F. Gastaldello

A Chandra View of Dark Matter in Early-Type Galaxies

We present a Chandra study of mass profiles in seven elliptical galaxies, of which three have galaxy-scale and four have group-scale halos, demarcated at 1013 M☉. These represent the best available data for nearby objects with comparable X-ray luminosities. We measure approximately flat mass-to-light (M/L) profiles within an optical half-light radius (Reff), rising by an order of magnitude at ~10 Reff, which confirms the presence of dark matter (DM). The data indicate hydrostatic equilibrium, which is also supported by agreement with studies of stellar kinematics in elliptical galaxies. The data are well fitted by a model comprising an NFW DM profile and a baryonic component following the optical light. The distribution of DM halo concentration parameters (c) versus Mvir agrees with ΛCDM predictions and our observations of bright groups. Concentrations are slightly higher than expected, which is most likely a selection effect. Omitting the stellar mass drastically increases c, possibly explaining large concentrations found by some past observers. The stellar M/LK agree with population synthesis models, assuming a Kroupa IMF. Allowing adiabatic compression (AC) of the DM halo by baryons made M/L more discrepant, casting some doubt on AC. Our best-fitting models imply total baryon fractions ~0.04-0.09, consistent with models of galaxy formation incorporating strong feedback. The groups exhibit positive temperature gradients, consistent with the universal profiles found in other groups and clusters, whereas the galaxies have negative gradients, suggesting a change in the evolutionary history of the systems around Mvir 1013 M☉.

The X-Ray Concentration-Virial Mass Relation

We present the concentration (c)-virial mass (M) relation of 39 galaxy systems ranging in mass from individual early-type galaxies up to the most massive galaxy clusters, (0.06-20) × 1014 M☉. We selected for analysis the most relaxed systems possessing the highest quality data currently available in the Chandra and XMM-Newton public data archives. A power-law model fitted to the X-ray c-M relation requires at high significance (6.6 σ) that c decreases with increasing M, which is a general feature of CDM models. The median and scatter of the c-M relation produced by the flat, concordance ΛCDM model (Ωm = 0.3, σ8 = 0.9) agrees with the X-ray data, provided that the sample is comprised of the most relaxed, early-forming systems, which is consistent with our selection criteria. When allowing only σ8 to vary in the concordance model, the c-M relation requires 0.76 99% confidence) both open CDM models and flat CDM models with Ωm ≈ 1. This result provides novel evidence for a flat, low-Ωm universe with dark energy using observations only in the local (z 1) universe. Possible systematic errors in the X-ray mass measurements of a magnitude ≈10% suggested by CDM simulations do not change our conclusions.

Probing the Dark Matter and Gas Fraction in Relaxed Galaxy Groups with X-Ray Observations from Chandra and XMM-Newton

We present radial mass profiles within ~0.3rvir for 16 relaxed galaxy groups—poor clusters (kT range 1-3 keV) selected for optimal mass constraints from the Chandra and XMM-Newton data archives. After accounting for the mass of hot gas, the resulting mass profiles are described well by a two-component model consisting of dark matter, represented by an NFW model, and stars from the central galaxy. The stellar component is required only for eight systems, for which reasonable stellar mass-to-light ratios (M/LK) are obtained, assuming a Kroupa IMF. Modifying the NFW dark matter halo by adiabatic contraction does not improve the fit and yields systematically lower M/LK. In contrast to previous results for massive clusters, we find that the NFW concentration parameter (cvir) for groups decreases with increasing Mvir and is inconsistent with no variation at the 3 σ level. The normalization and slope of the cvir-Mvir relation are consistent with the standard ΛCDM cosmological model with σ8 = 0.9 (considering a 10% bias for early forming systems). The small intrinsic scatter measured about the cvir-Mvir relation implies that the groups represent preferentially relaxed, early forming systems. The mean gas fraction (f = 0.05 ± 0.01) of the groups measured within an overdensity Δ = 2500 is lower than for hot, massive clusters, but the fractional scatter (σf/f = 0.2) for groups is larger, implying a greater impact of feedback processes on groups, as expected.

Mass profiles and concentration-dark matter relation in X-ray luminous galaxy clusters

Context. Galaxy clusters represent valuable cosmological probes us ing tests that mainly rely on measurements of cluster masses and baryon fractions. X-ray observations represent one of the m ain tools for uncovering these quantities. Aims. We aim to constrain the cosmological parameters Ωm andσ8 using the observed distribution of the both values of the con centrations and dark mass within R200 and of the gas mass fraction within R500. Methods. We applied two different techniques to recover the profiles t he gas and dark mass, described according to the Navarro, Frenk & White (1997, ApJ, 490, 493) functional form, of a samp le of 44 X-ray luminous galaxy clusters observed with XMM-Newton in the redshift range0.1− 0.3. We made use of the spatially resolved spectroscopic data an d of the PSF–deconvolved surface brightness and assumed that hydrostatic equilibrium holds betwee n the intracluster medium and the gravitational potential. We evaluated several systematic uncertainties that affect our reconstr uction of the X-ray masses. Results. We measured the concentration c200, the dark massM200 and the gas mass fraction in all the objects of our sample, pro viding the largest dataset of mass parameters for galaxy clu sters in the redshift range 0.1 − 0.3. We confirm that a tight correlation betweenc200 andM200 is present and in good agreement with the predictions from nu erical simulations and previous observations. When we consider a subsample of relaxed clusters that host a l w entropy core, we measure a flatter c − M relation with a total scatter that is lower by 40 per cent. We conclude, however, th at the slope of thec−M relation cannot be reliably determined from the fitting over a narrow mass range as the one considered in the pr es nt work. From the distribution of the estimates of c200 andM200, with associated statistical (15–25%) and systematic (5–15 %) errors, we used the predicted values from semi-analytic p rescriptions calibrated through N-body numerical runs and obtain σ8 Ω m = 0.45 ± 0.01 (at 2σ level, statistical only) for the subsample of the clusters where the mass reconstruction has been obtai ned more robustly and σ8 Ω m = 0.39 ± 0.02 for the subsample of the 11 more relaxed LEC objects. With the further constrai n from the gas mass fraction distribution in our sample, we b reak the degeneracy in the σ8 − Ωm plane and obtain the best-fit values σ8 ≈ 1.0± 0.2 (0.83± 0.1 when the subsample of the more relaxed objects is considered) and Ωm = 0.26 ± 0.02. Conclusions. We demonstrate that the analysis of the distribution of the c200 −M200 − fgas values represents a mature and competitive technique in the present era of precision cosmology, e ven though it needs more detailed analysis of the output of la rger sets of cosmological numerical simulations to provide definitive a nd robust results.

The gas distribution in the outer regions of galaxy clusters

Aims. We present our analysis of a local (z = 0.04-0.2) sample of 31 galaxy clusters with the aim of measuring the density of the X-ray emitting gas in cluster outskirts. We compare our results with numerical simulations to set constraints on the azimuthal symmetry and gas clumping in the outer regions of galaxy clusters.

Mass profiles and c − MDM relation in X-ray luminous galaxy clusters

Context. Galaxy clusters represent valuable cosmological probes using tests that mainly rely on measurements of cluster masses and baryon fractions. X-ray observations represent one of the main tools for uncovering these quantities. Aims. We aim to constrain the cosmological parameters Ωm and σ8 using the observed distribution of the both values of the concentrations and dark mass within R200 and of the gas mass fraction within R500. Methods. We applied two different techniques to recover the profiles the gas and dark mass, described according to the Navarro, Frenk & White (1997, ApJ, 490, 493) functional form, of a sample of 44 X-ray luminous galaxy clusters observed with XMM-Newton in the redshift range 0.1−0.3. We made use of the spatially resolved spectroscopic data and of the PSF–deconvolved surface brightness and assumed that hydrostatic equilibrium holds between the intracluster medium and the gravitational potential. We evaluated several systematic uncertainties that affect our reconstruction of the X-ray masses. Results. We measured the concentration c200, the dark mass M200 and the gas mass fraction in all the objects of our sample, providing the largest dataset of mass parameters for galaxy clusters in the redshift range 0.1−0.3. We confirm that a tight correlation between c200 and M200 is present and in good agreement with the predictions from numerical simulations and previous observations. When we consider a subsample of relaxed clusters that host a low entropy core, we measure a flatter c − M relation with a total scatter that is lower by 40 per cent. We conclude, however, that the slope of the c − M relation cannot be reliably determined from the fitting over a narrow mass range as the one considered in the present work. From the distribution of the estimates of c200 and M200, with associated statistical (15–25%) and systematic (5–15%) errors, we used the predicted values from semi-analytic prescriptions calibrated through N-body numerical runs and obtain σ8 Ω 0.60±0.03 m = 0.45 ± 0.01 (at 2σ level, statistical only) for the subsample of the clusters where the mass reconstruction has been obtained more robustly and σ8 Ω 0.56±0.04 m = 0.39 ± 0.02 for the subsample of the 11 more relaxed LEC objects. With the further constraint from the gas mass fraction distribution in our sample, we break the degeneracy in the σ8 − Ωm plane and obtain the best-fit values σ8 ≈ 1.0 ± 0. 2( 0.83 ± 0.1 when the subsample of the more relaxed objects is considered) and Ωm = 0.26 ± 0.02. Conclusions. Analysis of the distribution of the c200 − M200 − fgas values represents a mature and competitive technique in the present era of precision cosmology, even though it needs more detailed analysis of the output of larger sets of cosmological numerical simulations to provide definitive and robust results.

NuSTAR Observations of the Bullet Cluster: Constraints on Inverse Compton Emission

The search for diffuse non-thermal inverse Compton (IC) emission from galaxy clusters at hard X-ray energies has been undertaken with many instruments, with most detections being either of low significance or controversial. Because all prior telescopes sensitive at E > 10 keV do not focus light and have degree-scale fields of view, their backgrounds are both high and difficult to characterize. The associated uncertainties result in lower sensitivity to IC emission and a greater chance of false detection. In this work, we present 266 ks NuSTAR observations of the Bullet cluster, which is detected in the energy range 3-30 keV. NuSTARs unprecedented hard X-ray focusing capability largely eliminates confusion between diffuse IC and point sources; however, at the highest energies, the background still dominates and must be well understood. To this end, we have developed a complete background model constructed of physically inspired components constrained by extragalactic survey field observations, the specific parameters of which are derived locally from data in non-source regions of target observations. Applying the background model to the Bullet cluster data, we find that the spectrum is well—but not perfectly—described as an isothermal plasma with kT = 14.2 ± 0.2 keV. To slightly improve the fit, a second temperature component is added, which appears to account for lower temperature emission from the cool core, pushing the primary component to kT ~ 15.3 keV. We see no convincing need to invoke an IC component to describe the spectrum of the Bullet cluster, and instead argue that it is dominated at all energies by emission from purely thermal gas. The conservatively derived 90% upper limit on the IC flux of 1.1 × 10^(–12) erg s^(–1) cm^(–2) (50-100 keV), implying a lower limit on B ≳ 0.2 μG, is barely consistent with detected fluxes previously reported. In addition to discussing the possible origin of this discrepancy, we remark on the potential implications of this analysis for the prospects for detecting IC in galaxy clusters in the future.

An XMM-Newton Study of the 401 Hz Accreting Pulsar SAX J1808.4-3658 in Quiescence

SAX J1808.4-3658 is a unique source, being the first low-mass X-ray binary showing coherent pulsations at a spin period comparable to that of millisecond radio pulsars. Here we present an XMM-Newton observation of SAX J1808.4-3658 in quiescence, the first that assessed its quiescent luminosity and spectrum with a good signal-to-noise ratio. XMM-Newton did not reveal other sources in the vicinity of SAX J1808.4-3658, likely indicating that the source was also detected by previous BeppoSAX and ASCA observations, even with large positional and flux uncertainties. We derive a 0.5-10 keV unabsorbed luminosity of LX = 5 ? 1031 ergs s-1, a relatively low value compared with other neutron star soft X-ray transient sources. At variance with other soft X-ray transients, the quiescent spectrum of SAX J1808.4-3658 was dominated by a hard (? ~ 1.5) power law with only a minor contribution (10%) from a soft blackbody component. If the power law originates in the shock between the wind of a turned-on radio pulsar and matter outflowing from the companion, then a spin-down to an X-ray luminosity conversion efficiency of ? ~ 10-3 is derived; this is in line with the value estimated from the eclipsing radio pulsar PSR J1740-5340. Within the deep crustal heating model, the faintness of the blackbody-like component indicates that SAX J1808.4-3658 likely hosts a massive neutron star (M 1.7 M?).

CONFIRMATION OF X-RAY ABSORPTION BY WARM-HOT INTERGALACTIC MEDIUM IN THE SCULPTOR WALL

In a previous paper, we reported a 3σ detection of an absorption line from the warm-hot intergalactic medium (WHIM) using the Chandra and XMM X-ray grating spectra of the blazar H2356-309, the sight line of which intercepts the Sculptor Wall, a large-scale superstructure of galaxies at z ∼ 0.03. To verify our initial detection, we obtained a deep (500 ks), follow-up exposure of H2356-309 as part of the Cycle-10Chandra Large Project Program. From a joint analysis of the Cycle-10 and previous (Cycle-8) Chandra grating data we detect the redshifted Ovii WHIM line at a significance level of 3.4σ , a substantial improvement over the 1.7σ level reported previously when using only the Cycle-8 data. The significance increases to 4.0σ when the existing XMM grating data are included in the analysis, thus confirming at higher significance the existence of the line at the redshift of the Sculptor Wall with an equivalent width of 28.5 ± 10.5 mA (90% confidence). We obtain a 90% lower limit on the Ovii column density of 0.8 × 10 16 cm −2 and a 90% upper limit on the Doppler b parameter of 460 km s −1 . Assuming the absorber is uniformly distributed throughout the ∼15 Mpc portion of the blazar’s sight line that intercepts the Sculptor Wall, that the Ovii column density is ≈2 × 10 16 cm −2 (corresponding to b 150 km −1 where the inferred column density is only weakly dependent on b), and that the oxygen abundance is 0.1 solar, we estimate a baryon over-density of ∼30 for the WHIM, which is consistent with the peak of the WHIM mass fraction predicted by cosmological simulations. The clear detection of Ovii absorption in the Sculptor Wall demonstrates the viability of using current observatories to study WHIM in the X-ray absorption spectra of blazars behind known large-scale structures.

A variable absorption feature in the X-ray spectrum of a magnetar

Soft-γ-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly rotating, isolated neutron stars that sporadically undergo episodes of long-term flux enhancement (outbursts) generally accompanied by the emission of short bursts of hard X-rays. This behaviour can be understood in the magnetar model, according to which these sources are mainly powered by their own magnetic energy. This is supported by the fact that the magnetic fields inferred from several observed properties of SGRs and AXPs are greater than—or at the high end of the range of—those of radio pulsars. In the peculiar case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing parameters, whereas a strong field has been proposed to reside in the stellar interior and in multipole components on the surface. Here we show that the X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which depend strongly on the star’s rotational phase. This line is interpreted as a proton cyclotron feature and its energy implies a magnetic field ranging from 2 × 1014 gauss to more than 1015 gauss.