Pierre-Olivier Petrucci

XIPE: the X-ray imaging polarimetry explorer

Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2–10 keV band in 105 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15–35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time.

Multiwavelength campaign on Mrk 509 IV. Optical-UV-X-ray variability and the nature of the soft X-ray excess

We present the analysis of XMM-Newton and Swift optical-UV and X-ray observations of the Seyfert-1/QSO Mrk 509, part of an unprecedented multi-wavelength campaign, investigating the nuclear environment of this AGN. The XMM-Newton data are from a series of 10 observations of about 60 ks each, spaced from each other by about 4 days, taken in Oct.-Nov. 2009. During our campaign, Mrk 509 was also observed with Swift for a period of about 100 days, monitoring the behaviour of the source before and after the XMM-Newton observations. With these data we have established the continuum spectrum in the optical-UV and X-ray bands and investigated its variability on the timescale of our campaign with a resolution time of a few days. In order to measure and model the continuum as far as possible into the UV, we also made use of Hubble Space Telescope (HST) cosmic origin spectrograph (COS) observations of Mrk 509 (part of our coordinated campaign) and of an archival Far Ultraviolet Spectroscopic Explorer (FUSE) observation. We have found that in addition to an X-ray power-law, the spectrum displays soft X-ray excess emission below 2 keV, which interestingly varies in association with the thermal optical-UV emission from the accretion disc. The change in the X-ray power-law component flux (albeit smaller than that of the soft excess), on the other hand, is uncorrelated to the flux variability of the soft X-ray excess and the disc component on the probed timescale. The results of our simultaneous broad-band spectral and timing analysis suggest that, on a resolution time of a few days, the soft X-ray excess of Mrk 509 is produced by the Comptonisation of the thermal optical-UV photons from the accretion disc by a warm (0.2 keV) optically thick (tau similar to 17) corona surrounding the inner regions of the disc. This makes Mrk 509, with a black hole mass of about 1-3 x 10(8) M-circle dot, the highest mass known system to display such behaviour and origin for the soft X-ray excess.

Luminosity-dependent unification of active galactic nuclei and the X-ray Baldwin effect

The existence of an anti-correlation between the equivalent width (EW) of the narrow core of the iron Kline and the luminosity of the continuum (i.e. the X-ray Baldwin effect) in type-I active galactic nuclei has been confirmed over the last years by several studies carried out with XMM-Newton, Chandra and Suzaku. However, so far no general consensus on the origin of this trend has been reached. Several works have proposed the decrease of the covering factor of the molecular torus with the luminosity (in the framework of the luminosity-dependent unification models) as a possible explanation for the X-ray Baldwin effect. Using the fraction of obscured sources measured by recent X-ray and IR surveys as a proxy of the half-opening angle of the torus, and the recent Monte- Carlo simulations of the X-ray radiation reprocessed by a structure with a spherical-toroidal geometry by Ikeda et al. ( 2009) and Brightman & Nandra (2011), we test the hypothesis that the X-ray Baldwin effect is related to the decrease of the half-opening angle of the torus with the luminosity. Simulating the spectra of an unabsorbed population with a luminosity-dependent covering factor of the torus as predicted by recent X-ray surveys, we find that th is mechanism is able to explain the observed X-ray Baldwin effect. Fitting the simulated data with a log-linear L 2−10 keV− EW relation, we found that in the Seyfert regime (L 2−10 keV≤ 10 44.2 erg s −1 ) luminosity- dependent unification produces a slope consistent with the o bservations for average values of the equatorial column densities of the torus of log N T H & 23.1, and can reproduce both the slope and the intercept for log N T H ≃ 23.2. Lower values of N T H are obtained considering the decrease of the covering factor of the torus with the luminosity extrapolated from IR observations (22.9. log N T. 23). In the quasar regime (L 2−10 keV > 10 44.2 erg s −1 ) a decrease of the covering factor of the torus with the luminosity slower than that observed in the Seyfert regime (as found by recent hard X-ray surveys) is able to reproduce the observations for 23.2. log N T. 24.2.

An inhomogeneous jet model for the rapid variability of TeV blazars

ABSTRACT We present a new time-dependent inhomogeneous jet model of non-thermal blazaremission, which reproduces the entire spectral energy distribution together with therapid gamma-ray variability. Ultra-relativistic leptons are injected at the base of ajet and propagate along the jet structure. We assume continuous reacceleration andcooling, producing a relativistic quasi-maxwellian (or ”pile-up”) particle energy distri-bution. The synchrotron and Synchrotron-Self Compton jet emissivity are computedat each altitude. Klein-Nishina effects as well as intrinsic gamma-gamma absorptionare included in the computation. Due to the pair production optical depth, consider-able particle density enhancement can occur, particularly during flaring states.Time-dependent jet emission can be computed by varying the particle injection, but due tothe sensitivity of pair production process, only small variations of the injected densityare required during the flares. The stratification of the jet emission, together with apile-up distribution, allows significantly lower bulk Lorentz factors, compared to one-zone models. Applying this model to the case of PKS 2155–304 and its big TeV flareobserved in 2006, we can reproduce simultaneously the average broad band spectrumof this source as well as the TeV spectra and TeV light curve of the flare with bulkLorentz factor lower than 15.Key words: galaxies: active – BL Lacertae objects: individual: PKS 2155–304 –galaxies: jets – gamma-rays: theory – radiation mechanisms: nonthermal

Reflection at large distance from the central engine in Seyferts

We consider the possibility that most of the reflection component, observed in the hard X-ray spectra of Seyfert galaxies, could be formed on an extended medium, at large distance from the central source of primary radiation (e.g. on a torus). Then, the reflector cannot respond to the rapid fluctuations of the primary source. The observed reflected flux is controlled by the time-averaged primary spectrum rather than the instantaneous (observed) one. We show that this effect strongly influences the spectral fit parameters derived under the assumption of a reflection component consistent with the primary radiation. We find that a pivoting primary power-law spectrum with a nearly constant Comptonized luminosity may account for the reported correlation between the reflection amplitude R and the spectral index r, and simultaneously produces an iron line EW that is nearly independent of r. We emphasize the effects of the modelling of the primary component on the determination of the reflection amplitude, and show that in NGC 5548, when these effects are taken into account, the RXTE data are consistent with the reflection features being produced mainly at large distance from the central source.

The role of the disc magnetization on the hysteresis behaviour of X-ray binaries

We present a framework for understanding the dynamical and spectral properties of X-ray Binaries, where the presence of an organized large scale magnetic field plays a major role. Such a field is threading the whole accretion disk with an amplitude measured by the disk magnetization � (r, t) = B 2 z /(� oPtot), where Ptot is the total, gas and radiation, pressure. Below a transition radius rJ, a jet emitting disk (the JED) is settled and drives self-collimated non relativistic jets. Beyond rJ, no jet is produced despite the presence of the magnetic field and a standard accretion disc (the SAD) is

Photoionization instability of the Fe K absorbing plasma in the neutron star transient AX J1745.6-2901

AX J1745.6-2901 is a low-mass X-ray binary with an accreting neutron star, showing clear evidence for highly ionized absorption. Strong ionized Fe Kα and Kβ absorption lines are always observed during the soft state, whereas they disappear during the hard state. We computed photoionization stability curves for the hard and the soft states, under different assumptions on the adopted spectral energy distributions and the physical parameters of the plasma. We observe that the ionized absorber always lies on a stable branch of the photoionization stability curve during the soft state, while it becomes unstable during the hard state. This suggests that photoionization instability plays a key role in defining the observable properties of the ionized absorber. The same process might explain the disappearance of the high ionization absorber/wind during the hard state in other accreting neutron stars and black holes.

Multi-wavelength campaign on NGC 7469 - I. The rich 640 ks RGS spectrum

Aims. Outflows in active galaxies (AGNs) are common, although their launching mechanism, location, and physical impact on the host galaxy remain controversial. We conducted a multi-wavelength six-month campaign to observe the nearby Seyfert galaxy NGC 7469 with several observatories in order to better understand and quantify the outflow in this AGN.Methods. We report on the time-integrated line-resolved X-ray spectrum of NGC 7469 obtained with the Reflection Grating Spectrometer (RGS) on board XMM-Newton. We used the RGS spectrum to discern the many AGN outflow components and applied a global fit to obtain their physical parameters. Results. We find that the AGN wind can be well described by three narrow velocity components at ~–650, –950, and –2050 km s-1. The RGS clearly resolves the –2050 km s-1 component in C5+ Ly α, while the –650 km s-1 and –950 km s-1 velocities are blended. Similar velocities (±200 km s-1) are resolved in the UV. The H-equivalent column densities of these components are, respectively, NH ~ 7 × 1020, 2.2 × 1021, and 1020 cm-2, for a total of ~3 × 1021 cm-2, which was also measured in 2004, indicating the absorber did not significantly change. The –650 km s-1 component shows a broad ionization distribution (−1 ≲ log ξ ≲ 2,ξ being the ionization parameter in erg s-1 cm). We identify a photo-ionized emission component blue-shifted by ~–450 km s-1, somewhat broad (FWHM = 1400 km s-1), and with −1 ≲ log ξ ≲ 1 erg s-1 cm, which we ascribe to the same outflow that produces the absorption lines. We also find a collisionally ionized component at kT = 0.35 keV that we associate with the circum-nuclear star-formation activity of NGC 7469, as it follows the LFIR/LX ≈ 104 relation found in star forming galaxies. The elemental abundance ratios of C, N, Ne, S, and Fe to O in the outflow tend to be between one and two times solar. Preliminary estimates of the absorber distance from the AGN center suggest it is at least a few pc away from the center, but more advanced methods need to be applied in order to obtain better constraints.Conclusions. The complex X-ray spectrum of NGC 7469 demonstrates the richness of high energy phenomena taking place in AGN cores. The subtle spectroscopic differences between the various components require deep, high-resolution observations, such as the present RGS spectrum, if one is to resolve them and perform quantitative plasma diagnostics.Key words: galaxies: active / quasars: absorption lines / galaxies: Seyfert / galaxies: individual: NGC 7469

Pair models revivified for high energy emission of AGNs

We argue that although the high energy emission of radio-loud and radio-weak AGNs are markedly different, they all exhibit remarkable features (either spectral break or upper cut-off) around or just below MeV energy. The critical energy is correlated with the expected Doppler factor determined by radio characteristics of these objects. This supports the idea that high energy emission is closely linked to pair production, with absorption or annihilation features. In the case of radio-loud blasars, gamma-gamma absorption explains remarkably well the position and the magnitude of the spectral break, with very few ad-hoc assumption on the particle distribution. In the case of Seyfert galaxies, a static non thermal source, associating pair production and reacceleration, can explain the main features of high energy spectra. A self-consistent model associating anisotropic Inverse Compton process and disk reillumination predicts unambiguously the X/UV ratio, which is highly orientation dependant. We discuss some ...

VITRUV, a second-generation VLTI instrument for aperture synthesis imaging with eight telescopes

The science objectives of VITRUV is to investigate the morphology of compact astrophysical objects in optical wavelengths like the environment of AGN, star forming regions, stellar surfaces. This instrument will take full advantage of the VLTI site with 4 very large telescopes and 4 auxiliary telescopes. The instrument concept is to built aperture synthesis images like the millimeter-wave radiointerferometer of the IRAM Plateau de Bure. VITRUV coupled to the VLTI will have similar and even better resolution than ALMA. The astrophysical specifications although not yet finalized will be a temporal resolution of the order of 1 day, spectral resolution from 100 to 30,000, image dynamic from 100 to 1,000, a field of view of 1 arcsec for an initial wavlength coverage from 1 to 2.5 microns that could be extended from 0.5 to 5 microns. The technology that is contemplated at this stage is integrated optics.