Correcting for lensing bias in the Hubble diagram

Gravitational lensing will cause a dispersion in the Hubble diagram for high redshift sources. This effect will introduce a bias in the cosmological parameter determination using the distance-redshift relation for Type Ia supernovae. In this note we show how one can diagnose and correct for this bias when doing precision cosmology with supernovae.Comment: 5 pages, 5 figures, accepted for publication in A&

Estimating dust distances to Type Ia supernovae from colour excess time-evolution

We present a new technique to infer dust locations towards reddened Type Ia supernovae and to help discriminate between an interstellar and a circumstellar origin for the observed extinction. Using Monte Carlo simulations, we show that the time-evolution of the light-curve shape and especially of the colour excess \ebv~places strong constraints on the distance between dust and the supernova. We apply our approach to two highly-reddened Type Ia supernovae for which dust distance estimates are available in the literature: SN 2006X and SN 2014J. For the former, we obtain a time-variable $E(B-V)$ and from this derive a distance of $27.5^{+9.0}_{-4.9}$ or $22.1^{+6.0}_{-3.8}$ pc depending on whether dust properties typical of the Large Magellanic Cloud (LMC) or the Milky Way (MW) are used. For the latter, instead, we obtain a constant $E(B-V)$ consistent with dust at distances larger than 50 and 38 pc for LMC$-$ and MW$-$type dust, respectively. Values thus extracted are in excellent agreement with previous estimates for the two supernovae. Our findings suggest that dust responsible for the extinction towards these supernovae is likely to be located within interstellar clouds. We also discuss how other properties of reddened Type Ia supernovae $-$ such as their peculiar extinction and polarization behaviour and the detection of variable, blue-shifted sodium features in some of these events $-$ might be compatible with dust and gas at interstellar-scale distances.Comment: 13 pages, 8 figures; accepted for publication in MNRAS; dust distance values updated to match the published version; conclusions unchange

Constraints on the origin of the first light from SN2014J

We study the very early lightcurve of supernova 2014J (SN 2014J) using the high-cadence broad-band imaging data obtained by the Kilodegree Extremely Little Telescope (KELT), which fortuitously observed M 82 around the time of the explosion, starting more than two months prior to detection, with up to 20 observations per night. These observations are complemented by observations in two narrow-band filters used in an H$\alpha$ survey of nearby galaxies by the intermediate Palomar Transient Factory (iPTF) that also captured the first days of the brightening of the \sn. The evolution of the lightcurves is consistent with the expected signal from the cooling of shock heated material of large scale dimensions, \gsim 1 R_{\odot}. This could be due to heated material of the progenitor, a companion star or pre-existing circumstellar environment, e.g., in the form of an accretion disk. Structure seen in the lightcurves during the first days after explosion could also originate from radioactive material in the outer parts of an exploding white dwarf, as suggested from the early detection of gamma-rays. The model degeneracy translates into a systematic uncertainty of $\pm 0.3$ days on the estimate of the first light from SN 2014J.Comment: Accepted by ApJ. Companion paper by Siverd et al, arXiv:1411.415

Testing for redshift evolution of Type Ia supernovae using the strongly lensed PS1-10afx at z=1.4z=1.4

The light from distant supernovae (SNe) can be magnified through gravitational lensing when a foreground galaxy is located along the line of sight. This line-up allows for detailed studies of SNe at high redshift that otherwise would not be possible. Spectroscopic observations of lensed high-redshift Type Ia supernovae (SNe Ia) are of particular interest since they can be used to test for evolution of their intrinsic properties. The use of SNe Ia for probing the cosmic expansion history has proven to be an extremely powerful method for measuring cosmological parameters. However, if systematic redshift-dependent properties are found, their usefulness for future surveys could be challenged. We investigate whether the spectroscopic properties of the strongly lensed and very distant SN Ia PS1-10afx at $z=1.4$ deviates from the well-studied populations of normal SNe Ia at nearby or intermediate distance. We created median spectra from nearby and intermediate-redshift spectroscopically normal SNe Ia from the literature at -5 and +1 days from light-curve maximum. We then compared these median spectra to those of PS1-10afx. We do not find signs of spectral evolution in PS1-10afx. The observed deviation between PS1-10afx and the median templates are within what is found for SNe at low- and intermediate-redshift. There is a noticeable broad feature centred at $\rm \lambda\sim 3500$~\AA{}, which is present only to a lesser extent in individual low and intermediate redshift SN Ia spectra. From a comparison with a recently developed explosion model, we find this feature to be dominated by iron peak elements, in particular, singly ionized cobalt and chromium.Comment: accepted for publication in section 4. Extragalactic astronomy of Astronomy and Astrophysic

The cosmological constant and the relaxed universe

We study the role of the cosmological constant (CC) as a component of dark energy (DE). It is argued that the cosmological term is in general unavoidable and it should not be ignored even when dynamical DE sources are considered. From the theoretical point of view quantum zero-point energy and phase transitions suggest a CC of large magnitude in contrast to its tiny observed value. Simply relieving this disaccord with a counterterm requires extreme fine-tuning which is referred to as the old CC problem. To avoid it, we discuss some recent approaches for neutralising a large CC dynamically without adding a fine-tuned counterterm. This can be realised by an effective DE component which relaxes the cosmic expansion by counteracting the effect of the large CC. Alternatively, a CC filter is constructed by modifying gravity to make it insensitive to vacuum energy.Comment: 6 pages, no figures, based on a talk presented at PASCOS 201

Herschel limits on far-infrared emission from circumstellar dust around nearby Type Ia supernovae

We report upper limits on dust emission at far-infrared (IR) wavelengths from three nearby Type Ia supernovae: SNe 2011by, 2011fe and 2012cg. Observations were carried out at 70 um and 160 um with the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory. None of the supernovae were detected in the far-IR, allowing us to place upper limits on the amount of pre-existing dust in the circumstellar environment. Due to its proximity, SN 2011fe provides the tightest constraints, M_dust < 7 * 10^-3 M_sun at a 3 sigma-level for dust temperatures T_dust ~500 K assuming silicate or graphite dust grains of size a = 0.1 um. For SNe 2011by and 2012cg the corresponding upper limits are less stringent, with M_dust < 0.1 M_sun for the same assumptions.Comment: 6 pages, 3 figures, 1 table. Accepted for publication in MNRA

Probing gas and dust in the tidal tail of NGC 5221 with the type Ia supernova iPTF16abc

Context. Type Ia supernovae (SNe Ia) can be used to address numerous questions in astrophysics and cosmology. Due to their well known spectral and photometric properties, SNe Ia are well suited to study gas and dust along the lines-of-sight to the explosions. For example, narrow Na I D and Ca II H&K absorption lines can be studied easily, because of the well-defined spectral continuum of SNe Ia around these features. Aims. We study the gas and dust along the line-of-sight to iPTF16abc, which occurred in an unusual location, in a tidal arm, 80 kpc from centre of the galaxy NGC 5221. Methods. Using a time-series of high-resolution spectra, we examine narrow Na I D and Ca II H&K absorption features for variations in time, which would be indicative for circumstellar (CS) matter. Furthermore, we take advantage of the well known photometric properties of SNe Ia to determine reddening due to dust along the line-of-sight. Results. From the lack of variations in Na I D and Ca II H&K, we determine that none of the detected absorption features originate from the CS medium of iPTF16abc. While the Na I D and Ca II H&K absorption is found to be optically thick, a negligible amount of reddening points to a small column of interstellar dust. Conclusions. We find that the gas along the line-of-sight to iPTF16abc is typical of what might be found in the interstellar medium (ISM) within a galaxy. It suggests that we are observing gas that has been tidally stripped during an interaction of NGC 5221 with one of its neighbouring galaxies in the past $\sim10^9$ years. In the future, the gas clouds could become the locations of star formation. On a longer time scale, the clouds might diffuse, enriching the circum-galactic medium (CGM) with metals. The gas profile along the line-of-sight should be useful for future studies of the dynamics of the galaxy group containing NGC 5221.Comment: 8 pages, 6 figure

The peculiar extinction law of SN2014J measured with The Hubble Space Telescope

The wavelength-dependence of the extinction of Type Ia SN2014J in the nearby galaxy M82 has been measured using UV to near-IR photometry obtained with the Hubble Space Telescope, the Nordic Optical Telescope, and the Mount Abu Infrared Telescope. This is the first time that the reddening of a SN Ia is characterized over the full wavelength range of $0.2$-$2$ microns. A total-to-selective extinction, $R_V\geq3.1$, is ruled out with high significance. The best fit at maximum using a Galactic type extinction law yields $R_V = 1.4\pm0.1$. The observed reddening of SN2014J is also compatible with a power-law extinction, $A_{\lambda}/A_V = \left( {\lambda}/ {\lambda_V} \right)^{p}$ as expected from multiple scattering of light, with $p=-2.1\pm0.1$. After correction for differences in reddening, SN2014J appears to be very similar to SN2011fe over the 14 broad-band filter light curves used in our study.Comment: Accepted for publication in ApJ

Type Ia supernova Hubble diagram with near-infrared and optical observations

We main goal of this paper is to test whether the NIR peak magnitudes of SNe Ia could be accurately estimated with only a single observation obtained close to maximum light, provided the time of B band maximum and the optical stretch parameter are known. We obtained multi-epoch UBVRI and single-epoch J and H photometric observations of 16 SNe Ia in the redshift range z=0.037-0.183, doubling the leverage of the current SN Ia NIR Hubble diagram and the number of SNe beyond redshift 0.04. This sample was analyzed together with 102 NIR and 458 optical light curves (LCs) of normal SNe Ia from the literature. The analysis of 45 well-sampled NIR LCs shows that a single template accurately describes them if its time axis is stretched with the optical stretch parameter. This allows us to estimate the NIR peak magnitudes even with one observation obtained within 10 days from B-band maximum. We find that the NIR Hubble residuals show weak correlation with DM_15 and E(B-V), and for the first time we report a possible dependence on the J_max-H_max color. The intrinsic NIR luminosity scatter of SNe Ia is estimated to be around 0.10 mag, which is smaller than what can be derived for a similarly heterogeneous sample at optical wavelengths. In conclusion, we find that SNe Ia are at least as good standard candles in the NIR as in the optical. We showed that it is feasible to extended the NIR SN Ia Hubble diagram to z=0.2 with very modest sampling of the NIR LCs, if complemented by well-sampled optical LCs. Our results suggest that the most efficient way to extend the NIR Hubble diagram to high redshift would be to obtain a single observation close to the NIR maximum. (abridged)Comment: 39 pages, 15 figures, accepted by A&