arXiv subjects

Joel N. Bregman

Publications and source records attributed to Joel N. Bregman.

At least 127 records · Page 7Linked to original sources

arXiv2005

OVI in Elliptical Galaxies: Indicators of Cooling Flows

Early-type galaxies often contain a hot X-ray emitting interstellar medium (3-8E6 K) with an apparent radiative cooling time much less than a Hubble time. If unopposed by a heating mechanism, the gas will radiatively cool to temperatures <= 10E4 K at a rate proportional to L_X/T_X, typically 0.03-1 M_solar yr^-1. We can test if gas is cooling through the 3E5 K range by observing the OVI doublet, whose luminosity is proportional to the cooling rate. Here we report on a study of an unbiased sample of 24 galaxies, obtaining Far Ultraviolet Spectroscopic Explorer spectra to complement the X-ray data of ROSAT} and Chandra. The OVI line emission was detected in about 40% of the galaxies and at a luminosity level similar to the prediction from the cooling flow model. There is a correlation between Mdot_OVI and Mdot_X, although there is significant dispersion about the relationship, where the OVI is brighter or dimmer than expected by a factor of three or more. If the cooling flow picture is to be retained, this dispersion requires that cooling flows be time-dependent, as might occur by the activity of an AGN. However, of detected objects, those with the highest or lowest values of Mdot_OVI/Mdot_X are not systematically hot or cool, as one might predict from AGN heating.

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arXiv2005

Constraints on Intracluster Gas Bulk Motions in Clusters of Galaxies with ASCA

The recent detection of Doppler shifts of X-ray spectral lines in the intracluster gas of the Centaurus and Perseus clusters of galaxies with ASCA has raised the question of the frequency of such velocity gradients and their relation to other cluster properties. To address this and improve the sample of ICM velocity gradient candidates we carried out a systematic search of the ASCA archive for clusters with suitable observing conditions that allow full azimuthal mapping of gas velocities near the central regions. Here we present the results of the azimuthal velocity distribution of a sample consisting of the best" observed clusters for velocity analysis. Our sample encompasses clusters with different physical and morphological characteristics, including both well behaved "cooling flow" and dynamically active clusters. The gas temperatures of the clusters in our sample range from 1.4 keV to 6.3 keV. We find velocity substructures in ~ 15% of the clusters in our sample on the order of a few x 1E3 km/s. The significance of the velocity gradients' is greater than 99% in Abell 576 and RXJ0419.6+0225, the clusters where velocity gradients are most significant. The velocity gradients are consistent with transitory and/or rotational bulk motion. The velocity gradients in these clusters are unlikely to be due to projected temperature anisotropies, ASCA PSF, intrachip gain variations, and flux contamination from other sources. Chandra images of these clusters do not show obvious cluster alignments within the field of view covered by ASCA. We also noticed a high frequency (~50%) of anisotropic temperature gradients in the core of "cooling flow" clusters, suggesting the presence of frequent dynamical activity in the core of galaxy clusters.

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arXiv2005

Optical studies of ultra-luminous X-ray sources in nearby galaxies

Optical studies of ultra-luminous X-ray sources is an essential step in understanding the nature of these enigmatic sources, and in this paper we report our studies of five ULXs in NGC4559, NGC5194, NGC1313 and NGC628 observed with the Hubble Space Telescope and the 6.4 Magellan/Baade telescope. The ULX in NGC4559 is identified with four blue and red supergiants within a $0\farcs2$ error circle, in a star forming region that is younger than $10^{7.4}$ years. ULX-3 in NGC5194 is located on the rim of a star cluster with a few faint stars within the $0\farcs3$ error circle, though the secondary in this system is expected to be a low mass ($0.3M_\odot$) star below detection limits given its two hour period. ULX-5 in NGC5194 is located on a spiral arm with seven objects within the $0\farcs8$ error circle, which can be improved with future observations. Both ULXs in NGC5194 are in regions younger than $10^{7.8}$ years. The ULXs in NGC1313 and NGC628 was observed with the Baade telescope. The ULX in NGC1313 is identified with one object with R-I color bluer than the bluest stellar objects, indicative of non-thermal emission in the R band, or dramatic variability between two observations, or errors in measurements. The ULX in NGC628 is located in a bubble nebula and identified with a few extended objects that are probably young stellar clusters. Future observations with better spatial resolutions are needed to clarify the optical counterparts and their nature for the ULXs in NGC1313 and NGC628.

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arXiv2005

Ultra-luminous X-ray Sources in nearby galaxies from ROSAT HRI observations I. data analysis

X-ray observations have revealed in other galaxies a class of extra-nuclear X-ray point sources with X-ray luminosities of $10^{39}$--$10^{41}$ erg/sec, exceeding the Eddington luminosity for stellar mass X-ray binaries. These ultra-luminous X-ray sources (ULXs) may be powered by intermediate mass black hole of a few thousand $M_\odot$ or stellar mass black holes with special radiation processes. In this paper, we present a survey of ULXs in 313 nearby galaxies with $D_{25}$$>$$1^\prime$ within 40 Mpc with 467 ROSAT HRI archival observations. The HRI observations are reduced with uniform procedures, refined by simulations that help define the point source detection algorithm employed in this survey. A sample of 562 extragalactic X-ray point sources with $L_X = 10^{38}$--$10^{43}$ erg/sec is extracted from 173 survey galaxies, including 106 ULX candidates within the $D_{25}$ isophotes of 63 galaxies and 110 ULX candidates between 1--2$\times D_{25}$ of 64 galaxies, from which a clean sample of 109 ULXs is constructed to minimize the contamination from foreground or background objects. In a subsequent paper, the statistic properties of the survey will be studied to calculate the occurrence frequencies and luminosity functions for ULXs in different types of galaxies to shed light on the nature of these enigmatic sources.

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arXiv2005

A Two Hour Quasi-Period in an Ultra-luminous X-Ray source in NGC628

Quasi-periodic oscillations and X-ray spectroscopy are powerful probes of black hole masses and accretion disks, and here we apply these diagnostics to an ultraluminous X-ray source (ULX) in the spiral galaxy NGC628 (M74). This object was observed four times over two years with the Chandra X-ray Observatory and XMM-Newton, with three long observations showing dramatic variability, distinguished by a series of outbursts with a quasi-period (QPO) of 4,000-7,000 seconds. This is unique behavior among both ULXs and Galactic X-ray binaries due to the combination of its burst-like peaks and deep troughs, its long quasi-periods, its high variation amplitudes of $>90$%, and its substantial variability between observations. The X-ray spectra is fitted by an absorbed accretion disk plus a power-law component, suggesting the ULX was in a spectral state analogous to the Low Hard state or the Very High state of Galactic black hole X-ray binaries. A black hole mass of $\sim2$--$20\times10^3 M_\odot$ is estimated from the $f_b$--$M_\bullet$ scaling relation found in the Galactic X-ray binaries and active galactic nuclei.

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arXiv2005

Ultra-luminous X-ray Sources in nearby galaxies from ROSAT HRI observations II. statistical properties

The statistical properties of the ultra-luminous X-ray source (ULX) populations extracted from the ROSAT HRI survey of X-ray point sources in nearby galaxies in Paper I are studied to reveal connections between the ULX phenomenon and survey galaxy properties. The $logN$--$logS$ relation is used to calculate and remove the influence of false ULXs from the background and/or foreground objects. Study of the luminosity functions shows that the regular ULXs below $10^{40}$ erg/sec are an extension of the ordinary HMXB/LMXB population below $10^{39}$ erg/sec in the late-type galaxies, and that the extreme ULXs above $10^{40}$ erg/sec might be a different population from the regular ULXs. This survey confirms statistically that the ULX phenomenon is closely connected to star formation activities, since ULXs preferentially occur in late-type galaxies rather than in early-typTwo ULX populations, the HMXB-like ULXs as an extension of the ordinary HMXB population associated with the young stellar population and the LMXB-like ULXs as an extension of the ordinary LMXB population associated with the old stellar population, are both required to account for the total ULX population. It is found that the LMXB-like ULXs dominate the ULX population at low star formation rates, while HMXB-like ULXs dominate at high star formation rates. However, an accurate quantitative description of the relative fractions of HMXB-like ULXs and LMXB-like ULXs is impossible in the current HRI survey due to the source blending effects, and a Chandra survey of the X-ray point sources in nearby galaxies would be required for this purpose. e galaxies, and ULXs in late-type galaxies tend to trace the spiral arms.

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arXiv2004

High-Velocity Clouds in M 83 and M 51

Various scenarios have been proposed to explain the origin of the Galactic high-velocity clouds, predicting different distances and implying widely varying properties for the Galaxy's gaseous halo. To eliminate the difficulties of studying the Galactic halo from within, we have embarked on a program to study anomalous neutral gas in external galaxies, and here we present the results for two nearby, face-on spiral galaxies, M 83 and M 51. Significant amounts of anomalous-velocity HI are detected in deep VLA 21-cm observations, including an extended, slowly rotating disk and several discrete HI clouds. Our detection algorithm reaches a limiting HI source mass of 7x10^5 solar masses, and it allows for detailed statistical analysis of the false detection rate. We use this to place limits on the HVC mass distributions in these galaxies and the Milky Way; if the HVC populations are similar, then the Galactocentric HVC distances must be less than about 25 kpc.

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arXiv2004

Hot Baryons in Supercluster Filaments

Most of the baryons in the local universe are ``missing'' in that they are not in galaxies or in the previously detected gaseous phases. These missing baryons are predicted to be in a moderately hot phase, 10^5-10^7 K, largely in the form of giant cosmic filaments that connect the denser virialized clusters and groups of galaxies. Models show that the highest covering fraction of such filaments occurs in superclusters. To determine whether such filaments exist, we have begun a project to search for UV absorption against AGNs projected behind possible supercluster filaments. Using data from the HST and FUSE archives along with new observations, we have detected UV absorption within about 1300 km/s of seven supercluster sightlines out of a sample of eight. The likelihood of such detections being generated by chance is less than 10^-4.

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arXiv2004

Cosmic Filaments in Superclusters

Large-scale structure calculations show that modest overdensity filaments will connect clusters of galaxies and these filaments are reservoirs of baryons, mainly in gaseous form. To determine whether such filaments exist, we have examined the UV absorption line properties of three AGNs projected behind possible filaments in superclusters of galaxies; the AGNs lie within 3 Mpc of the centerlines of loci connecting clusters. All three lines of sight show absorption in Lyα, Lyβ, or/and OVI at redshifts within about 1300 km/s of the nearby galaxy clusters that would define the closest filaments. For one AGN, the absorption line redshifts are close to the emission line redshift of the AGN, so we cannot rule out self-absorption for this object. These absorption line associations with superclusters are unlikely to have occurred by chance, a result consistent with the presence of cosmic filaments within superclusters.

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arXiv2003

The Lack of Very Ultraluminous X-ray Sources in Early-type Galaxies

We have searched for ultraluminous X-ray sources (ULXs) in a sample of 28 elliptical and S0 galaxies observed with Chandra. We find that the number of X-ray sources detected at a flux level that would correspond to a 0.3-10 keV X-ray luminosity of ~2 x 10^39 ergs/s or greater (for which we have used the designation very ultraluminous X-ray sources; VULXs) at the distance of each galaxy is equal to the number of expected foreground/background objects. In addition, the VULXs are uniformly distributed over the Chandra field of view rather than distributed like the optical light of the galaxies, strengthening the argument that the high flux sources are unassociated with the galaxies. We have also taken the VULX candidate list of Colbert and Ptak and determined the spatial distribution of VULXs in early-type galaxies and late-type galaxies separately. While the spiral galaxy VULXs are clearly concentrated toward the centers of the galaxies, the early-type galaxy VULXs are distributed randomly over the ROSAT HRI field of view, again indicating that they are not associated with the galaxies themselves. We conclude that with the exception of two rare high luminosity objects within globular clusters of the elliptical galaxy NGC1399, VULXs are generally not found within old stellar systems. However, we do find a significant population of sources with luminosities of 1-2 x 10^39 ergs/s that reside within the sample galaxies that can be explained by accretion onto 10-20 solar mass black holes. Given our results, we propose that ULXs be defined as X-ray sources with L_X(0.3-10 keV) > 2 x 10^39 ergs/s.

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arXiv2003

Small-Scale structure in the Galactic ISM: Implications for Galaxy Cluster Studies

Observations of extragalactic objects need to be corrected for Galactic absorption and this is often accomplished by using the measured 21 cm HI column. However, within the beam of the radio telescope there are variations in the HI column that can have important effects in interpreting absorption line studies and X-ray spectra at the softest energies. We examine the HI and DIRBE/IRAS data for lines of sight out of the Galaxy, which show evidence for HI variations in of up to a factor of three in 1 degree fields. Column density enhancements would preferentially absorb soft X-rays in spatially extended objects and we find evidence for this effect in the ROSAT PSPC observations of two bright clusters of galaxies, Abell 119 and Abell 2142. For clusters of galaxies, the failure to include column density fluctuations will lead to systematically incorrect fits to the X-ray data in the sense that there will appear to be a very soft X-ray excess. This may be one cause of the soft X-ray excess in clusters, since the magnitude of the effect is comparable to the observed values.

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arXiv2002

X-ray Spectral Properties of Low-Mass X-ray Binaries in Nearby Galaxies

We investigated the X-ray spectral properties of a collection of low-mass X-ray binaries (LMXBs) within a sample of 15 nearby early-type galaxies observed with Chandra. We find that the spectrum of the sum of the sources in a given galaxy is remarkably similar from galaxy to galaxy when only sources with X-ray luminosities less than 10^39 ergs/s (0.3-10 keV) are considered. Fitting these lower luminosity sources in all galaxies simultaneously yielded a best-fit power law exponent of Gamma = 1.56 +/- 0.02 (or kT_brem = 7.3 +/- 0.3 keV). This is the tightest constraint to date on the spectral properties of LMXBs in external galaxies. There is no apparent difference in the spectral properties of LMXBs that reside inside and outside globular clusters. We demonstrate how the uniformity of the spectral properties of LMXBs can lead to more accurate determinations of the temperature and metallicity of the hot gas in galaxies. Although few in number in any given galaxy, sources with luminosities of 1-2 x 10^39 ergs/s are present in 10 of the galaxies. The spectra of these luminous sources are softer than the spectra of the rest of the sources, and are consistent with the spectra of Galactic black hole X-ray binary candidates when they are in their very high state. The spectra of these sources are very different than those of ultraluminous X-ray sources (ULXs) found within spiral galaxies, suggesting that the two populations of X-ray luminous objects have different formation mechanisms. The number of sources with apparent luminosities above 2 x 10^39 ergs/s is equal to the number of expected background AGN and thus appear to not be associated with the galaxy, indicating that very luminous sources are absent or very rare in early-type galaxies. (Abridged)

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arXiv2002

The optical counterpart of a Ultraluminous X-Ray Object in M81

Ultraluminous X-Ray Objects are off-nucleus point sources with $L_X=10^{39}$-$10^{41}$ erg/s and are among the most poorly understood X-ray sources. To help understand their nature, we are trying to identify their optical counterparts by combining images from the Hubble Space Telescope and the Chandra Observatory. Here we report an optical counterpart for ULX NGC 3031 X-11, which has average X-ray luminosity of $\sim2\times10^{39}$ erg/s and has varied by a factor of 40% over the last 20 years. We find a unique optical counterpart with the magnitude and color of an O8V star and we identify this as the secondary in a binary system. The primary is believed to be a black hole of approximately 18 $M_\odot$, based on analyses of ACIS and ASCA spectra. This binary system probably is powered by mass transfer from the O8V star onto the black hole, with the Roche Lobe equal to the stellar radius. This model predicts an orbital period of $\sim1.8$ days, which can be tested by future observations.

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arXiv2002

An Ultra-luminous X-Ray Object with a 2 hour period in M51

Ultra-luminous X-Ray Objects are off-nucleus point sources with $L_X=10^{39}$-$10^{41}$ erg/s but the nature of such systems are largely unidentified. Here we report a 2.1 hour period observed in a Chandra ACIS observation for ULX M51 X-7, which is located on the edge of a young star cluster in the star forming region in a spiral arm. In two ACIS observations separated by one year, the ULX changed from a high-hard to a low-soft spectral state, in contrast to most Galactic low mass X-ray binaries. Based on its period and spectral behaviors, we suggest that this ULX is a low mass X-ray binary system, with a dwarf companion of 0.2-0.3 $M_\odot$ and a compact accretor, either a neutron star or a black hole, whose mass is not well constrained. Relativistic beaming effects are likely involved to produce the observed high X-ray luminosities given its low accretion rate as inferred from a sustainable accretion scenario via Roche lobe overflow.

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arXiv2002

A Scattered Light Echo around SN 1993J in M81

A light echo around SN 1993J was observed 8.2 years after explosion by a HST WFPC2 observation, adding to the small family of supernovae with light echoes. The light echo was formed by supernova light scattered from a dust sheet, which lies 220 parsecs away from the supernova, 50 parsecs thick along the line of sight, as inferred from radius and width of the light echo. The dust inferred from the light echo surface brightness is 1000 times denser than the intercloud dust. The graphite to silicate fraction can not be determined by our BVI photometric measurements, however, a pure graphite model can be excluded based on comparison with the data. With future observations, it will be possible to measure the expansion rate of the light echo, from which an independent distance to M81 can be obtained.

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arXiv2002

Untangling the X-ray Emission From the Sa Galaxy NGC1291 With Chandra

We present a Chandra ACIS-S observation of the nearby bulge-dominated Sa galaxy NGC1291. The X-ray emission from the bulge resembles the X-ray emission from a sub-class of elliptical and S0 galaxies with low L_X/L_B luminosity ratios. The X-ray emission is composed of a central point-like nucleus, ~50 point sources that are most likely low mass X-ray binaries (LMXBs), and diffuse gas detectable out to a radius of 120" (5.2 kpc). The diffuse gas has a global temperature of 0.32^{+0.04}_{-0.03} keV and metallicity of 0.06 +/- 0.02 solar, and both quantities marginally decrease with increasing radius. The hot gas fills the hole in the HI distribution, and the softening of the spectrum of the X-ray gas with radius might indicate a thermal coupling of the hot and cold phases of the interstellar medium as previously suggested. The integrated X-ray luminosity of the LMXBs, once normalized by the optical luminosity, is a factor of 1.4 less than in the elliptical galaxy NGC4697 or S0 galaxy NGC1553. The difference in L_{X,stellar}/L_B between the galaxies appears to be because of a lack of very bright sources in NGC1291. No sources above 3 x 10^38 ergs/s were found in NGC1291 when ~7 were expected from scaling from NGC4697 and NGC1553. The cumulative L_{X,stellar}/L_B value including only sources below 1.0 x 10^38 ergs/s is remarkably similar between NGC1291 and NGC4697, if a recent surface brightness fluctuation-determined distance is assumed for NGC4697. If this is a common feature of the LMXB population in early-type systems, it might be used as a distance indicator. Finally, a bright, variable (1.6-3.1 x 10^39 ergs/s) source was detected at the optical center of the galaxy. Its spectrum shows excess soft emission superimposed on a highly absorbed power law component, similar to what has been found in several other low luminosity AGN (ABRIDGED).

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arXiv2002

Prospects for Detection of Intracluster Gas Bulk Velocities Through the Sunyaev-Zel'dovich Effect

Intracluster gas velocity gradients have been recently detected by the authors in the Centaurus cluster (Abell 3526) using the Doppler shift of X-ray spectral lines with ASCA Solid-state Imaging Spectrometers. The velocity gradient was found to be maximum along a line roughly perpendicular to the direction of the incoming subgroup Cen 45 and has a correspondent velocity difference of ~ (3.4+-1.1) x 1000 km/s within a ~10'-diameter region centered on the cD galaxy NGC 4696. Such bulk velocities should Comptonize the Cosmic Microwave Background Radiation producing variations of intensity and temperature that can be detectable in the near future with bolometers such as BOLOCAM . In this paper we realistically estimate the expected CMBR Comptonization for the central region of Abell 3526, using ASCA and ROSAT data to constrain the S-Z parameter expectations.

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arXiv2002

Mid-IR Observations of Mass Loss in Elliptical Galaxies

Early-type galaxies exhibit thermal and molecular resonance emission from dust that is shed and heated through stellar mass loss as a subset of the population moves through the AGB phase of evolution. Because this emission can give direct insight into stellar evolution in addition to galactic stellar mass loss and ISM injection rates, we conducted a program to search for this signature emission with CAM on ISO. We obtained 6-15 micron imaging observations in six narrow bands for nine elliptical galaxies; every galaxy is detected in every band. For wavelengths shorter than 9 microns, the spectra are well matched by a blackbody, originating from the K and M stars that dominate the integrated light of elliptical galaxies. However, at wavelengths between 9 and 15 microns, the galaxies display excess emission relative to the stellar photospheric radiation. Additional data taken with the fine resolution circular variable filter on one source clearly shows broad emission from 9 to 15 microns, peaking around 10 microns. This result is consistent with the known, broad silicate feature at 9.7 microns, originating in the circumstellar envelopes of AGB stars. This emission is compared with studies of Galactic and LMC AGB stars to derive cumulative mass loss rates. In general, these mass loss rates agree with the expected ~0.8 solar masses per year value predicted by stellar evolutionary models. Both the photospheric and circumstellar envelope emission follow a de Vaucouleurs' R^{1/4} law, supporting the conclusion that the mid-infrared excess emission originates in the stellar component of the galaxies and acts as a tracer of AGB mass loss and mass injection into the ISM.

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