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Raimundo Lopes de Oliveira

Publications and source records attributed to Raimundo Lopes de Oliveira.

30 records · Page 2Linked to original sources
arXiv2018

SU Lyn: diagnosing the boundary layer with UV and hard X-ray data

Symbiotic stars in which the symbiotic phenomenon is powered solely by accretion, often at an average rate that is higher than in cataclysmic variable stars, provide an important opportunity to diagnose boundary layers around disk-accreting white dwarfs. Here we investigate SU Lyncis, a recently discovered example of a purely accretion-powered symbiotic star, using the first reliable X-ray spectroscopy, obtained with NuSTAR, and UV photometry obtained with Swift. SU Lyn has hard, thermal, X-ray emission that is strongly affected by a variable local absorber - that has little impact on the UV emission. Its X-ray spectrum is described well using a plasma cooling from $k$T $\approx$ 21 keV, with a 3 to 30 keV luminosity of approximately 4.9$\times$10$^{32}$ ergs s$^{-1}$. The spectrum is also consistent with the presence of reflection with an amplitude of 1.0, although in that case, the best-fit plasma temperature is 20-25% lower. The UV to X-ray luminosity ratio of SU Lyn changed significantly between 2015 and 2016. We interpret this as a consequence of a drop by almost 90% in the accretion rate. Whereas the UV luminosity of the disk responded linearly, the luminosity of the optically thin (hard X-ray) emission from the boundary layer remained roughly constant because the boundary layer changed from partially optically thick to almost completely optically thin. Under this interpretation, we place a lower limit on the white dwarf mass of 0.7 M$_{\odot}$ (0.8 M$_{\odot}$ if we neglect reflection).

astro-ph.SR↗
arXiv2015

The origin of the puzzling hard X-ray emission of $γ$ Cassiopeiae

Massive B and Be stars produce X-rays from shocks in high velocity winds with temperatures of a few million degrees and maximum X-ray luminosities of $\approx$ 10$^{31}$ erg/s. Surprisingly, a sub-group of early Be stars exhibits > 20 times hotter X-ray temperatures and > 10 times higher X-ray luminosities than normal. This group of Be stars, dubbed Gamma-Cas analogs, contains about 10 known objects. The origin of this bizarre behavior has been extensively debated in the past decades. Two mechanisms have been put forward, accretion of circumstellar disk matter onto an orbiting white dwarf, or magnetic field interaction between the star and the circumstellar disk (Smith & Robinson 1999). We show here that the X-ray and optical emissions of the prototype of the class, Gamma-Cas, are very well correlated on year time scales with no significant time delay. Since the expected migration time from internal disk regions that emit most of the optical flux to the orbit of the companion star is of several years, the simultaneity of the high energy and optical fluxes variations indicates that X-ray emission arises from close to the star. The systematic lack of magnetic field detection reported in recent spectro-polarimetric surveys of Be stars is consistent with the absence of strong magnetic wind braking in these fast spinning stars but put strong constraints on the possible origin of the magnetic field. We propose that in Gamma-Cas the magnetic field emerges from equatorially condensed subsurface convecting layers the thickness of which steeply increases with rotation rate and that Gamma-Cas and its analogs are the most massive and closest to critical rotation Be stars

astro-ph.HE↗
arXiv2012

Characterization of the X-ray light curve of the gamma Cas-like B1e star HD110432

HD 110432 (BZ Cru; B1Ve) is the brightest member of a small group of "gamma Cas analogs" that emit copious hard X-ray flux, punctuated by ubiquitous "flares." To characterize the X-ray time history of this star, we made a series of six RXTE multi-visit observations in 2010 and an extended observation with the XMM-Newton in 2007. We analyzed these new light curves along with three older XMM-Newton observations from 2002--2003. Distributed over five months, the RXTE observations were designed to search for long X-ray modulations over a few months. These observations indeed suggest the presence of a long cycle with P = 226 days and an amplitude of a factor of two. We also used X-ray light curves constructed from XMM-Newton observations to characterize the lifetimes, strengths, and interflare intervals of 1615 flare-like events in the light curves. After accounting for false positive events, we infer the presence of 955 (2002-2003) and 386 (2007) events we identified as flares. Similarly, as a control we measured the same attributes for an additional group of 541 events in XMM-Newton light curves of gamma Cas, which after a similar correction yielded 517 flares. We found that the flare properties of HD 110432 are mostly similar to our control group. In both cases the distribution of flare strengths are best fit with log-linear relations. Both the slopes of these distributions and the flaring frequencies themselves exhibit modest fluctuations. We discovered that some flares in the hard X-ray band of HD 110432 were weak or unobserved in the soft band and vice versa. The light curves also occasionally show rapid curve drop offs that are sustained for hours. We discuss the existence of the long cycle and these flare properties in the backdrop of two rival scenarios to produce hard X-rays, a magnetic star-disk interaction and the accretion of blobs onto a secondary white dwarf.

astro-ph.HE↗
arXiv2011

Finding Fossil Groups: Optical Identification and X-ray Confirmation

We report the discovery of 12 new fossil groups of galaxies, systems dominated by a single giant elliptical galaxy and cluster-scale gravitational potential, but lacking the population of bright galaxies typically seen in galaxy clusters. These fossil groups (FGs), selected from the maxBCG optical cluster catalog, were detected in snapshot observations with the Chandra X-ray Observatory. We detail the highly successful selection method, with an 80% success rate in identifying 12 FGs from our target sample of 15 candidates. For 11 of the systems, we determine the X-ray luminosity, temperature, and hydrostatic mass, which do not deviate significantly from expectations for normal systems, spanning a range typical of rich groups and poor clusters of galaxies. A small number of detected FGs are morphologically irregular, possibly due to past mergers, interaction of the intra-group medium (IGM) with a central AGN, or superposition of multiple massive halos. Two-thirds of the X-ray-detected FGs exhibit X-ray emission associated with the central BCG, although we are unable to distinguish between AGN and extended thermal galaxy emission using the current data. This sample, a large increase in the number of known FGs, will be invaluable for future planned observations to determine FG temperature, gas density, metal abundance, and mass distributions, and to compare to normal (non-fossil) systems. Finally, the presence of a population of galaxy-poor systems may bias mass function determinations that measure richness from galaxy counts. When used to constrain power spectrum normalization and Ω_m, these biased mass functions may in turn bias these results.

astro-ph.CO↗
arXiv2011

On the mass-to-light ratios of fossil groups. Are they simply dark clusters?

Defined as X-ray bright galaxy groups with large differences between the luminosities of their brightest and second brightest galaxies, "fossil groups" are believed to be some of the oldest galaxy systems in the universe. They have therefore been the subject of much recent research. In this work we present a study of 10 fossil group candidates with an average of 33 spectroscopically confirmed members per group, making this the deepest study of its type to-date. We also use this data to perform an analysis of the luminosity function of our sample of fossil groups. We confirm the high masses previously reported for many of fossil systems, finding values more similar to those of clusters than of groups. We also confirm the high dynamical mass-to-light ratios reported in many previous studies. While our results are consistent with previous studies in many ways, our interpretation is not. This is because we show that, while the luminosities of the BCGs in these systems are consistent with their high dynamical masses, their richnesses (total number of galaxies above some canonical value) are extremely low. This leads us to suggest a new interpretation of fossil systems in which the large differences between the luminosities of their brightest and second brightest galaxies are simply the result the high BCG luminosities and low richnesses, while the high masses and low richnesses also explain the high mass-to-light ratios. Our results therefore suggest that fossil systems can be characterised as cluster-like in their masses and BCG luminosities, but possessing the richnesses and optical luminosities of relatively poor groups. These findings are not predicted by any of the current models for the formation of fossil groups. Therefore, if this picture is confirmed, current ideas about the formation and evolution of fossil systems will need to be reformulated.

astro-ph.CO↗
arXiv2011

A hard and variable X-ray emission from the massive emission line star HD 157832

We report the discovery of a hard-thermal (T ~ 130 MK) and variable X-ray emission from the Be star HD 157832, a new member of the puzzling class of gamma-Cas-like Be/X-ray systems. Recent optical spectroscopy reveals the presence of a large/dense circumstellar disc seen at intermediate/high inclination. With a B1.5V spectral type, HD 157832 is the coolest gamma-Cas analog known. In addition, its non detection in the ROSAT all-sky survey shows that its average soft X-ray luminosity varied by a factor larger than ~ 3 over a time interval of 14 yr. These two remarkable features, ``low'' effective temperature and likely high X-ray variability turn HD 157832 into a promising object for understanding the origin of the unusually high temperature X-ray emission in these systems.

astro-ph.SR↗
arXiv2010

gamma Cassiopeiae: an X-ray Be star with personality

gamma Cassiopeiae (gCas) is a B0.5e star with peculiar X-ray emission properties and yet the prototype of its own small class. In this paper we examine the X-ray spectra for a 2004 XMM-Newton observation and a previously published 2001 Chandra observation. In both cases the spectra can be modeled with 3 or 4 thermal components, which appear be discrete in temperature and spatially distinct. The dominant component, having kT ~ 12 keV contributes most (~80-90%) of the flux. The secondary components have temperatures in the range of 2-3 keV to 0.1 keV; these values can shift in time. Importantly, we find that the strong absorption of soft X-rays in 2001 is absent in 2004, meaning that an absorbing column in front of the source has moved off the star or has been removed. Other differences include a reduced Fe abundance from the ionized lines of the FeKalpha complex (even more subsolar than the 2001 observation), a decrease of the Fe K and possibly of the Si K fluorescence features, and from the NVII and NeX H-alpha lines, a possible overabundance of N and Ne. Also, we note common characteristics in both spectra that seem to set gCas apart from HD110342, another member of this subclass studied in detail. In this sense these stars have different "personalities." For example, for gCas rapid X-ray flaring and slower changes in the light curve are only seldomly accompanied by variations in hardness, and the hot X-ray component remains nearly constant in temperature. Moreover, the light curve shows recurrent "lulls" in flux, suggesting that a relaxation cycle is operates as part of the (unknown) X-ray generation process.

astro-ph.HE↗
arXiv2009

Revisiting the fossil group candidates UGC 842 and NGC 6034

We present a new insight on NGC 6034 and UGC 842, two groups of galaxies previously reported in the literature as being fossil groups. The study is based on optical photometry and spectroscopy obtained with the CTIO Blanco telescope and Sloan Digital Sky Survey archival data. We find that NGC 6034 is embedded in a large structure, dominated by three rich clusters and other small groups. Its first and next four ranked galaxies have magnitude differences in the r band and projected distances which violate the optical criteria to classify it as a fossil group. We confirm that the UGC 842 group is a fossil group, but with about half the velocity dispersion that is reported in previous works. The velocity distribution of its galaxies reveals the existence of two structures in its line of sight, one with sigmaV ~ 223 km/s and another with sigmaV ~ 235 km/s, with a difference in velocity of ~820 km/s. The main structure is dominated by passive galaxies, while these represent ~60% of the second structure. The X-ray temperature for the intragroup medium of a group with such a velocity dispersion is expected to be kT ~0.5-1 keV, against the observed value of kT ~1.9 keV reported in the literature. This result makes UGC 842 a special case among fossil groups because (1) it represents more likely the interaction between two small groups, which warms the intragroup medium and/or (2) it could constitute evidence that member galaxies lost energy in the process of spiraling toward the group center, and decreased the velocity dispersion of the system. As far as we know, UGC 842 is the first low-mass fossil group studied in detail.

astro-ph.CO↗
arXiv2007

On the X-ray and optical properties of the Be star HD 110432: a very hard-thermal X-ray emitter

(ABRIDGED) - HD 110432 is the first proposed, and best studied, member of a growing group of Be stars with X-ray properties similar to gamma Cas. These stars exhibit hard-thermal X-ray emissions (kT ~> 7 keV) that are variable on all measurable timescales. In this work we present X-ray spectral and timing properties of HD 110432 from three XMM-Newton observations in addition to new optical spectroscopic observations. Its X-ray spectrum, complex and timing dependent, is well described in each observation by three thermal plasmas with temperatures ranging between 0.4-0.7, 3-6, and 21-37 keV. Thus, HD 110432 has the hottest thermal plasma of any known Be star. A sub-solar iron abundance (~ 0.3-0.5 x ZFe_sun) is derived for the hottest plasma. The star has a moderate 0.2-12 keV luminosity of ~ 5 x 10^32 erg/s. Recurrent flare-like events on time scales as short as ~ 10 seconds are superimposed over a slowly ~ 5-10 x 10^3 seconds varying basal flux, followed by similarly rapid hardness variabilities. There is no evidence for coherent oscillations, and an upper limit of ~ 2.5% is derived on the pulsed fraction for short pulsations from 0.005 to 2.5 Hz. In the optical region the strong and quasi-symmetrical profile of the Halpha line (EW \~ -60 Angs.) as well as the detection of several metallic lines in emission strongly suggest a dense and/or large circumstellar disk. HD 110432 has several properties reminiscence of the cataclysmic variables such as a very hot X-ray temperature and its detailed spectral features. This suggests that it might be a Be star harbouring an accreting white dwarf. On the other hand, accumulating evidence of magnetic activities in the literature supports the surface-disk of the star as being the X-ray site.

astro-ph↗
arXiv2006

New Gamma Cas-like objects: X-ray and optical observations of SAO49725 and HD161103

A growing number of early Be stars exhibit unusually hard X-ray spectra and luminosities intermediate between those typical of early type stars and those emitted by most Be/X-ray binaries in quiescence. We report on XMM-Newton and optical observations of two such Be stars, SAO 49725 and HD 161103. The nature of the hard-thermal X-ray emission is discussed in the light of the models proposed for Gamma Cas, magnetic disc-star interaction or accretion onto a compact companion object - neutron star or white dwarf. These two new objects added to similar cases discovered in XMM-Newton surveys point at the emergence of a new class of Gamma Cas analogs.

astro-ph↗
arXiv2005

X-ray and optical properties of new Gamma Cas-like objects discovered in X-ray surveys

A growing number of early Be stars discovered in X-ray surveys exhibit X-ray luminosities intermediate between those of normal stars and those of most Be/X-ray binaries in quiescence. Their X-ray spectra are also much harder than those of shocked wind OB stars and can be best fitted by a thin thermal plasma with T ~ 10^8 K, added to a cooler and much fainter thermal component. An iron line complex including a fluorescence component is detected in many cases. There is no evidence for coherent pulsations in any of these systems but strong variability on time scales as short as 100 s is usually observed. Large oscillations with quasi-periods of the order of one hour or more are detected in the X-ray light curves of several sources, but have so far failed to prove to be strictly periodic. The optical and X-ray properties of these new objects strikingly resemble those of the so far unique and enigmatic Be star Gamma-Cas and define a new class of X-ray emitters. We discuss the possible origin of the X-ray emission in the light of the models proposed for Gamma-Cas, magnetic disc-star interaction or accretion onto a compact companion object -- neutron star or white dwarf.

astro-ph↗