X-ray Spectral and Variability Properties of Low-Mass AGN

We study the X-ray properties of a sample of 14 optically-selected low-mass AGN whose masses lie within the range 1E5 -2E6 M(solar) with XMM-Newton. Only six of these low-mass AGN have previously been studied with sufficient quality X-ray data, thus, we more than double the number of low-mass AGN observed by XMM-Newton with the addition of our sample. We analyze their X-ray spectral properties and variability and compare the results to their more massive counterparts. The presence of a soft X-ray excess is detectable in all five objects which were not background dominated at 2-3 keV. Combined with previous studies, this gives a total of 8 low-mass AGN with a soft excess. The low-mass AGN exhibit rapid, short-term variability (hundreds to thousands of seconds) as well as long-term variability (months to years). There is a well-known anti-correlation between black hole mass and variability amplitude (normalized excess variance). Comparing our sample of low-mass AGN with this relation we find that all of our sample lie below an extrapolation of the linear relation. Such a flattening of the relation at low masses (below about 1E6 M(solar)) is expected if the variability in all AGN follows the same shape power spectrum with a break frequency that is dependent on mass. Finally, we also found two objects that show significant absorption in their X-ray spectrum, indicative of type 2 objects, although they are classified as type 1 AGN based on optical spectra.Comment: 12 pages, 5 figures, 7 tables, accepted for publication in MNRA

Pure cycles in flexible robotic cells

Cataloged from PDF version of article.In this study, an m-machine flexible robotic manufacturing cell consisting of CNC machines is considered. The flexibility of the machines leads to a new class of robot move cycles called the pure cycles. We first model the problem of determining the best pure cycle in an m-machine cell as a special travelling salesman problem in which the distance matrix consists of decision variables as well as parameters.We focus on two specific cycles among the huge class of pure cycles.We prove that, in most of the regions, either one of these two cycles is optimal. For the remaining regions we derive worst case performances of these cycles.We also prove that the set of pure cycles dominates the flowshop-type robot move cycles considered in the literature. As a design problem, we consider the number of machines in a cell as a decision variable. We determine the optimal number of machines that minimizes the cycle time for given cell parameters such as the processing times, robot travel times and the loading/unloading times of the machines. 2007 Elsevier Ltd. All rights reserved

Scheduling in a three-machine robotic flexible manufacturing cell

Cataloged from PDF version of article.In this study, we consider a flexible manufacturing cell (FMC) processing identical parts on which the loading and unloading of machines are made by a robot. The machines used in FMCs are predominantly CNC machines and these machines are flexible enough for performing several operations provided that the required tools are stored in their tool magazines. Traditional research in this area considers a flowshop type system. The current study relaxes this flowshop assumption which unnecessarily limits the number of alternatives. In traditional robotic cell scheduling literature, the processing time of each part on each machine is a known parameter. However, in this study the processing times of the parts on the machines are decision variables. Therefore, we investigated the productivity gain attained by the additional flexibility introduced by the FMCs. We propose new lower bounds for the 1-unit and 2-unit robot move cycles (for which we present a completely new procedure to derive the activity sequences of 2-unit cycles in a three-machine robotic cell) under the new problem domain for the flowshop type robot move cycles. We also propose a new robot move cycle which is a direct consequence of process and operational flexibility of CNC machines.We prove that this proposed cycle dominates all 2-unit robot move cycles and present the regions where the proposed cycle dominates all 1-unit cycles.We also present a worst case performance bound of using this proposed cycle. 2005 Elsevier Ltd. All rights reserved

Swift/UVOT grism monitoring of NGC 5548 in 2013: an attempt at MgII reverberation mapping

Reverberation-mapping-based scaling relations are often used to estimate the masses of black holes from single-epoch spectra of AGN. While the radius-luminosity relation that is the basis of these scaling relations is determined using reverberation mapping of the H$\beta$ line in nearby AGN, the scaling relations are often extended to use other broad emission lines, such as MgII, in order to get black hole masses at higher redshifts when H$\beta$ is redshifted out of the optical waveband. However, there is no radius-luminosity relation determined directly from MgII. Here, we present an attempt to perform reverberation mapping using MgII in the well-studied nearby Seyfert 1, NGC 5548. We used Swift to obtain UV grism spectra of NGC 5548 once every two days from April to September 2013. Concurrent photometric UV monitoring with Swift provides a well determined continuum lightcurve that shows strong variability. The MgII emission line, however, is not strongly correlated with the continuum variability, and there is no significant lag between the two. We discuss these results in the context of using MgII scaling relations to estimate high-redshift black hole masses.Comment: 8 pages, 7 figures, accepted for publication in Ap

In Their Own Words: Exploring Family Pathways to Housing Instability.

Purpose/Background: For every family experiencing literal homelessness, there are approximately five families who are precariously housed; doubled-up with friends or family, or living in hotels or temporary housing. These families experience higher rates of acute and chronic illness, emergency room utilization, and higher rates of negative behavioral and mental health outcomes than their stably-housed peers. Efforts to address housing instability focus on financial assistance only when a family is on the brink of losing their housing. This study aims to uncover pathways to family housing instability, allowing mothers facing homelessness to share the life events that led to their housing difficulties in order to identify points of intervention in a more up-stream, community-based approach. Methods: I employed a qualitative narrative approach, utilizing Smith & Liehr’s Story Theory to elicit individual accounts of the life events women felt contributed to their housing instability. Individual stories were then organized into narratives, in which I identified themes. I then shared these findings with participants and asked that they verify or correct findings in order to validate themes and key details. Results: Sixteen women with children facing eviction who qualified for assistance from a HUD emergency services grant completed 2 interviews. Fourteen of the 16 participants were survivors of domestic violence (either in their childhood home or adult relationships), and most participants reported estrangement or abuse from their families of origin. Women were able to articulate the struggles they had faced in their life, but few were able to connect past trauma and violence to current circumstances, even when their stories depicted a likely relationship between the past and present. Women articulated a need for peer support and mentoring to prevent similar housing instability for other women and teenagers at risk. The frequency and intensity of trauma experienced by the research participants also revealed a need for trauma-informed housing support services. Conclusions: The results of this study offer insight into the experiences of families facing housing instability. This research is a starting point for developing long-term community-based strategies to reduce risk of housing instability through targeted interventions that are meaningful and sustainable to communities.PhDNursingUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108818/1/lgulteki_1.pd

A Single Circumbinary Disk in the HD 98800 Quadruple System

We present sub-arcsecond thermal infrared imaging of HD 98800, a young quadruple system composed of a pair of low-mass spectroscopic binaries separated by 0.8'' (38 AU), each with a K-dwarf primary. Images at wavelengths ranging from 5 to 24.5 microns show unequivocally that the optically fainter binary, HD 98800B, is the sole source of a comparatively large infrared excess upon which a silicate emission feature is superposed. The excess is detected only at wavelengths of 7.9 microns and longer, peaks at 25 microns, and has a best-fit black-body temperature of 150 K, indicating that most of the dust lies at distances greater than the orbital separation of the spectroscopic binary. We estimate the radial extent of the dust with a disk model that approximates radiation from the spectroscopic binary as a single source of equivalent luminosity. Given the data, the most-likely values of disk properties in the ranges considered are R_in = 5.0 +/- 2.5 AU, DeltaR = 13+/-8 AU, lambda_0 = 2(+4/-1.5) microns, gamma = 0+/-2.5, and sigma_total = 16+/-3 AU^2, where R_in is the inner radius, DeltaR is the radial extent of the disk, lambda_0 is the effective grain size, gamma is the radial power-law exponent of the optical depth, tau, and sigma_total is the total cross-section of the grains. The range of implied disk masses is 0.001--0.1 times that of the moon. These results show that, for a wide range of possible disk properties, a circumbinary disk is far more likely than a narrow ring.Comment: 11 page Latex manuscript with 3 postscript figures. Accepted for publication in Astrophysical Journal Letters. Postscript version of complete paper also available at http://www.hep.upenn.edu/PORG/web/papers/koerner00a.p

The Globular Cluster/Central Black Hole Connection in Galaxies

We explore the relation between the total globular cluster population in a galaxy (N_GC) and the the mass of its central black hole (M_BH). Using a sample of 33 galaxies, twice as large as the original sample discussed by Burkert & Tremaine (2010), we find that N_GC for elliptical and spiral galaxies increases in almost precisely direct proportion to M_BH. The S0-type galaxies by contrast do not follow a clear trend, showing large scatter in M_BH at a given N_GC. After accounting for observational measurement uncertainty, we find that the mean relation defined by the E and S galaxies must also have an intrinsic or "cosmic" scatter of +-0.2 in either logN_GC or logM_BH. The residuals from this correlation show no trend with globular cluster specific frequency. We suggest that these two types of galaxy subsystems (central black hole and globular cluster system) may be closely correlated because they both originated at high redshift during the main epoch of hierarchical merging, and both require extremely high-density conditions for formation. Lastly, we note that roughly 10% of the galaxies in our sample (one E, one S, and two S0) deviate strongly from the main trend, all in the sense that their M_BH is at least 10x smaller than would be predicted by the mean relation.Comment: 7 pages, 4 figures; accepted for publication in MNRA

Multi-Wavelength Coverage of State Transitions in the New Black Hole X-Ray Binary Swift J1910.2-0546

Understanding how black holes accrete and supply feedback to their environment is one of the outstanding challenges of modern astrophysics. Swift J1910.2-0546 is a candidate black hole low-mass X-ray binary that was discovered in 2012 when it entered an accretion outburst. To investigate the binary configuration and the accretion morphology we monitored the evolution of the outburst for ~3 months at X-ray, UV, optical (B,V,R,I), and near-infrared (J,H,K) wavelengths using Swift and SMARTS. The source evolved from a hard to a soft X-ray spectral state with a relatively cold accretion disk that peaked at ~0.5 keV. A Chandra/HETG spectrum obtained during this soft state did not reveal signatures of an ionized disk wind. Both the low disk temperature and the absence of a detectable wind could indicate that the system is viewed at relatively low inclination. The multi-wavelength light curves revealed two notable features that appear to be related to X-ray state changes. Firstly, a prominent flux decrease was observed in all wavebands ~1-2 weeks before the source entered the soft state. This dip occurred in (0.6-10 keV) X-rays ~6 days later than at longer wavelengths, which could possibly reflect the viscous time scale of the disk. Secondly, about two weeks after the source transitioned back into the hard state, the UV emission significantly increased while the X-rays steadily decayed. We discuss how these observations may reflect changes in the accretion flow morphology, perhaps related to the quenching/launch of a jet or the collapse/recovery of a hot flow.Comment: 8 pages, 5 figures, 1 table. To be published in Ap

Astro2020 Science White Paper: The Local Relics of of Supermassive Black Hole Seeds

We have compelling evidence for stellar-mass black holes (BHs) of ~5-80 M_sun that form through the death of massive stars. We also have compelling evidence for so-called supermassive BHs (10^5-10^10 M_sun) that are predominantly found in the centers of galaxies. We have very good reason to believe there must be BHs with masses in the gap between these ranges: the first ~10^9 M_sun BHs are observed only hundreds of millions of years after the Big Bang, and all theoretically viable paths to making supermassive BHs require a stage of "intermediate" mass. However, no BHs have yet been reliably detected in the 100-10}^5 M_sun mass range. Uncovering these intermediate-mass BHs of 10^3-10^5 M_sun is within reach in the coming decade. In this white paper we highlight the crucial role that 30-m class telescopes will play in dynamically detecting intermediate-mass black holes, should they exist.Comment: Science White Paper Submitted for the Astro2020 Decadal Survey on Astronomy and Astrophysic