higher density samples. The most luminousred galaxies (R<~-21) are absent from the lowest density regions at the2.5 σ level.
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