arXiv subjects

C. Combet

Publications and source records attributed to C. Combet.

At least 91 records · Page 5Linked to original sources

arXiv2014

Planck 2013 results. VI. High Frequency Instrument data processing

We describe the processing of the 531 billion raw data samples from the High Frequency Instrument (hereafter HFI), which we performed to produce six temperature maps from the first 473 days of Planck-HFI survey data. These maps provide an accurate rendition of the sky emission at 100, 143, 217, 353, 545, and 857 GHz with an angular resolution ranging from 9.7 to 4.6 arcmin. The detector noise per (effective) beam solid angle is respectively, 10, 6, 12 and 39 microKelvin in HFI four lowest frequency channel (100--353 GHz) and 13 and 14 kJy/sr for the 545 and 857 GHz channels. Using the 143 GHz channel as a reference, these two high frequency channels are intercalibrated within 5% and the 353 GHz relative calibration is at the percent level. The 100 and 217 GHz channels, which together with the 143 GHz channel determine the high-multipole part of the CMB power spectrum (50 < l <2500), are intercalibrated at better than 0.2 %.

astro-ph.CO↗

arXiv2014

Planck 2013 results. I. Overview of products and scientific results

The ESA's Planck satellite, dedicated to studying the early Universe and its subsequent evolution, was launched 14 May 2009 and has been scanning the microwave and submillimetre sky continuously since 12 August 2009. This paper gives an overview of the mission and its performance, the processing, analysis, and characteristics of the data, the scientific results, and the science data products and papers in the release. The science products include maps of the CMB and diffuse extragalactic foregrounds, a catalogue of compact Galactic and extragalactic sources, and a list of sources detected through the SZ effect. The likelihood code used to assess cosmological models against the Planck data and a lensing likelihood are described. Scientific results include robust support for the standard six-parameter LCDM model of cosmology and improved measurements of its parameters, including a highly significant deviation from scale invariance of the primordial power spectrum. The Planck values for these parameters and others derived from them are significantly different from those previously determined. Several large-scale anomalies in the temperature distribution of the CMB, first detected by WMAP, are confirmed with higher confidence. Planck sets new limits on the number and mass of neutrinos, and has measured gravitational lensing of CMB anisotropies at greater than 25 sigma. Planck finds no evidence for non-Gaussianity in the CMB. Planck's results agree well with results from the measurements of baryon acoustic oscillations. Planck finds a lower Hubble constant than found in some more local measures. Some tension is also present between the amplitude of matter fluctuations derived from CMB data and that derived from SZ data. The Planck and WMAP power spectra are offset from each other by an average level of about 2% around the first acoustic peak.

astro-ph.CO↗

arXiv2014

Planck intermediate results. XIX. An overview of the polarized thermal emission from Galactic dust

This paper presents the large-scale polarized sky as seen by Planck HFI at 353 GHz, which is the most sensitive Planck channel for dust polarization. We construct and analyse large-scale maps of dust polarization fraction and polarization direction, while taking account of noise bias and possible systematic effects. We find that the maximum observed dust polarization fraction is high (pmax > 18%), in particular in some of the intermediate dust column density (AV < 1mag) regions. There is a systematic decrease in the dust polarization fraction with increasing dust column density, and we interpret the features of this correlation in light of both radiative grain alignment predictions and fluctuations in the magnetic field orientation. We also characterize the spatial structure of the polarization angle using the angle dispersion function and find that, in nearby fields at intermediate latitudes, the polarization angle is ordered over extended areas that are separated by filamentary structures, which appear as interfaces where the magnetic field sky projection rotates abruptly without apparent variations in the dust column density. The polarization fraction is found to be anti-correlated with the dispersion of the polarization angle, implying that the variations are likely due to fluctuations in the 3D magnetic field orientation along the line of sight sampling the diffuse interstellar medium.We also compare the dust emission with the polarized synchrotron emission measured with the Planck LFI, with low-frequency radio data, and with Faraday rotation measurements of extragalactic sources. The two polarized components are globally similar in structure along the plane and notably in the Fan and North Polar Spur regions. A detailed comparison of these three tracers shows, however, that dust and cosmic rays generally sample different parts of the line of sight and confirms that much of the variation observed in the Planck data is due to the 3D structure of the magnetic field.

astro-ph.GA↗

arXiv2014

Planck 2013 results. VIII. HFI photometric calibration and mapmaking

This paper describes the processing applied to the HFI cleaned time-ordered data to produce photometrically calibrated maps. HFI observes the sky over a broad range of frequencies, from 100 to 857 GHz. To get the best accuracy on the calibration on such a large range, two different photometric calibration schemes have to be used. The 545 and 857 \GHz\ data are calibrated using Uranus and Neptune flux density measurements, compared with models of their atmospheric emissions to calibrate the data. The lower frequencies (below 353 GHz) are calibrated using the cosmological microwave background dipole.One of the components of this anisotropy results from the orbital motion of the satellite in the Solar System, and is therefore time-variable. Photometric calibration is thus tightly linked to mapmaking, which also addresses low frequency noise removal. The 2013 released HFI data show some evidence for apparent gain variations of the HFI bolometers' detection chain. These variations were identified by comparing observations taken more than one year apart in the same configuration. We developed an effective correction to limit its effect on calibration, and assess its accuracy. We present several methods used to estimate the precision of the photometric calibration. We distinguish relative (from one detector to another, or from one frequency to another) and absolute uncertainties. In both cases, we found that these uncertainties range from a few $10^{-3}$ to several per cents from 100 to 857 GHz. We describe the pipeline producing the maps from the HFI timelines, based on the photometric calibration parameters and we detail the scheme used to a posteriori set the zero level of the maps. We also briefly discuss the cross-calibration between HFI and the SPIRE instrument on board Herschel. We finally summarize the basic characteristics of the set of the HFI maps from the 2013 Planck data release.

astro-ph.CO↗

arXiv2013

Planck 2013 results. IX. HFI spectral response

The Planck High Frequency Instrument (HFI) spectral response was determined through a series of ground based tests conducted with the HFI focal plane in a cryogenic environment prior to launch. The main goal of the spectral transmission tests was to measure the relative spectral response (including out-of-band signal rejection) of all HFI detectors. This was determined by measuring the output of a continuously scanned Fourier transform spectrometer coupled with all HFI detectors. As there is no on-board spectrometer within HFI, the ground-based spectral response experiments provide the definitive data set for the relative spectral calibration of the HFI. The spectral response of the HFI is used in Planck data analysis and component separation, this includes extraction of CO emission observed within Planck bands, dust emission, Sunyaev-Zeldovich sources, and intensity to polarization leakage. The HFI spectral response data have also been used to provide unit conversion and colour correction analysis tools. Verifications of the HFI spectral response data are provided through comparisons with photometric HFI flight data. This validation includes use of HFI zodiacal emission observations to demonstrate out-of-band spectral signal rejection better than 10^8. The accuracy of the HFI relative spectral response data is verified through comparison with complementary flight-data based unit conversion coefficients and colour correction coefficients. These coefficients include those based upon HFI observations of CO, dust, and Sunyaev-Zeldovich emission. General agreement is observed between the ground-based spectral characterization of HFI and corresponding in-flight observations, within the quoted uncertainty of each; explanations are provided for any discrepancies.

astro-ph.IM↗

arXiv2013

Planck 2013 results. XIII. Galactic CO emission

Rotational transition lines of CO play a major role in molecular radio astronomy and in particular in the study of star formation and the Galactic structure. Although a wealth of data exists in the Galactic plane and some well-known molecular clouds, there is no available CO high sensitivity all-sky survey to date. Such all-sky surveys can be constructed using the \Planck\ HFI data because the three lowest CO rotational transition lines at 115, 230 and 345 GHz significantly contribute to the signal of the 100, 217 and 353 GHz HFI channels respectively. Two different component separation methods are used to extract the CO maps from Planck HFI data. The maps obtained are then compared to one another and to existing external CO surveys. From these quality checks the best CO maps in terms of signal to noise and/or residual foreground contamination are selected. Three sets of velocity-integrated CO emission maps are produced: Type 1 maps of the CO (1-0), (2-1), and (3-2) rotational transitions with low foreground contamination but moderate signal-to-noise ratio; Type 2 maps for the (1-0) and (2-1) transitions with a better signal-to-noise ratio; and one Type 3 map, a line composite map with the best signal-to-noise ratio in order to locate the faintest molecular regions. The maps are described in detail. They are shown to be fully compatible with previous surveys of parts of the Galactic Plane and also of fainter regions out of the Galactic plane. The Planck HFI velocity-integrated CO maps for the (1-0), (2-1), and (3-2) rotational transitions provide an unprecedented all-sky CO view of the Galaxy. These maps are also of great interest to monitor potential CO contamination on CMB \Planck\ studies.

astro-ph.GA↗

arXiv2013

Planck 2013 results. XV. CMB power spectra and likelihood

We present the Planck likelihood, a complete statistical description of the two-point correlation function of the CMB temperature fluctuations. We use this likelihood to derive the Planck CMB power spectrum over three decades in l, covering 2 <= l <= 2500. The main source of error at l <= 1500 is cosmic variance. Uncertainties in small-scale foreground modelling and instrumental noise dominate the error budget at higher l's. For l < 50, our likelihood exploits all Planck frequency channels from 30 to 353 GHz through a physically motivated Bayesian component separation technique. At l >= 50, we employ a correlated Gaussian likelihood approximation based on angular cross-spectra derived from the 100, 143 and 217 GHz channels. We validate our likelihood through an extensive suite of consistency tests, and assess the impact of residual foreground and instrumental uncertainties on cosmological parameters. We find good internal agreement among the high-l cross-spectra with residuals of a few uK^2 at l <= 1000. We compare our results with foreground-cleaned CMB maps, and with cross-spectra derived from the 70 GHz Planck map, and find broad agreement in terms of spectrum residuals and cosmological parameters. The best-fit LCDM cosmology is in excellent agreement with preliminary Planck polarisation spectra. The standard LCDM cosmology is well constrained by Planck by l <= 1500. For example, we report a 5.4 sigma deviation from n_s /= 1. Considering various extensions beyond the standard model, we find no indication of significant departures from the LCDM framework. Finally, we report a tension between the best-fit LCDM model and the low-l spectrum in the form of a power deficit of 5-10% at l <~ 40, significant at 2.5-3 sigma. We do not elaborate further on its cosmological implications, but note that this is our most puzzling finding in an otherwise remarkably consistent dataset. (Abridged)

astro-ph.CO↗

arXiv2012

Disentangling cosmic-ray and dark-matter induced gamma-rays in galaxy clusters?

Galaxy clusters are among the best targets for indirect dark matter detection in gamma-rays, despite the large astrophysical background expected from these objects. Detection is now within reach of current observatories (Fermi-LAT or Cerenkov telescopes); however, assessing the origin of this signal might be difficult. We investigate whether the behaviour of the number of objects per `flux' bin (logN-logF) and that of the stacked signal could be used as a signature of the dominant process at stake.We use the CLUMPY code to integrate the signal from decaying or annihilating dark matter and cosmic rays along the line of sight. We assume the standard NFW profile for the dark matter density and rely on a parametrised emissivity for the cosmic-ray component. In this context, the consequences of stacking are explored using the MCXC meta-catalogue of galaxy clusters. We find the value of the slope of the logN-logF power law (or the increase of the signal with the number of stacked objects) to be a clear diagnosis to disentangle decaying dark matter from cosmic-ray induced gamma-rays. For dark matter annihilation, depending on the signal boost from the substructures, it is either similar to the cosmic-ray signal (no boost) or similar to the decay case (large boosts). The shift between the brightest object and its followers also depends on the signal origin. For annihilation, this shift and the stacked signal are poorly constrained because of the large uncertainty affecting the boost. We also underline that the angular dependence of the annihilation signal is not universal because of the substructure contribution.

astro-ph.HE↗

arXiv2012

gamma-rays from annihilating dark matter in galaxy clusters: stacking vs single source analysis

Clusters of galaxies are potentially important targets for indirect searches for dark matter annihilation. Here we reassess the detection prospects for annihilation in massive halos, based on a statistical investigation of 1743 clusters in the new Meta-Catalog of X-ray Clusters. We derive a new limit for the extra-galactic dark matter annihilation background of at least 20% of that originating from the Galaxy for an integration angle of 0.1 deg. The number of clusters scales as a power law with their brightness, suggesting that stacking may provide a significant improvement over a single target analysis. The mean angle containing 80% of the dark-matter signal for the sample is ~0.15 deg, indicating that instruments with this angular resolution or better would be optimal for a cluster annihilation search based on stacking. A detailed study based on the Fermi-LAT performance and position-dependent background, suggests that stacking may result in a factor ~2 improvement in sensitivity, depending on the source selection criteria. Based on the expected performance of CTA, we find no improvement with stacking, due to the requirement for pointed observations. We note that several potentially important targets: Opiuchius, A2199, A3627 (Norma) and CIZAJ1324.7-5736 may be disfavoured due to a poor contrast with respect to the Galactic dark-matter signal. The use of the homogenised MCXC meta-catalogue provides a robust ranking of the targets, although the absolute value of their signal depends on the exact dark matter substructure content. For conservative assumptions, we find that galaxy clusters (with or without stacking) can probe down to 1e-25-1e-24 cm3/s for dark matter masses in the range 10 GeV-100 GeV. For more favourable substructure configurations, ~1e-26 cm3/s may be reached.

astro-ph.HE↗

arXiv2012

Decaying dark matter: a stacking analysis of galaxy clusters to improve on current limits

We show that a stacking approach to galaxy clusters can improve current limits on decaying dark matter by a factor $\gtrsim 5-100$, with respect to a single source analysis, for all-sky instruments such as Fermi-LAT. Based on the largest sample of X-ray-selected galaxy clusters available to date (the MCXC meta-catalogue), we provide all the astrophysical information, in particular the astrophysical term for decaying dark matter, required to perform an analysis with current instruments.

astro-ph.CO↗

arXiv2012

CLUMPY: a code for gamma-ray signals from dark matter structures

We present the first public code for semi-analytical calculation of the gamma-ray flux astrophysical J-factor from dark matter annihilation/decay in the Galaxy, including dark matter substructures. The core of the code is the calculation of the line of sight integral of the dark matter density squared (for annihilations) or density (for decaying dark matter). The code can be used in three modes: i) to draw skymaps from the Galactic smooth component and/or the substructure contributions, ii) to calculate the flux from a specific halo (that is not the Galactic halo, e.g. dwarf spheroidal galaxies) or iii) to perform simple statistical operations from a list of allowed DM profiles for a given object. Extragalactic contributions and other tracers of DM annihilation (e.g. positrons, antiprotons) will be included in a second release.

astro-ph.HE↗

arXiv2011

Dark matter profiles and annihilation in dwarf spheroidal galaxies: prospectives for present and future gamma-ray observatories - I. The classical dSphs

Due to their large dynamical mass-to-light ratios, dwarf spheroidal galaxies (dSphs) are promising targets for the indirect detection of dark matter (DM) in gamma-rays. We examine their detectability by present and future gamma-ray observatories. The key innovative features of our analysis are: (i) We take into account the angular size of the dSphs; while nearby objects have higher gamma ray flux, their larger angular extent can make them less attractive targets for background-dominated instruments. (ii) We derive DM profiles and the astrophysical J-factor (which parameterises the expected gamma-ray flux, independently of the choice of DM particle model) for the classical dSphs directly from photometric and kinematic data. We assume very little about the DM profile, modelling this as a smooth split-power law distribution, with and without sub-clumps. (iii) We use a Markov Chain Monte Carlo (MCMC) technique to marginalise over unknown parameters and determine the sensitivity of our derived J-factors to both model and measurement uncertainties. (iv) We use simulated DM profiles to demonstrate that our J-factor determinations recover the correct solution within our quoted uncertainties. Our key findings are: (i) Sub-clumps in the dSphs do not usefully boost the signal; (ii) The sensitivity of atmospheric Cherenkov telescopes to dSphs within 20 kpc with cored halos can be up to ~50 times worse than when estimated assuming them to be point-like. Even for the satellite-borne Fermi-LAT the sensitivity is significantly degraded on the relevant angular scales for long exposures, hence it is vital to consider the angular extent of the dSphs when selecting targets; (iii) No DM profile has been ruled out by current data, but using a prior on the inner dark matter cusp slope 0<=gamma<=1 provides J-factor estimates accurate to a factor of a few if an appropriate angular scale [abridged]

astro-ph.HE↗

arXiv2011

Dark matter in the classical dwarf spheroidal galaxies: a robust constraint on the astrophysical factor for gamma-ray flux calculations

We present a new analysis of the relative detectability of dark matter annihilation in the Milky Way's eight classical dwarf spheroidal satellite galaxies. Ours is similar to previous analyses in that we use Markov-Chain Monte Carlo techniques to fit dark matter halo parameters to empirical velocity dispersion profiles via the spherical Jeans equation, but more general in the sense that we do not adopt priors derived from cosmological simulations. We show that even without strong constraints on the shapes of dSph dark matter density profiles (we require only that the inner profile satisfies -lim(r->0) [dlnrho/dln r] <=1), we obtain a robust and accurate constraint on the astrophysical component of a prospective dark matter annihilation signal, provided that the integration angle is approximately twice the projected half-light radius of the dSph divided by distance to the observer, alpha_int~2rh/d. Using this integration angle, which represents a compromise between maximizing prospective flux and minimizing uncertainty in the dSph's dark matter distribution, we calculate the relative detectability of the classical dSphs by ground- and space-based gamma-ray observatories.

astro-ph.HE↗

arXiv2009

The outburst duration and duty-cycle of GRS 1915+105

The extraordinarily long outburst of GRS 1915+105 makes it one of the most remarkable low-mass X-ray binaries (LMXBs). It has been in a state of constant outburst since its discovery in 1992, an eruption which has persisted ~100 times longer than those of more typical LXMBs. The long orbital period of GRS 1915+105 implies that it contains large and massive accretion disc which is able to fuel its extreme outburst. In this paper, we address the longevity of the outburst and quiescence phases of GRS 1915+105 using Smooth Particle Hydrodynamics (SPH) simulations of its accretion disc through many outburst cycles. Our model is set in the two-alpha framework and includes the effects of the thermo-viscous instability, tidal torques, irradiation by central X-rays and wind mass loss. We explore the model parameter space and the examine the impact of the various ingredients. We predict that the outburst of GRS 1915+105 should last a minimum of 20 years and possibly up to ~100 years if X-ray irradiation is very significant. The predicted recurrence times are of the order of 10^4 years, making the X-ray duty cycle a few 0.1%. Such a low duty cycle may mean that GRS 1915+105 is not an anomaly among the more standard LMXBs and that many similar, but quiescent, systems could be present in the Galaxy.

astro-ph.SR↗

arXiv2009

Primordial Globular Clusters, X-Ray Binaries & Cosmological Reionisation

Globular clusters are dense stellar systems that have typical ages of ~13 billion years, implying that they formed at redshifts of z>~6. Massive stars in newly formed or primordial globular clusters could have played an important role during the epoch of cosmological reionisation (z>~6) as sources of energetic, neutral hydrogen ionising UV photons. We investigate whether or not these stars could have been as important in death as sources of energetic X-ray photons as they were during their main sequence lives. Most massive stars are expected to form in binaries, and an appreciable fraction of these (as much as ~30%) will evolve into X-ray luminous (L_X~10^38 erg/s) high-mass X-ray binaries (HMXBs). These sources would have made a contribution to the X-ray background at z>~6. Using Monte Carlo models of a globular cluster, we estimate the total X-ray luminosity of a population of HMXBs. We compare and contrast this with the total UV luminosity of the massive stars during their main sequence lives. For reasonable estimates, we find that the bolometric luminosity of the cluster peaks at ~10^42 erg/s during the first few million years, but declines to ~10^41 erg/s after ~5 million years as the most massive stars evolve off the main sequence. From this time onwards, the total bolometric luminosity is dominated by HMXBs and falls gradually to ~10^40 erg/s after ~50 million years. Assuming a power-law spectral energy distribution for the HMXBs, we calculate the effective number of neutral hydrogen ionisations per HMXB and show that HMXBs can be as important as sources of ionising radiation as massive stars. Finally we discuss the implications of our results for modelling galaxy formation at high redshift and the prospects of using globular clusters as probes of reionisation.

astro-ph.CO↗

arXiv2009

Transport of exotic anti-nuclei: II- Antiproton and Antideuteron astrophysical uncertainties

We use a 1D propagation model to study the dependence of the pbar and dbar exotic fluxes on the transport parameters. The simple analytical solutions allow us i) to clarify the origin of the astrophysical uncertainties, and ii) to compare two models used for {\em signal} predictions, namely the constant and the linear Galactic wind models. We also study how these uncertainties should be reduced using forthcoming nuclear cosmic ray data. We confirm that the degeneracy of the transport parameters for a given propagation model leads to very different fluxes for primary antinuclei (~10^2). However, we show that with forthcoming data, these uncertainties could be greatly reduced (~2). As the precision will increase, the astrophysical uncertainty could then be dominated by our ignorance of the correct spatial dependence for some of the transport parameters: for instance, the constant and the linear wind models do not predict the same amount of exotic pbar at low energy.

astro-ph↗

arXiv2007

The radial structure of protostellar accretion disks: influence of jets

The radial structure of accretion disks is a fundamental issue regarding star and planet formation. Many theoretical studies, focussing on different aspects such as e.g. disk emissivity or ionization, have been conducted in the context of the Standard Accretion Disk (SAD) model, where no jet is present. We wish to calculate the structure of YSO accretion disks in an approach that takes into account the presence of the protostellar jets. The radial structure of these Jet Emitting Disks (JED) should then be compared to that of standard accretion disks. The analytical treatment used in this work is very similar to that of standard accretion disks but is using the parameter space of Magnetised Accretion-Ejection Structures that include the jet torque on the underlying disk. In this framework, the analytical expressions of key quantities, such as mid-plane temperatures, surface densities or disk aspect ratio are derived. It is found that JEDs present a structure very different from the SADs and that can be observationally tested. The implications on planet formation in the inner regions of accretion disks are briefly discussed. We also supply sets of analytical formulae, valid in different opacity regimes, for the disk quantities. These expressions can be readily used for any work where the disk structure is needed as an input for the model.

astro-ph↗

arXiv2007

Transport of exotic anti-nuclei: I- Fast formulae for antiproton fluxes

The Galactic secondary cosmic ray anti-proton flux calculated with different propagation models is fairly consistent with data, and the associated propagation uncertainty is small. This is not the case for any anti-proton exotic component of the dark matter halo. Detailed propagation models are mandatory if the ultimate goal is to explain an excess. However, simpler and faster approximate formulae for anti-protons are an attractive alternative to quickly check that a given dark matter model is not inconsistent with the anti-proton observed flux. This paper provides such formulae. In addition, they could be used to put constraints on new physics in this channel, where an extensive scan of a large parameter space could otherwise be quite expensive in computer ressources.

astro-ph↗