Constraints on a strong X-ray flare in the Seyfert galaxy MCG-6-30-15

We discuss implications of a strong flare event observed in the Seyfert galaxy MCG-6-30-15 assuming that the emission is due to localized magnetic reconnection. We conduct detailed radiative transfer modeling of the reprocessed radiation for a primary source that is elevated above the disk. The model includes relativistic effects and Keplerian motion around the black hole. We show that for such a model setup the observed time-modulation must be intrinsic to the primary source. Using a simple analytical model we then investigate time delays between hard and soft X-rays during the flare. The model considers an intrinsic delay between primary and reprocessed radiation, which measures the geometrical distance of the flare source to the reprocessing sites. The observed time delays are well reproduced if one assumes that the reprocessing happens in magnetically confined, cold clouds.Comment: 4 pages, 2 figures, proceedings of a talk given at the symposium 238 at the IAU General Assembly 200

The primordial deuterium abundance at z = 2.504 from a high signal-to-noise spectrum of Q1009+2956

The spectrum of the $z_{\rm em} = 2.63$ quasar Q1009+2956 has been observed extensively on the Keck telescope. The Lyman limit absorption system $z_{\rm abs} = 2.504$ was previously used to measure D/H by Burles & Tytler using a spectrum with signal to noise approximately 60 per pixel in the continuum near Ly {\alpha} at $z_{\rm abs} = 2.504$. The larger dataset now available combines to form an exceptionally high signal to noise spectrum, around 147 per pixel. Several heavy element absorption lines are detected in this LLS, providing strong constraints on the kinematic structure. We explore a suite of absorption system models and find that the deuterium feature is likely to be contaminated by weak interloping Ly {\alpha} absorption from a low column density H I cloud, reducing the expected D/H precision. We find D/H = $2.48^{+0.41}_{-0.35}\times10^{-5}$ for this system. Combining this new measurement with others from the literature and applying the method of Least Trimmed Squares to a statistical sample of 15 D/H measurements results in a "reliable" sample of 13 values. This sample yields a primordial deuterium abundance of (D/H)$_{\rm p} = (2.545 \pm 0.025)\times10^{-5}$. The corresponding mean baryonic density of the Universe is $\Omega_{\rm b}h^2 = 0.02174\pm0.00025$. The quasar absorption data is of the same precision as, and marginally inconsistent with, the 2015 CMB Planck (TT+lowP+lensing) measurement, $\Omega_{\rm b}h^2 = 0.02226\pm0.00023$. Further quasar and more precise nuclear data are required to establish whether this is a random fluctuation.Comment: accepted by MNRAS, 18 pages, 12 figures, 6 table

Power Allocation Games in Wireless Networks of Multi-antenna Terminals

We consider wireless networks that can be modeled by multiple access channels in which all the terminals are equipped with multiple antennas. The propagation model used to account for the effects of transmit and receive antenna correlations is the unitary-invariant-unitary model, which is one of the most general models available in the literature. In this context, we introduce and analyze two resource allocation games. In both games, the mobile stations selfishly choose their power allocation policies in order to maximize their individual uplink transmission rates; in particular they can ignore some specified centralized policies. In the first game considered, the base station implements successive interference cancellation (SIC) and each mobile station chooses his best space-time power allocation scheme; here, a coordination mechanism is used to indicate to the users the order in which the receiver applies SIC. In the second framework, the base station is assumed to implement single-user decoding. For these two games a thorough analysis of the Nash equilibrium is provided: the existence and uniqueness issues are addressed; the corresponding power allocation policies are determined by exploiting random matrix theory; the sum-rate efficiency of the equilibrium is studied analytically in the low and high signal-to-noise ratio regimes and by simulations in more typical scenarios. Simulations show that, in particular, the sum-rate efficiency is high for the type of systems investigated and the performance loss due to the use of the proposed suboptimum coordination mechanism is very small

Iron lines from transient and persisting hot spots on AGN accretion disks

[abridged] We model the X-ray reprocessing from a strong co-rotating flare above an accretion disk in active galactic nuclei. We explore the horizontal structure and evolution of the underlying hot spot. To obtain the spectral evolution seen by a distant observer, we apply a general relativity ray-tracing technique. We concentrate on the energy band around the iron K-line, where the relativistic effects are most pronounced. Persistent flares lasting for a significant fraction of the orbital time scale and short, transient flares are considered. In our time-resolved analysis, the spectra recorded by a distant observer depend on the position of the flare/spot with respect to the central black hole. If the flare duration significantly exceeds the light travel time across the spot, then the spot horizontal stratification is unimportant. On the other hand, if the flare duration is comparable to the light travel time across the spot radius, the lightcurves exhibit a typical asymmetry in their time profiles. The sequence of dynamical spectra proceeds from more strongly to less strongly ionized re-emission. At all locations within the spot the spectral intensity increases towards edge-on emission angles, revealing the limb brightening effect. Future X-ray observatories with significantly larger effective collecting areas will enable to spectroscopically map out the azimuthal irradiation structure of the accretion disk and to localize persistent flares. If the hot spot is not located too close to the marginally stable orbit of the black hole, it will be possible to probe the reflecting medium via the sub-structure of the iron K-line. Indications for transient flares will only be obtained from analyzing the observed lightcurves on the gravitational time scale of the accreting supermassive black hole.Comment: 15 pages, 8 figures, accepted by Astronomy & Astrophysic

Double-Peaked Broad Emission Lines in NGC 4450 and Other LINERs

Spectra taken with HST reveal that NGC 4450 emits Balmer emission lines with displaced double peaks and extremely high-velocity wings. This characteristic line profile, previously seen in a few nearby LINERs and in a small fraction of broad-line radio galaxies, can be interpreted as a kinematic signature of a relativistic accretion disk. We can reproduce the observed profile with a model for a disk with a radial range of 1000-2000 gravitational radii and inclined by 27 degrees along the line of sight. The small-aperture HST data also allow us to detect, for the first time, the featureless continuum at optical wavelengths in NGC 4450; the nonstellar nucleus is intrinsically very faint, with M_B = -11.2 mag for D = 16.8 Mpc. We have examined the multiwavelength properties of NGC 4450 collectively with those of other low-luminosity active nuclei which possess double-peaked broad lines and find a number of common features. These objects are all classified spectroscopically as "type 1" LINERs or closely related objects. The nuclear luminosities are low, both in absolute terms and relative to the Eddington rates. All of them have compact radio cores, whose strength relative to the optical nuclear emission places them in the league of radio-loud active nuclei. The broad-band spectral energy distributions of these sources are most notable for their deficit of ultraviolet emission compared to those observed in luminous Seyfert 1 nuclei and quasars. The double-peaked broad-line radio galaxies Arp 102B and Pictor A have very similar attributes. We discuss how these characteristics can be understood in the context of advection-dominated accretion onto massive black holes.Comment: To appear in The Astrophysical Journal. Latex, 15 pages, embedded figures and tabl

Is the Broad-Line Region Clumped or Smooth? Constraints from the H alpha Profile in NGC 4395, the Least Luminous Seyfert 1 Galaxy

The origin and configuration of the gas which emits broad lines in Type I active galactic nuclei is not established yet. The lack of small-scale structure in the broad emission-line profiles is consistent with a smooth gas flow, or a clumped flow with many small clouds. An attractive possibility for the origin of many small clouds is the atmospheres of bloated stars, an origin which also provides a natural mechanism for the cloud confinement. Earlier studies of the broad-line profiles have already put strong lower limits on the minimum number of such stars, but these limits are sensitive to the assumed width of the lines produced by each cloud. Here we revisit this problem using high-resolution Keck spectra of the H alpha line in NGC 4395, which has the smallest known broad-line region (~10^14 cm). Only a handful of the required bloated stars (each having r~10^14 cm) could fit into the broad-line region of NGC 4395, yet the observed smoothness of the H alpha line implies a lower limit of ~10^4-10^5 on the number of discrete clouds. This rules out conclusively the bloated-stars scenario, regardless of any plausible line-broadening mechanisms. The upper limit on the size of the clouds is ~10^12 cm, which is comparable to the size implied by photoionization models. This strongly suggests that gas in the broad-line region is structured as a smooth rather than a clumped flow, most likely in a rotationally dominated thick disk-like configuration. However, it remains to be clarified why such a smooth, gravity-dominated flow generates double-peaked emission lines only in a small fraction of active galactic nuclei.Comment: 12 pages, including 3 figures, accepted for publication in The Astrophysical Journa

CENP-A Is Dispensable for Mitotic Centromere Function after Initial Centromere/Kinetochore Assembly

Human centromeres are defined by chromatin containing the histone H3 variant CENP-A assembled onto repetitive alphoid DNA sequences. By inducing rapid, complete degradation of endogenous CENP-A, we now demonstrate that once the first steps of centromere assembly have been completed in G1/S, continued CENP-A binding is not required for maintaining kinetochore attachment to centromeres or for centromere function in the next mitosis. Degradation of CENP-A prior to kinetochore assembly is found to block deposition of CENP-C and CENP-N, but not CENP-T, thereby producing defective kinetochores and failure of chromosome segregation. Without the continuing presence of CENP-A, CENP-B binding to alphoid DNA sequences becomes essential to preserve anchoring of CENP-C and the kinetochore to each centromere. Thus, there is a reciprocal interdependency of CENP-A chromatin and the underlying repetitive centromere DNA sequences bound by CENP-B in the maintenance of human chromosome segregation