The 'Magnificent Seven' (M7) are a group of radio-quiet isolated neutron stars discovered in the soft X-rays through their purely thermal surface emission. Owing to the large inferred magnetic fields ...(B ... 10 super( 13) G), radiation from these sources is expected to be substantially polarized, independently of the mechanism actually responsible for the thermal emission. A large observed polarization degree (PD) is, however, expected only if quantum-electrodynamic (QED) polarization effects are present in the magnetized vacuum around the star. The detection of a strong linearly polarized signal would therefore provide the first observational evidence of QED effects in the strong-field regime. While polarization measurements in the soft X-rays are not feasible yet, optical polarization measurements are within reach also for quite faint targets, like the M7 which have optical counterparts with magnitudes ...26-28. Here, we report on the measurement of optical linear polarization for the prototype, and brightest member, of the class, RX J1856.5-3754 (V ~ 25.5), the first ever for one of the M7, obtained with the Very Large Telescope. We measured a PD = 16.43 plus or minus 5.26 per cent and a polarization position angle PA = 145 ...39 plus or minus 9 ...44, computed east of the North Celestial Meridian. The PD that we derive is large enough to support the presence of vacuum birefringence, as predicted by QED. (ProQuest: ... denotes formulae/symbols omitted.)
ABSTRACT
We predict linear polarization for a radioactively powered kilonova following the merger of a black hole and a neutron star. Specifically, we perform 3D Monte Carlo radiative transfer ...simulations for two different models, both featuring a lanthanide-rich dynamical ejecta component from numerical-relativity simulations while only one including an additional lanthanide-free disc-wind component. We calculate polarization spectra for nine different orientations at 1.5, 2.5, and 3.5 d after the merger and in the $0.1\!-\!2\, \mu$m wavelength range. We find that both models are polarized at a detectable level 1.5 d after the merger while show negligible levels thereafter. The polarization spectra of the two models are significantly different. The model lacking a disc wind shows no polarization in the optical, while a signal increasing at longer wavelengths and reaching $\sim 1\!-\!6{{\ \rm per\ cent}}$ at $2\, \mu$m depending on the orientation. The model with a disc-wind component, instead, features a characteristic ‘double-peak’ polarization spectrum with one peak in the optical and the other in the infrared. Polarimetric observations of future events will shed light on the debated neutron richness of the disc-wind component. The detection of optical polarization would unambiguously reveal the presence of a lanthanide-free disc-wind component, while polarization increasing from zero in the optical to a peak in the infrared would suggest a lanthanide-rich composition for the whole ejecta. Future polarimetric campaigns should prioritize observations in the first ∼48 h and in the $0.5\!-\!2\, \mu$m range, where polarization is strongest, but also explore shorter wavelengths/later times where no signal is expected from the kilonova and the interstellar polarization can be safely estimated.
ABSTRACT
Classical Cepheids (DCEPs) are the most important primary indicators of the extragalactic distance scale. Establishing the dependence on metallicity of their period–luminosity and ...period–Wesenheit (PLZ/PWZ) relations has deep consequences on the calibration of secondary distance indicators that lead to the final estimate of the Hubble constant (H0). We collected high-resolution spectroscopy for 47 DCEPs plus 1 BL Her variables with HARPS-N@TNG and derived accurate atmospheric parameters, radial velocities, and metal abundances. We measured spectral lines for 29 species and characterized their chemical abundances, finding very good agreement with previous results. We re-determined the ephemerides for the program stars and measured their intensity-averaged magnitudes in the V, I, J, H, Ks bands. We complemented our sample with literature data and used the Gaia Early Data Release 3 (EDR3) to investigate the PLZ/PWZ relations for Galactic DCEPs in a variety of filter combinations. We find that the solution without any metallicity term is ruled out at more than the 5σ level. Our best estimate for the metallicity dependence of the intercept of the PLKs, PWJKs, PWVKs, and PWHVI relations with three parameters is −0.456 ± 0.099, −0.465 ± 0.071, −0.459 ± 0.107, and −0.366 ± 0.089 mag dex−1, respectively. These values are significantly larger than the recent literature. The present data are still inconclusive to establish whether or not also the slope of the relevant relationships depends on metallicity. Applying a correction to the standard zero-point offset of the Gaia parallaxes has the same effect of reducing by ∼22 per cent the size of the metallicity dependence on the intercept of the PLZ/PWZ relations.
ABSTRACT
Multimessenger astronomy received a great boost following the discovery of kilonova (KN) AT2017gfo, the optical counterpart of the gravitational wave source GW170817 associated with the ...short gamma-ray burst GRB 170817A. AT2017gfo was the first KN that could be extensively monitored in time using both photometry and spectroscopy. Previously, only few candidates have been observed against the glare of short GRB afterglows. In this work, we aim to search the fingerprints of AT2017gfo-like KN emissions in the optical/NIR light curves of 39 short GRBs with known redshift. For the first time, our results allow us to study separately the range of luminosity of the blue and red components of AT2017gfo-like kilonovae in short GRBs. In particular, the red component is similar in luminosity to AT2017gfo, while the blue KN can be more than 10 times brighter. Finally, we exclude a KN as luminous as AT2017gfo in GRBs 050509B and 061201.
The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical-near-infrared source ...powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named 'macronovae' or 'kilonovae', are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst at redshift z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source GW170817 and γ-ray burst GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides.
The emergence of passive galaxies in the early Universe results from the delicate interplay among the different physical processes responsible for their rapid assembly and the abrupt shut-down of ...their star formation activity. Investigating the individual properties and demographics of early passive galaxies improves our understanding of these mechanisms. In this work we present a follow-up analysis of the
z
> 3 passive galaxy candidates selected by Merlin et al. (2019, MNRAS, 490, 3309) in the CANDELS fields. We begin by first confirming the accuracy of their passive classification by exploiting their sub-millimetre emission to demonstrate the lack of ongoing star formation. Using archival ALMA observations we are able to confirm at least 61% of the observed candidates as passive. While the remainder lack sufficiently deep data for confirmation, we are able to validate the entire sample in a statistical sense. We then estimate the stellar mass function (SMF) of all 101 passive candidates in three redshift bins from
z
= 5 to
z
= 3. We adopt a stepwise approach that has the advantage of taking into account photometric errors, mass and selection completeness issues, as well as the Eddington bias, without any a posteriori correction. We observe a pronounced evolution in the SMF around
z
∼ 4, indicating that we are witnessing the emergence of the passive population at this epoch. Massive (
M
> 10
11
M
⊙
) passive galaxies, only accounting for a small (< 10%) fraction of galaxies at
z
> 4, become dominant at later epochs. Thanks to a combination of photometric quality, sample selection, and methodology, we overall find a higher density of passive galaxies than in previous works. The comparison with theoretical predictions, despite a qualitative agreement (at least for some of the models considered), denotes a still incomplete understanding of the physical processes responsible for the formation of these galaxies. Finally, we extrapolate our results to predict the number of early passive galaxies expected in surveys carried out with future facilities.
ABSTRACT
Classical Cepheids (DCEPs) are the first fundamental step in the calibration of the cosmological distance ladder. Furthermore, they represent powerful tracers in the context of Galactic ...studies. We have collected high-resolution spectroscopy with UVES@VLT for a sample of 65 DCEPs. The majority of them are the faintest DCEPs ever observed in the Milky Way. For each target, we derived accurate atmospheric parameters, radial velocities, and abundances for 24 different species. The resulting iron abundances range between +0.3 and −1.1 dex with the bulk of stars at Fe/H ∼ −0.5 dex. Our sample includes the most metal-poor DCEPs observed so far with high-resolution spectroscopy. We complement our sample with literature data obtaining a complete sample of 637 DCEPs and use Gaia Early Data Release 3 (EDR3) photometry to determine the distance of the DCEPs from the period–Wesenheit–Metallicity relation. Our more external data trace the Outer arm at Galactocentric radius (RGC) ∼ 16–18 kpc which appears significantly warped. We investigate the metallicity gradient of the Galactic disc using this large sample, finding a slope of −0.060 ± 0.002 dex kpc−1, in very good agreement with previous results based both on DCEPs and open clusters. We also report a possible break in the gradient at RGC = 9.25 kpc with slopes of −0.063 ± 0.007 and −0.079 ± 0.003 dex kpc−1 for the inner and outer sample, respectively. The two slopes differ by more than 1σ. A more homogeneous and extended DCEPs sample is needed to further test the plausibility of such a break.
Abstract
Although many exoplanets have been indirectly detected in recent years, direct imaging of them with ground-based telescopes remains challenging. In the presence of atmospheric fluctuations, ...it is ambitious to resolve the high brightness contrasts at the small angular separation between the star and its potential partners. Post-processing of telescope images has become an essential tool to improve the resolvable contrast ratios. This paper contributes a post-processing algorithm for fast-cadence imaging, which deconvolves sequences of telescope images. The algorithm infers a Bayesian estimate of the astronomical object, as well as the atmospheric optical path length, including its spatial and temporal structures. For this, we utilize physics-inspired models for the object, the atmosphere, and the telescope. The algorithm is computationally expensive but allows us to resolve high contrast ratios despite short observation times and no field rotation. We test the performance of the algorithm with pointlike companions synthetically injected into a real data set acquired with the SHARK-VIS pathfinder instrument at the LBT telescope. Sources with brightness ratios down to 6 × 10
−4
to the star are detected at 185 mas separation with a short observation time of 0.6 s.
Context.
Classical Cepheids (DCEPs) are the most important primary indicators for the extragalactic distance scale. Establishing the dependence on metallicity of their period–luminosity and ...period–Wesenheit (PL and PW) relations has deep consequences for the estimate of the Hubble constant (
H
0
).
Aims.
We investigate the dependence on metal abundance (Fe/H) of the PL and PW relations for Galactic DCEPs.
Methods.
We combined proprietary and literature photometric and spectroscopic data, gathering a total sample of 413 Galactic DCEPs (372 fundamental mode, DCEP_F, and 41 first-overtone, DCEP_1O) and constructed new metallicity-dependent PL and PW relations in the near-infrared adopting the astrometry-based luminosity.
Results.
We find indications that the slopes of the PL(
K
S
) and PW(
J
,
K
S
) relations for Galactic DCEPs might depend on metallicity on the basis of the comparison with the Large Magellanic Cloud relationships. Therefore we used a generalized form of the PL and PW relations to simultaneously take the metallicity dependence of the slope and intercept of these relations into account.
Conclusions.
We calculated PL and PW relations that for the first time explicitly include a metallicity dependence of the slope and intercept terms. The quality of the available data is insufficient, however, and we cannot yet present conclusive results, but they are relevant from a methodological point of view. The new relations are linked to the geometric measurement of the distance to the Large Magellanic Cloud and allowed us to estimate a
Gaia
DR2 parallax zero-point offset Δ
ϖ
= 0.0615 ± 0.004 mas from the dataset of DCEPs used in this work.
Abstract
We present the results of the spectroscopic and photometric follow-up of two field galaxies that were selected as possible stellar counterparts of local high-velocity clouds. Our analysis ...shows that the two systems are distant (D > 20 Mpc) dwarf irregular galaxies unrelated to the local H i clouds. However, the newly derived distance and structural parameters reveal that the two galaxies have luminosities and effective radii very similar to the recently identified ultra diffuse galaxies (UDGs). At odds with classical UDGs, they are remarkably isolated, having no known giant galaxy within ∼2.0 Mpc. Moreover, one of them has a very high gas content compared to galaxies of similar stellar mass, with a H i
to stellar mass ratio
$M_{\rm H\, {\small {I}}}/M_{\rm \star }\sim 90$
, typical of almost-dark dwarfs. Expanding on this finding, we show that extended dwarf irregulars overlap the distribution of UDGs in the MV
versus log r
e plane and that the sequence including dwarf spheroidals, dwarf irregulars and UDGs appears as continuously populated in this plane. This may suggest an evolutionary link between dwarf irregulars and UDGs.
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