arXiv subjects

Rosanne Di Stefano

Publications and source records attributed to Rosanne Di Stefano.

At least 55 records · Page 3Linked to original sources

arXiv2011

X-ray emission from the Sombrero galaxy: a galactic-scale outflow

Based on new and archival Chandra observations of the Sombrero galaxy (M 104), we study the diffuse X-ray emission in and around its massive stellar bulge. The 2-6 keV unresolved emission from the bulge region closely follows the K-band star light and most likely arises from unresolved stellar sources. At lower energies, however, the unresolved emission reaches a galactocentric radius of at least 23 kpc, significantly beyond the extent of the starlight, clearly indicating the presence of diffuse hot gas. We isolate the emission of the gas by properly accounting for the emission from unresolved stellar sources, predominantly cataclysmic variables and coronally active binaries, whose quasi-universal X-ray emissivity is recently established. We find a gas temperature of ~0.6 keV with little variation across the field of view, except for a lower temperature of ~0.3 keV along the stellar disk. We measure a total intrinsic 0.3-2 keV luminosity of ~2e39 erg/s, which is comparable to the prediction by the latest galaxy formation models for disk galaxies as massive as Sombrero. However, such numerical models do not fully account for internal feedback processes, such as nuclear feedback and stellar feedback, against accretion from the intergalactic medium. On the other hand, we find no evidence for either the nucleus or the very modest star-forming activities in the disk to be a dominant heating source for the diffuse gas. We also show that neither the expected energy released by Type Ia supernovae nor the expected mass returned by evolved stars is recovered by observations. We argue that in Sombrero a galactic-scale subsonic outflow of hot gas continuously removes much of the "missing" energy and mass input from the bulge region. The observed density and temperature distributions of such an outflow, however, continues to pose challenges to theoretical studies.

astro-ph.CO↗

arXiv2010

X-ray emission from the Sombrero galaxy: discrete sources

We present a study of discrete X-ray sources in and around the bulge-dominated, massive Sa galaxy, Sombrero (M104), based on new and archival Chandra observations with a total exposure of ~200 ks. With a detection limit of L_X = 1E37 erg/s and a field of view covering a galactocentric radius of ~30 kpc (11.5 arcminute), 383 sources are detected. Cross-correlation with Spitler et al.'s catalogue of Sombrero globular clusters (GCs) identified from HST/ACS observations reveals 41 X-rays sources in GCs, presumably low-mass X-ray binaries (LMXBs). We quantify the differential luminosity functions (LFs) for both the detected GC and field LMXBs, whose power-low indices (~1.1 for the GC-LF and ~1.6 for field-LF) are consistent with previous studies for elliptical galaxies. With precise sky positions of the GCs without a detected X-ray source, we further quantify, through a fluctuation analysis, the GC LF at fainter luminosities down to 1E35 erg/s. The derived index rules out a faint-end slope flatter than 1.1 at a 2 sigma significance, contrary to recent findings in several elliptical galaxies and the bulge of M31. On the other hand, the 2-6 keV unresolved emission places a tight constraint on the field LF, implying a flattened index of ~1.0 below 1E37 erg/s. We also detect 101 sources in the halo of Sombrero. The presence of these sources cannot be interpreted as galactic LMXBs whose spatial distribution empirically follows the starlight. Their number is also higher than the expected number of cosmic AGNs (52+/-11 [1 sigma]) whose surface density is constrained by deep X-ray surveys. We suggest that either the cosmic X-ray background is unusually high in the direction of Sombrero, or a distinct population of X-ray sources is present in the halo of Sombrero.

astro-ph.CO↗

arXiv2010

The Progenitors of Type Ia Supernovae: II. Are they Double-Degenerate Binaries? The Symbiotic Channel

In order for a white dwarf (WD) to achieve the Chandrasekhar mass, M_C, and explode as a Type Ia supernova (SNIa), it must interact with another star, either accreting matter from or merging with it. The failure to identify the types of binaries which produce SNeIa is the "progenitor problem". Its solution is required if we are to utilize the full potential of SNeIa to elucidate basic cosmological and physical principles. In single-degenerate models, a WD accretes and burns matter at high rates. Nuclear-burning WDs (NBWDs) with mass close to M_C are hot and luminous, potentially detectable as supersoft x-ray sources (SSSs). In previous work we showed that > 90-99% of the required number of progenitors do not appear as SSSs during most of the crucial phase of mass increase. The obvious implication is that double-degenerate (DD) binaries form the main class of progenitors. We show in this paper, however, that many binaries that later become DDs must pass through a long-lived NBWD phase during which they are potentially detectable as SSSs. The paucity of SSSs is therefore not a strong argument in favor of DD models. Those NBWDs that are the progenitors of DD binaries are likely to appear as symbiotic binaries for intervals > 10^6 years. In fact, symbiotic pre-DDs should be common, whether or not the WDs eventually produce SNeIa. The key to solving the progenitor problem lies in understanding the appearance of NBWDs. Most do not appear as SSSs most of the time. We therefore consider the evolution of NBWDs to address the question of what their appearance may be and how we can hope to detect them.

astro-ph.CO↗

arXiv2010

Transits and Lensing by Compact Objects in the Kepler Field: Disrupted Stars Orbiting Blue Stragglers

Kepler's first major discoveries are two hot objects orbiting stars in its field. These may be the cores of stars that have each been eroded or disrupted by a companion star. The companion, which is the star monitored today, is likely to have gained mass from its now-defunct partner, and can be considered to be a blue straggler. KOI-81 is almost certainly the product of stable mass transfer; KOI-74 may be as well, or it may be the first clear example of a blue straggler created throughthree-body interactions. We show that mass transfer binaries are common enough that Kepler should discover ~1000 white dwarfs orbiting main sequence stars. Most, like KOI-74 and KOI-81, will be discovered through transits, but many will be discovered through a combination of gravitational lensing and transits, while lensing will dominate for a subset. In fact, some events caused by white dwarfs will have the appearance of "anti-transits" --i.e., short-lived enhancements in the amount of light received from the monitored star. Lensing and other mass measurements methods provide a way to distinguish white dwarf binaries from planetary systems. This is important for the success of Kepler's primary mission, in light of the fact that white dwarf radii are similar to the radii of terrestrial planets, and that some white dwarfs will have orbital periods that place them in the habitable zones of their stellar companions. By identifying transiting and/or lensing white dwarfs, Kepler will conduct pioneering studies of white dwarfs and of the end states of mass transfer. It may also identify orbiting neutron stars or black holes. The calculations inspired by the discovery of KOI-74 and KOI-81 have implications for ground-based wide-field surveys as well as for future space-based surveys.

astro-ph.GA↗

arXiv2009

Measuring the Accreting Stellar and Intermediate Mass Black Hole Populations in the Galaxy and Local Group

The population of stellar black holes (SBHs) in the Galaxy and galaxies generally is poorly known in both number and distribution. SBHs are the fossil record of the massive stars in galaxy evolution and may have produced some (if not all) of the intermediate mass (\gsim100\Msun) black holes (IMBHs) and, in turn, the central supermassive black holes (SMBHs) in galactic nuclei. For the first time, a Galaxy-wide census of accreting black holes, and their more readily recognizable tracer population, accreting neutron stars (NSs), could be measured with a wide-field hard X-ray imaging survey and soft X-ray and optical/IR prompt followup -- as proposed for the EXIST mission.

astro-ph.HE↗

arXiv2009

A Search for Asteroids, Moons, and Rings Orbiting White Dwarfs

Do white dwarfs host asteroid systems? Although several lines of argument suggest that white dwarfs may be orbited by large populations of asteroids, transits would provide the most direct evidence. We demonstrate that the Kepler mission has the capability to detect transits of white dwarfs by asteroids. Because white-dwarf asteroid systems, if they exist, are likely to contain many asteroids orbiting in a spatially extended distribution, discoveries of asteroid transits can be made by monitoring only a small number of white dwarfs, compatible with Kepler's primary mission, which is to monitor stars with potentially habitable planets. Possible future missions that survey ten times as many stars with similar sensitivity and minute-cadence monitoring can establish the characteristics of asteroid systems around white dwarfs, such as the distribution of asteroid sizes and semimajor axes. Transits by planets would be more dramatic, but the probability that they will occur is lower. Ensembles of planetary moons and/or the presence of rings around planets can also produce transits detectable by Kepler. The presence of moons and rings can significantly increase the probability that Kepler will discover planets orbiting white dwarfs, even while monitoring only a small number of them.

astro-ph.EP↗

arXiv2009

Short-duration lensing events: II. Expectations and Protocols

Ongoing microlensing observations by OGLE and MOA regularly identify and conduct high-cadence sampling of lensing events with Einstein diameter crossing time, tau_E, of 16 or fewer days. Events with estimated values of tau_E of one to two days have been detected. Short duration events tend to be generated by low-mass lenses or by lenses with high transverse velocities. We compute the expected rates, demonstrate the expected ranges of parameters for lenses of different mass, and develop a protocol for observing and modeling short-duration events. Relatively minor additions to the procedures presently used will increase the rate of planet discovery, and also discover or place limits on the population of high-speed dim stars and stellar remnants in the vicinity of the Sun.

astro-ph.EP↗

arXiv2009

Short-duration lensing events: I. wide-orbit planets? free-floating low-mass objects? or high-velocity stars?

Short duration lensing events tend to be generated by low-mass lenses or by lenses with high transverse velocities. Furthermore, for any given lens mass and speed, events of short duration are preferentially caused by nearby lenses (mesolenses) that can be studied in detail, or else by lenses so close to the source star that finite-source-size effects may be detected, yielding information about both the Einstein ring radius and the surface of the lensed star. Planets causing short-duration events may be in orbits with any orientation, and may have semimajor axes smaller than an AU, or they may reach the outer limits of their planetary systems, in the region corresponding to the Solar System's Oort Cloud. They can have masses larger than Jupiter's or smaller than Pluto's. Lensing therefore has a unique potential to expand our understanding of planetary systems. A particular advantage of lensing is that it can provide precision measurements of system parameters, including the masses of and projected separation between star and planet. We demonstrate how the parameters can be extracted and show that a great deal can be learned. For example, it is remarkable that the gravitational mass of nearby free-floating planet-mass lenses can be measured by complementing observations of a photometric event with deep images that detect the planet itself. A fraction of short events may be caused by high-velocity stars located within a kpc. Many high-velocity lenses are likely to be neutron stars that received large natal kicks. Other high-speed stars may be members of the halo population. Still others may be hypervelocity stars that have been ejected from the Galactic Center, or runaway stars escaped from close binaries, possibly including the progenitor binaries of Type Ia supernovae.

astro-ph.EP↗

arXiv2009

The Progenitors of Type Ia Supernovae: Are They Supersoft Sources?

In a canonical model, the progenitors of Type Ia supernovae (SNe Ia) are accreting, nuclear-burning white dwarfs (NBWDs), which explode when the white dwarf reaches the Chandrasekhar mass, M_C. Such massive NBWDs are hot (kT ~100 eV), luminous (L ~ 10^{38} erg/s), and are potentially observable as luminous supersoft X-ray sources (SSSs). During the past several years, surveys for soft X-ray sources in external galaxies have been conducted. This paper shows that the results falsify the hypothesis that a large fraction of progenitors are NBWDs which are presently observable as SSSs. The data also place limits on sub-M_C models. While Type Ia supernova progenitors may pass through one or more phases of SSS activity, these phases are far shorter than the time needed to accrete most of the matter that brings them close to M_C.

astro-ph.HE↗

arXiv2009

LSST Science Book, Version 2.0

A survey that can cover the sky in optical bands over wide fields to faint magnitudes with a fast cadence will enable many of the exciting science opportunities of the next decade. The Large Synoptic Survey Telescope (LSST) will have an effective aperture of 6.7 meters and an imaging camera with field of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over 20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a total point-source depth of r~27.5. The LSST Science Book describes the basic parameters of the LSST hardware, software, and observing plans. The book discusses educational and outreach opportunities, then goes on to describe a broad range of science that LSST will revolutionize: mapping the inner and outer Solar System, stellar populations in the Milky Way and nearby galaxies, the structure of the Milky Way disk and halo and other objects in the Local Volume, transient and variable objects both at low and high redshift, and the properties of normal and active galaxies at low and high redshift. It then turns to far-field cosmological topics, exploring properties of supernovae to z~1, strong and weak lensing, the large-scale distribution of galaxies and baryon oscillations, and how these different probes may be combined to constrain cosmological models and the physics of dark energy.

astro-ph.IM↗

arXiv2009

Populations of Supersoft X-ray Sources: Novae, tidal disruption, Type Ia supernovae, accretion-induced collapse, ionization, and intermediate-mass black holes?

Observations of hundreds of supersoft x-ray sources (SSSs) in external galaxies have shed light on the diversity of the class and on the natures of the sources. SSSs are linked to the physics of Type Ia supernovae and accretion-induced collapse, ultraluminous x-ray sources and black holes, the ionization of the interstellar medium, and tidal disruption by supermassive black holes. The class of SSSs has an extension to higher luminosities: ultraluminous SSSs have luminosities above 10^39 erg/s. There is also an extension to higher energies: quasisoft x-ray sources (QSSs) emit photons with energies above 1 eV, but few or none with energies above 2 keV. Finally, a significant fraction of the SSSs found in external galaxies switch states between observations, becoming either quasisoft or hard. For many systems ``supersoft'' refers to a temporary state; SSSs are sources, possibly including a variety of fundamentally different system types, that pass through such a state. We review those results derived from extragalactic data and related theoretical work that are most surprising and that suggest directions for future research.

astro-ph.CO↗

arXiv2009

The Dynamic X-ray Sky of the Local Universe

Over the next decade, we can expect time domain astronomy to flourish at optical and radio wavelengths. In parallel with these efforts, a dedicated transient "machine" operating at higher energies (X-ray band through soft gamma-rays) is required to reveal the unique subset of events with variable emission predominantly visible above 100 eV. Here we focus on the transient phase space never yet sampled due to the lack of a sensitive, wide-field and triggering facility dedicated exclusively to catching high energy transients and enabling rapid coordinated multi-wavelength follow-up. We first describe the advancements in our understanding of known X-ray transients that can only be enabled through such a facility and then focus on the classes of transients theoretically predicted to be out of reach of current detection capabilities. Finally there is the exciting opportunity of revealing new classes of X-ray transients and unveiling their nature through coordinated follow-up observations at longer wavelengths.

astro-ph.HE↗

arXiv2008

An Ultraluminous Supersoft X-ray Source in M81: An Intermediate-Mass Black Hole?

Ultraluminous supersoft X-ray sources (ULSSS) exhibit supersoft spectra with blackbody temperatures of 50-100 eV and bolometric luminosities above $10^{39}$ erg s$^{-1}$, and are possibly intermediate mass black holes (IMBHs) of $\ge10^3 M_\odot$ or massive white dwarfs that are progenitors of type Ia supernovae. In this letter we report our optical studies of such a source in M81, M81-ULS1, with HST archive observations. M81-ULS1 is identified with a point-like object, the spectral energy distribution of which reveals a blue component in addition to the companion of an AGB star. The blue component is consistent with the power-law as expected from the geometrically-thin accretion disk around an IMBH accretor, but inconsistent with the power-law as expected from the X-ray irradiated flared accretion disk around a white dwarf accretor. This result is strong evidence that M81-ULS1 is an IMBH instead of a white dwarf.

astro-ph↗

arXiv2008

Discovery and Study of Nearby Habitable Planets with Mesolensing

We demonstrate that gravitational lensing can be used to discover and study planets in the habitable zones of nearby dwarf stars. If appropriate software is developed, a new generation of monitoring programs will automatically conduct a census of nearby planets in the habitable zones of dwarf stars. In addition, individual nearby dwarf stars can produce lensing events at predictable times; careful monitoring of these events can discover any planets located in the zone of habitability. Because lensing can discover planets (1) in face-on orbits, and (2) in orbit around the dimmest stars, lensing techniques will provide complementary information to that gleaned through Doppler and/or transit investigations. The ultimate result will be a comprehensive understanding of the variety of systems with conditions similar to those that gave rise to life on Earth.

astro-ph↗

arXiv2007

Beyond Caustic Crossings: Properties of Binary Microlensing Light Curves

Binary microlensing light curves have a variety of morphologies. Many are indistinguishable from point lens light curves. Of those that deviate from the point lens form, caustic crossing light curves have tended to dominate identified binary lens events. Other distinctive signatures of binary lenses include significant asymmetry, multiple peaks, and repeating events. We have quantified, using high resolution simulations, the theoretically expected relative numbers of each type of binary lens event, based on its measurable characteristics. We find that a microlensing survey with current levels of photometric uncertainty and sampling should find at least as many non-caustic crossing binary lens events as caustic crossing events; in future surveys with more sensitive photometry, the contribution of distinctive non-caustic crossing events will be even greater. To try to explain why caustic crossing light curves appear to be so dominant among the published binary lensing events, we investigate the influence of several physical effects, including blending, sampling rate, and various binary populations.

astro-ph↗

arXiv2006

Ten Facts of Life for Distant Supersoft Sources

First discovered in the Magellanic Clouds and in the Milky Way, the largest pools of luminous supersoft X-ray sources (SSSs) now known lie in M31 and in more distant galaxies. Hundreds of newly-discovered SSSs are helping us to test models for Type Ia supernovae and to identify SSSs that may represent a wider range of physical systems, including accreting intermediate-mass black holes. In this short report we list ten intriguing facts about distant SSSs.

astro-ph↗

arXiv2006

Dwarf Galaxies of the Local Group

XMM-Newton and Chandra have ushered in a new era for the study of dwarf galaxies in the Local Group. We provide an overview of the opportunities, challenges, and some early results. The large number of background sources relative to galaxy sources is a major theme. Despite this challenge, the identification of counterparts has been possible, providing hints that the same mechanisms producing X-ray sources in larger galaxies are active in dwarf galaxies. A supersoft X-ray source within 2" of the supermassive black hole in M32 may be a remnant of the tidal disruption of a giant, although other explanations cannot be ruled out.

astro-ph↗

arXiv2005

Mesolensing: I. High Probability Lensing Without Large Optical Depth

In a variety of astronomical situations, there is a relatively high probability that a single isolated lens will produce a detectable event. The high probability is caused by some combination of a large Einstein angle, fast angular motion, and a dense background field. We refer to high-probability lenses as mesolenses. The mesolensing regime is complementary to the regime in which the optical depth is high, in that it applies to isolated lenses instead of to dense lens fields. Mesolensing is complementary to microlensing, because it is well suited to the study of different lens populations, and also because different observing and analysis strategies are required to optimize the discovery and study of mesolenses. Planetary and stellar masses located within 1-2 kpc are examples of mesolenses. We show that their presence can be detected against a wide variety of background fields, using distinctive signatures of both time variability and spatial effects. Time signatures can be identical to those of microlensing, but can also include baseline jitter, extreme apparent chromaticity, events well fit by lens models in which several independent sources are simultaneously lensed, and sequences of events. Spatial signatures include astrometric lensing of surface brightness fluctuations, as well as patterns of time variability that sweep across the background field as the lens moves. Wide-field monitoring programs, such as Pan-STARRS and LSST, are well-suited to the study of mesolensing. In addition, high resolution observations of the region behind a known mass traveling across a background field can use lensing effects to measure the lens mass and study its multiplicity.

astro-ph↗