A changing-look AGN to be probed by X-ray polarimetry

Active galactic nuclei (AGN) produce the highest intrinsic luminosities in the Universe from within a compact region. The central engine is thought to be powered by accretion onto a supermassive black hole. A fraction of this huge release of energy influences the evolution of the host galaxy, and in particular, star formation. Thus, AGN are key astronomical sources not only because they play an important role in the evolution of the Universe, but also because they constitute a laboratory for extreme physics. However, these objects are under the resolution limit of current telescopes. Polarimetry is a unique technique capable of providing us with information on physical AGN structures. The incoming new era of X-ray polarimetry will give us the opportunity to explore the geometry and physical processes taking place in the innermost regions of the accretion disc. Here we exploit this future powerful tool in the particular case of changing-look AGN, which are key for understanding the complexity of AGN physics.Comment: 9 pages, 1 figures, published by Galaxies under the special issue "The Bright Future of Astronomical X-ray Polarimetry

The Cotton, Simon-Mars and Cotton-York Tensors in Stationary Spacetimes

The Cotton-York and Simon-Mars tensors in stationary vacuum spacetimes are studied in the language of the congruence approach pioneered by Hawking and Ellis. Their relationships with the Papapetrou field defined by the stationary Killing congruence and with a recent characterization of the Kerr spacetime in terms of the alignment between of the principal null directions of the Weyl tensor with those of the Papapetrou field are also investigated in this more transparent language.Comment: 14 pages latex(2e) iopart style, no figure

The ionised X-ray outflowing torus in ESO 323-G77: low-ionisation clumps confined by homogeneous warm absorbers

We report on the long- and short-term X-ray spectral analysis of the polar-scattered Seyfert 1.2 galaxy ESO 323-G77, observed in three epochs between 2006 and 2013 with Chandra and XMM-Newton. Four high-resolution Chandra observations give us a unique opportunity to study the properties of the absorbers in detail, as well as their short time-scale (days) variability. From the rich set of absorption features seen in the Chandra data, we identify two warm absorbers with column densities and ionisations that are consistent with being constant on both short and long time-scales, suggesting that those are the signature of a rather homogeneous and extended outflow. A third absorber, ionised to a lesser degree, is also present and it replaces the strictly neutral absorber that is ubiquitously inferred from the X-ray analysis of obscured Compton-thin sources. This colder absorber appears to vary in column density on long time-scales, suggesting a non-homogeneous absorber. Moreover, its ionisation responds to the nuclear luminosity variations on time-scales as short as a few days, indicating that the absorber is in photoionisation equilibrium with the nuclear source on these time-scales. All components are consistent with being co-spatial and located between the inner and outer edges of the so-called dusty, clumpy torus. Assuming co-spatiality, the three phases also share the same pressure, suggesting that the warm / hot phases confine the colder, most likely clumpy, medium. We discuss further the properties of the outflow in comparison with the lower resolution XMM-Newton data.Comment: 16 pages, 7 figures, 6 tables. Accepted for publication in MNRA

Gravitational waves from newly born, hot neutron stars

We study the gravitational radiation associated to the non--radial oscillations of newly born, hot neutron stars. The frequencies and damping times of the relevant quasi--normal modes are computed for two different models of proto--neutron stars, at different times of evolution, from its birth until it settles down as a cold neutron star. We find that the oscillation properties of proto--neutron stars are remarkably different from those of their cold, old descendants and that this affects the characteristic features of the gravitational signal emitted during the post-collapse evolution. The consequences on the observability of these signals by resonant--mass and interferometric detectors are analyzed. We find that gravitational waves from the pulsations of a newborn proto--neutron star in the galaxy could be detected with a signal to noise ratio of 5 by the first generation interferometers, if the energy stored in the modes is greater than $\sim 10^{-8} M_\odot c^2$, or by a resonant antenna if it is greater than $\sim 10^{-4} M_\odot c^2$. In addition since at early times the frequency of the spacetime modes is much lower than that of a cold neutron star, they would be also detectable with the same signal to noise ratio if a comparable amount of energy is radiated into these modes.Comment: Minor changes in Section 4.1, Table 3 and Figure 5. Accepted for publication in MNRA

SUZAKU BROADBAND SPECTROSCOPY OF SWIFT J1753.5–0127 IN THE LOW-HARD STATE

We present Suzaku observations of the Galactic black hole candidate Swift J1753.5–0127 in the low-hard state (LHS). The broadband coverage of Suzaku enables us to detect the source over the energy range 0.6-250 keV. The broadband spectrum (2-250 keV) is found to be consistent with a simple power-law (Γ ~ 1.63). In agreement with previous observations of this system, a significant excess of soft X-ray flux is detected consistent with the presence of a cool accretion disk. Estimates of the disk inner radius infer a value consistent with the innermost stable circular orbit (ISCO; R [subscript in] [< over ~] 6R[subscript g] , for certain values of, e.g., N [subscript H], i), although we cannot conclusively rule out the presence of an accretion disk truncated at larger radii (R [subscript in] ~ 10-50R[subscript g] ). A weak, relativistically broadened iron line is also detected, in addition to disk reflection at higher energy. However, the iron-K line profile favors an inner radius larger than the ISCO (R [subscript in] ~ 10-20R[subscript g] ). The implications of these observations for models of the accretion flow in the LHS are discussed.United States. National Aeronautics and Space AdministrationJapan Aerospace Exploration AgencyGoddard Space Flight CenterUnited States. National Aeronautics and Space Administration (Grant NNX08AC20G

The size of the X-ray emitting region in SWIFT J2127.4+5654 via a broad line region cloud X-ray eclipse

We present results obtained from the time-resolved X-ray spectral analysis of the Narrow-Line-Seyfert 1 galaxy SWIFT J2127.4+5654 during a ~130 ks XMM-Newton observation. We reveal large spectral variations, especially during the first ~90 ks of the XMM-Newton exposure. The spectral variability can be attributed to a partial eclipse of the X-ray source by an intervening low-ionization/cold absorbing structure (cloud) with column density N_H = 2.0^{+0.2}_{-0.3}e22 cm^-2 which gradually covers and then uncovers the X-ray emitting region with covering fraction ranging from zero to ~43 per cent. Our analysis enables us to constrain the size, number density, and location of the absorbing cloud with good accuracy. We infer a cloud size (diameter) of $D_c < 1.5e13 cm, corresponding to a density of n_c > 1.5e9 cm^-3 at a distance of R_c > 4.3e16 cm from the central black hole. All of the inferred quantities concur to identify the absorbing structure with one single cloud associated with the broad line region of SWIFT J2127.4+5654. We are also able to constrain the X-ray emitting region size (diameter) to be D_s < 2.3e13 cm which, assuming the black hole mass estimated from single-epoch optical spectroscopy (1.5e7 M_sun), translates into D_s < 10.5 gravitational radii (r_g) with larger sizes (in r_g) being associated with smaller black hole masses, and viceversa. We also confirm the presence of a relativistically distorted reflection component off the inner accretion disc giving rise to a broad relativistic Fe K emission line and small soft excess (small because of the high Galactic column density), supporting the measurement of an intermediate black hole spin in SWIFT J2127.4+5654 that was obtained from a previous Suzaku observation.Comment: 8 pages, 7 figures, accepted for publication in MNRA

Spectral energy distribution of hyperluminous infrared galaxies

Aims. The relationship between star formation and super-massive black hole growth is central to our understanding of galaxy formation and evolution. Hyperluminous infrared galaxies (HLIRG) are unique laboratories to investigate the connection between starburst (SB) and active galactic nuclei (AGN), because they exhibit extreme star-formation rates, and most of them show evidence of harbouring powerful AGN. Methods. Our previous X-ray study of a sample of HLIRG shows that the X-ray emission of most of these sources is dominated by AGN activity. To improve our estimate of the relative contribution of the AGN and SB emission to its total bolometric output, we have built multi-wavelength (from radio to X-rays) spectral energy distributions (SED) for these HLIRG and fitted standard empirical AGN and SB templates to these SED. Results. In broad terms, most sources are well fitted with this method, and we found AGN and SB contributions similar to those obtained by previous studies of HLIRG. We have classified the HLIRG SED into two groups, class A and class B. Class A HLIRG show a flat SED from the optical to the infrared energy range. Three out of seven class A sources can be modelled with a pure luminosity-dependent quasar template, while the rest of them require a type 1 AGN template and a SB template. The SB component is dominant in three out of four class A objects. Class B HLIRG show SED with a prominent and broad IR bump. These sources cannot easily be modelled with a combination of pure AGN and pure SB, they require templates of composite objects, suggesting that >∼50% of their emission comes from stellar formation processes. Conclusions. We propose that our sample is actually composed of three different populations: very luminous quasars (class A objects with negligible SB contribution), young galaxies going through their maximal star-formation period (class A objects with significant SB emission) and the high luminosity tail of the ultraluminous infrared galaxies population distribution (class B sources).We are grateful to the referee M. Rowan-Robinson for the constructive comments and suggestions that improved this paper. A.R. acknowledges support from a Universidad de Cantabria fellowship and from ASI grant n. ASI I/088/06/0. Financial support for A.R. and F.J.C. was provided by the Spanish Ministry of Education and Science, under projects ESP2003-00812 and ESP2006-13608-C02-01. FP acknowledges financial support under the project ASI INAF I/08/07/0. G.M. thanks the Ministerio de Ciencia e Innovación and CSIC for support through a Ramón y Cajal contract. This research made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This paper is based also on data from the VOSED tool at LAEFF. The 2.5 m Isaac Newton Telescope and its service programme are operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias

Rapid late-time X-ray brightening of the tidal disruption event OGLE16aaa

Stars that pass too close to a super-massive black hole may be disrupted by strong tidal forces. OGLE16aaa is one such tidal disruption event (TDE) which rapidly brightened and peaked in the optical/UV bands in early 2016 and subsequently decayed over the rest of the year. OGLE16aaa was detected in an XMM-Newton X-ray observation on June 9, 2016 with a flux slightly below the Swift/XRT upper limits obtained during the optical light curve peak. Between June 16-21, 2016, Swift/XRT also detected OGLE16aaa and based on the stacked spectrum, we could infer that the X-ray luminosity had jumped up by more than a factor of ten in just one week. No brightening signal was seen in the simultaneous optical/UV data to cause the X-ray luminosity to exceed the optical/UV one. A further XMM-Newton observation on November 30, 2016 showed that almost a year after the optical/UV peak, the X-ray emission was still at an elevated level, while the optical/UV flux decay had already leveled off to values comparable to those of the host galaxy. In all X-ray observations, the spectra were nicely modeled with a 50-70 eV thermal component with no intrinsic absorption, with a weak X-ray tail seen only in the November 30 XMM-Newton observation. The late-time X-ray behavior of OGLE16aaa strongly resembles the tidal disruption events ASASSN-15oi and AT2019azh. We were able to pinpoint the time delay between the initial optical TDE onset and the X-ray brightening to $182 \pm 5$ days, which may possibly represent the timescale between the initial circularization of the disrupted star around the super-massive black hole and the subsequent delayed accretion. Alternatively, the delayed X-ray brightening could be related to a rapid clearing of a thick envelope that covers the central X-ray engine during the first six months.Comment: 8 pages, 2 figures, Accepted for publication in A&