Taking stock of SLSN and LGRB host galaxy comparison using a complete sample of LGRBs

Long gamma-ray bursts (LGRBs) and superluminous supernovae (SLSNe) are both explosive transients with very massive progenitor stars. Clues about the nature of the progenitors can be found by investigating environments in which such transients occur. While studies of LGRB host galaxies have a long history, dedicated observational campaigns have only recently resulted in a high enough number of photometrically and spectroscopically observed SLSN hosts to allow statistically significant analysis of their properties. In this paper we make a comparison of the host galaxies of hydrogen-poor (H-poor) SLSNe and the Swift/BAT6 sample of LGRBs. In contrast to previous studies we use a complete sample of LGRBs and we address a special attention to the comparison methodology and the selection of SLSN sample whose data have been compiled from the available literature. At intermediate redshifts (0.3 < z < 0.7) the two classes of transients select galaxies whose properties (stellar mass, luminosity, star-formation rate, specific star-formation rate and metallicity) do not differ on average significantly. Moreover, the host galaxies of both classes of objects follow the fundamental metallicity relation and the fundamental plane of metallicity. In contrast to previous studies we show that at intermediate redshifts the emission line equivalent widths of the two populations are essentially the same and that the previous claims regarding the higher fraction of SLSN hosts among the extreme emission line galaxies with respect to LGRBs are mostly due to a larger fraction of strong-line emitters among SLSN hosts at z < 0.3, where samples of LGRB hosts are small and poorly defined.Comment: 7 pages, 4 figures, accepted to Astronomy & Astrophysic

A complete sample of bright Swift Long Gamma Ray Bursts: testing the spectral-energy correlations

We use a nearly complete sample of Gamma Ray Bursts (GRBs) detected by the Swift satellite to study the correlations between the spectral peak energy Ep of the prompt emission, the isotropic energetics Eiso and the isotropic luminosity Liso. This GRB sample is characterized by a high level of completeness in redshift (90%). This allows us to probe in an unbiased way the issue related to the physical origin of these correlations against selection effects. We find that one burst, GRB 061021, is an outlier to the Ep-Eiso correlation. Despite this case, we find strong Ep-Eiso and Ep-Liso correlations for the bursts of the complete sample. Their slopes, normalisations and dispersions are consistent with those found with the whole sample of bursts with measured redshift and Ep. This means that the biases present in the total sample commonly used to study these correlations do not affect their properties. Finally, we also find no evolution with redshift of the Ep-Eiso and Ep-Liso correlations.Comment: MNRAS in press, 9 pages, 4 figures, 2 tables. This version matches the published version in MNRA

Optical and X-ray Rest-frame Light Curves of the BAT6 sample

We present the rest-frame light curves in the optical and X-ray bands of an unbiased and complete sample of Swift long Gamma-Ray Bursts (GRBs), namely the BAT6 sample. The unbiased BAT6 sample (consisting of 58 events) has the highest level of completeness in redshift ($\sim$ 95%), allowing us to compute the rest-frame X-ray and optical light curves for 55 and 47 objects, respectively. We compute the X-ray and optical luminosities accounting for any possible source of absorption (Galactic and intrinsic) that could affect the observed fluxes in these two bands. We compare the behaviour observed in the X-ray and in the optical bands to assess the relative contribution of the emission during the prompt and afterglow phases. We unarguably demonstrate that the GRBs rest-frame optical luminosity distribution is not bimodal, being rather clustered around the mean value Log(L$_{R}$) = 29.9 $\pm$ 0.8 when estimated at a rest frame time of 12 hr. This is in contrast with what found in previous works and confirms that the GRB population has an intrinsic unimodal luminosity distribution. For more than 70% of the events the rest-frame light curves in the X-ray and optical bands have a different evolution, indicating distinct emitting regions and/or mechanisms. The X-ray light curves normalised to the GRB isotropic energy (E$_{\rm iso}$), provide evidence for X-ray emission still powered by the prompt emission until late times ($\sim$ hours after the burst event). On the other hand, the same test performed for the E$_{\rm iso}$-normalised optical light curves shows that the optical emission is a better proxy of the afterglow emission from early to late times.Comment: Accepted for publication in A&A: 10 pages, 5 figures, 2 table

GRB orphan afterglows in present and future radio transient surveys

Orphan Afterglows (OA) are slow transients produced by Gamma Ray Bursts seen off-axis that become visible on timescales of days/years at optical/NIR and radio frequencies, when the prompt emission at high energies (X and gamma rays) has already ceased. Given the typically estimated jet opening angle of GRBs theta_jet ~ 3 deg, for each burst pointing to the Earth there should be a factor ~ 700 more GRBs pointing in other directions. Despite this, no secure OAs have been detected so far. Through a population synthesis code we study the emission properties of the population of OA at radio frequencies. OAs reach their emission peak on year-timescales and they last for a comparable amount of time. The typical peak fluxes (which depend on the observing frequency) are of few micro Jy in the radio band with only a few OA reaching the mJy level. These values are consistent with the upper limits on the radio flux of SN Ib/c observed at late times. We find that the OA radio number count distribution has a typical slope -1.7 at high fluxes and a flatter (-0.4) slope at low fluxes with a break at a frequency-dependent flux. Our predictions of the OA rates are consistent with the (upper) limits of recent radio surveys and archive searches for radio transients. Future radio surveys like VAST/ASKAP at 1.4 GHz should detect ~ 3x10^-3 OA deg^-2 yr-1, MeerKAT and EVLA at 8.4 GHz should see ~ 3x10^-1 OA deg-2 yr-1. The SKA, reaching the micro Jy flux limit, could see up to ~ 0.2-1.5 OA deg^-2 yr^-1. These rates also depend on the duration of the OA above a certain flux limit and we discuss this effect with respect to the survey cadence.Comment: (10 pages, 5 figures, 1 table) Accepted for publication by PAS

Clustering of galaxies around GRB sight-lines

There is evidence of an overdensity of strong intervening MgII absorption line systems distributed along the lines of sight towards GRB afterglows relative to quasar sight-lines. If this excess is real, one should also expect an overdensity of field galaxies around GRB sight-lines, as strong MgII tends to trace these sources. In this work, we test this expectation by calculating the two point angular correlation function of galaxies within 120$^{\prime\prime}$ ($\sim470~h_{71}^{-1}~\mathrm{Kpc}$ at $\langle z\rangle \sim0.4$) of GRB afterglows. We compare the Gamma-ray burst Optical and Near-infrared Detector (GROND) GRB afterglow sample -- one of the largest and most homogeneous samples of GRB fields -- with galaxies and AGN found in the COSMOS-30 photometric catalog. We find no significant signal of anomalous clustering of galaxies at an estimated median redshift of $z\sim0.3$ around GRB sight-lines, down to $K_{\mathrm{AB}}<19.3$. This result is contrary to the expectations from the MgII excess derived from GRB afterglow spectroscopy, although many confirmed galaxy counterparts to MgII absorbers may be too faint to detect in our sample -- especially those at $z>1$. We note that the addition of higher sensitivity Spitzer IRAC or HST WFC3 data for even a subset of our sample would increase this survey's depth by several orders of magnitude, simultaneously increasing statistics and enabling the investigation of a much larger redshift space.}Comment: 10 pages, 6 figures. A&A accepte

Integral field spectroscopy with SINFONI of VVDS galaxies. II. The mass-metallicity relation at 1.2 < z < 1.6

This work aims to provide a first insight into the mass-metallicity (MZ) relation of star-forming galaxies at redshift z~1.4. To reach this goal, we present a first set of nine VVDS galaxies observed with the NIR integral-field spectrograph SINFONI on the VLT. Oxygen abundances are derived from empirical indicators based on the ratio between strong nebular emission-lines (Halpha, [NII]6584 and [SII]6717,6731). Stellar masses are deduced from SED fitting with Charlot & Bruzual (2007) population synthesis models, and star formation rates are derived from [OII]3727 and Halpha emission-line luminosities. We find a typical shift of 0.2-0.4 dex towards lower metallicities for the z~1.4 galaxies, compared to the MZ-relation in the local universe as derived from SDSS data. However, this small sample of eight galaxies does not show any clear correlation between stellar mass and metallicity, unlike other larger samples at different redshift (z~0, z~0.7, and z~2). Indeed, our galaxies lie just under the relation at z~2 and show a small trend for more massive galaxies to be more metallic (~0.1 logarithmic slope). There are two possible explanations to account for these observations. First, the most massive galaxies present higher specific star formation rates when compared to the global VVDS sample which could explain the particularly low metallicity of these galaxies as already shown in the SDSS sample. Second, inflow of metal-poor gas due to tidal interactions could also explain the low metallicity of these galaxies as two of these three galaxies show clear signatures of merging in their velocity fields. Finally, we find that the metallicity of 4 galaxies is lower by ~0.2 to 0.4 dex if we take into account the N/O abundance ratio in their metallicity estimate.Comment: 7 pages, 4 figures, accepted in A&A Comments: Comments: more accurate results with better stellar mass estimate

Effective absorbing column density in the gamma-ray burst afterglow X-ray spectra

We investigate the scaling relation between the observed amount of absorption in the X-ray spectra of Gamma Ray Burst (GRB) afterglows and the absorber redshift. Through dedicated numerical simulations of an ideal instrument, we establish that this dependence has a power law shape with index 2.4. However, for real instruments, this value depends on their low energy cut-off, spectral resolution and on the detector spectral response in general. We thus provide appropriate scaling laws for specific instruments. Finally, we discuss the possibility to measure the absorber redshift from X-ray data alone. We find that 10^5-10^6 counts in the 0.3-10 keV band are needed to constrain the redshift with 10% accuracy. As a test case we discuss the XMM-Newton observation of GRB 090618 at z=0.54. We are able to recover the correct redshift of this burst with the expected accuracy.Comment: MNRAS accepted. 6 figures. 3 table