Publications (23)16.95 Total impact
-
Article: Supersoft X-Ray Sources in M31. I. A Chandra Survey and an Extension to Quasi-soft Sources
[show abstract] [hide abstract]
ABSTRACT: We report on very soft X-ray sources (VSSs) in M31. In a survey that was most sensitive to soft sources in four 8' × 8' regions covered by Chandra's ACIS-S S3 CCD, we find 33 VSSs that appear to belong to M31. Fifteen VSSs have spectral characteristics mirroring the supersoft X-ray sources studied in the Magellanic Clouds and Milky Way (kTeff ≤ 100 eV); we therefore call these "classical" supersoft sources, or simply supersoft sources (SSSs). Eighteen VSSs may have either small (<10%) hard components or slightly higher effective temperatures (but still less than 350 eV). We refer to these VSSs as quasi-soft sources (QSSs). While hot white dwarf models may apply to SSSs, the effective temperatures of QSSs are too high, unless, e.g., the radiation emanates from only a small portion of surface. Two of the SSSs were first detected and identified as such through ROSAT observations. One SSS and one QSS may be identified with symbiotics and two SSSs with supernova remnants. Both SSSs and QSSs in the disk are found near star-forming regions, possibly indicating that they are young. VSSs in the outer disk and halo are likely to be old systems; in these regions, there are more QSSs than SSSs, which is opposite to what is found in fields closer to the galaxy center. The largest density of bright VSSs is in the bulge; some of the bulge sources are close enough to the nucleus to be remnants of the tidal disruption of a giant by the massive central black hole. By using Chandra data in combination with ROSAT and XMM observations, we find most VSSs to be highly variable, fading from or brightening toward detectability on timescales of months. There is evidence for VSSs with low luminosities (~1036 ergs s-1).The Astrophysical Journal 12/2008; 610(1):247. · 6.02 Impact Factor -
Article: X-Ray Point Sources in the Sombrero Galaxy: Very Soft Sources, the Globular Cluster/Low-Mass X-Ray Binary Connection, and an Overview
[show abstract] [hide abstract]
ABSTRACT: We report on the population of point sources discovered during an 18.5 ks Chandra ACIS-S observation of the Sombrero galaxy. We present the luminosity function and the spectra of the six brightest sources, consider correlations with globular clusters (GCs) and with planetary nebulae, and study the galaxy's population of very soft sources. We detected 122 sources. Twenty-two sources are identified as very soft; of these, five appear to be classical luminous supersoft X-ray sources (SSSs), while 17 may belong to the slightly harder class referred to as quasi-soft (QSSs). There is an overdensity of very soft sources within 2 kpc of the nucleus, which is itself the brightest X-ray source. Very soft sources are also found in the disk and halo, with one QSS in a globular cluster (GC). This source is somewhat harder than most SSSs; the energy distribution of its photons is consistent with what is expected from an accreting intermediate-mass black hole. Several sources in the Sombrero's halo are good candidates for SSS models in which the accretor is a nuclear-burning white dwarf. In total, 32 X-ray sources are associated with GCs. The majority of sources with luminosity greater than 1038 ergs s-1 are in GCs. These results for M104, an Sa galaxy, are similar to what has been found for elliptical galaxies and for the late-type spiral M31. We find that those optically bright GCs with X-ray sources house only the brightest X-ray sources. We find that, in common with other galaxies, there appears to be a positive connection between young (metal-rich) GCs and X-ray sources but that the brightest X-ray sources are equally likely to be in metal-poor GCs. The luminosity function of X-ray sources in GCs has a cut-off near the Eddington luminosity for a 1.4 M☉ object. We propose a model that can explain the trends seen in the data sets from the Sombrero and other galaxies. Thermal timescale mass transfer can occur in some of the younger clusters in which the turnoff mass is slightly greater than 0.8 M☉; multiplicity may play a role in some of the most massive clusters; accretion from giant stars may be the dominant mechanism in some older, less massive and less centrally concentrated clusters. Key elements of the model can be tested.The Astrophysical Journal 12/2008; 599(2):1067. · 6.02 Impact Factor -
Article: Chandra Observations of the M31 X-Ray Transient SWIFT J004320.5+411528
[show abstract] [hide abstract]
ABSTRACT: As part of our ongoing Chandra/HST M31 transient program we report on a Chandra ACIS observation of M31 containing the new transient source reported by Pietsch et al. (2008, ATel #1674). A single 5ks observation was performed on 2008-09-01.32 UT. The Chandra coordinates for this source (subject to the boresight correction) are RA(J2000) 00:43:20.710, Dec(J2000) +41:15:31.31. Although we cannot rule out an absorbed power law, an absorbed disk black body with nH = 8.4e20 atoms/cm2 and Tin = 0.7 keV represents the best physically motivated model that fits the data.The Astronomer's Telegram. 08/2008; 1693:1. -
Article: Chandra Observations of the M31 X-Ray Transients XMMU J004245.9+411036 and XMMU J004241.2+411821
[show abstract] [hide abstract]
ABSTRACT: As part of our ongoing Chandra/HST M31 transient program we report on Chandra ACIS observations of M31 containing the new transient sources reported by Pietsch et al. (2008, ATel #1647). Two 5ks observations were performed on 2008-05-31.47 UT (Obs. 1) and 2008-07-15.70 UT (Obs. 2), both prior to the observation of Pietsch et al. (2008, Obs. 3). XMMU J004245.9+411036 The source fell on a chip gap during part of Obs.The Astronomer's Telegram. 07/2008; 1658:1. -
Article: Chandra observations of the M31 transients XMMU J004215.8+411924 and SWIFT J004217.4+411532
[show abstract] [hide abstract]
ABSTRACT: We report on 3 Chandra ACIS-I observations containing the two M31 transient X-ray sources announced by Haberl et al. (2006, ATel #881) and followed up by Pietsch et al. (2006, ATel #899). These observations are part of our ongoing Chandra/HST M31 transient program XMMU J004215.8+411924: This High Mass X-ray Binary was first observed with Chandra for 5 ks on 2006-07-31.02 UT (10 days before its discovery with XMM).The Astronomer's Telegram. 11/2006; 969:1. -
Article: A Possible Detection of M31* with Chandra
[show abstract] [hide abstract]
ABSTRACT: Two independent sets of Chandra and HST images of the nuclear region of M31 allow registration of X-ray and optical images to 0.1''. This registration shows that none of the bright (10^{37} erg/s) X-ray sources near the nucleus is coincident with the central super-massive black hole, M31*. A 50ks Chandra HRC image shows 2.5 sigma evidence for a faint (3 x 10^{35} erg/s), apparently resolved source which is consistent with the position of the M31*. The Bondi radius of M31* is 0.9'', making it one of the few super-massive black holes with a resolvable accretion flow. This large radius and the previous detections of diffuse, X-ray emitting gas in the nuclear region make M31* one of the most secure cases for a radiatively inefficient accretion flow and place some of the most severe constraints on the radiative processes in such a flow. Comment: 14 pages, 5 figures, submitted to ApJ12/2004; -
Article: Two New X-ray/Optical/Radio Supernova Remnants in M31
[show abstract] [hide abstract]
ABSTRACT: We compare a deep (37 ks) Chandra ACIS-S image of the M31 bulge to Local Group Survey narrow-band optical data and Very Large Array (VLA) radio data of the same region. Our precisely registered images reveal two new optical shells with X-ray counterparts. These shells have sizes, [S II]/H-alpha flux ratios, and X-ray spectral properties typical of supernova remnants (SNRs) with ages of 9$^{+3}_{-4}$ and 17$^{+6}_{-9}$ kyr. Analysis of complementary VLA data reveals the radio counterparts, further confirming that they are SNRs. We discuss and compare the properties and morphologies of these SNRs at the different wavelengths. Comment: 18 pages, 5 figures, accepted for publication in ApJ07/2004; -
Article: X-ray Point Sources in the Sombrero Galaxy: Supersoft sources, the globular cluster/LMXB connection, and an overview
[show abstract] [hide abstract]
ABSTRACT: We report on the population of point sources discovered during an 18.5 ksec Chandra ACIS-S observation of the Sombrero Galaxy. We present the luminosity function, the spectra of the 6 brightest sources, consider correlations with globular clusters (GCs) and with planetary nebulae (PNe), and study the galaxy's population of SSSs. We detected 122 sources, 22 of them are identified as luminous supersoft X-ray sources (SSSs). There is an over density of SSSs within 1.5 kpc of the nucleus, which is itself the brightest X-ray source. SSSs are also found in the disk and halo, with 1 SSS in a globular cluster (GC). Several sources in Sombrero's halo are good candidates for SSS models in which the accretor is a nuclear-burning white dwarf. In total, 32 X-ray sources are associated with GCs. The majority of sources with luminosity > 1e38 erg/s are in GCs. These results for M104, an Sa galaxy, are similar to what has been found for elliptical galaxies and for the late-type spiral M31. We find that those optically bright GCs with X-ray sources house only the brightest X-ray sources. We find that, in common with other galaxies, there appears to be a positive connection between young (metal-rich) GCs and X-ray sources, but that the brightest X-ray sources are equally likely to be in metal-poor GCs. We propose a model which can explain the trends seen in the data sets from the Sombrero and other galaxies. Thermal-time scale mass transfer can occur in some of the the younger clusters in which the turn-off mass is slightly greater than $0.8 M_\odot$; multiplicity may play a role in some of the most massive clusters; accretion from giant stars may be the dominant mechanism in some older, less massive and less centrally concentrated clusters. Comment: 13 pages, 7 figures. ApJ submitted06/2003; -
Article: Chandra observations of the X-ray luminous star-forming galaxy merger Arp~299
[show abstract] [hide abstract]
ABSTRACT: We report results of a Chandra observation of the X-ray luminous star-forming galaxy Arp299 (NGC3690/IC694). We detect 18 discrete X-ray sources with luminosities above ~10^39 erg\s (0.5-8.0 keV band), which contribute ~40% of the total galactic emission in this band. The remaining emission is associated with a diffuse component spatially coincident with regions of widespread star-formation. We detect X-ray emission from both nuclei. One of the discrete sources within the complex nuclear region of NGC 3690 is found to have a very hard spectrum and therefore we associate it with the origin of the AGN-like spectrum that has also been detected at high X-ray energies using Beppo-SAX. Comment: 7 pages, 2 figures, to appear in ApJ Lett06/2003; -
Article: The Discovery of a Spatially-Resolved Supernova Remnant in M31 with Chandra
[show abstract] [hide abstract]
ABSTRACT: Chandra observations of M31 allow the first spatially resolved X-ray image of a supernova remnant (SNR) in an external spiral galaxy. CXOM31 J004327.7+411829 is a slightly elongated ring-shaped object with a diameter of ~11'' (42 pc). In addition, the X-ray image hints that the chemical composition of the SNR is spatial dependent. The X-ray spectrum of the SNR can be well fitted with a Raymond-Smith model or a non-equilibrium ionization model. Depending on the spectral model, the 0.3-7 keV luminosity is between 3.2x10^36 erg/s and 4.5x10^37 erg/s. The age of the SNR is estimated to be 3210-22300 years and the number density of ambient gas is ~0.003-0.3 cm^-3. This suggests that the local interstellar medium around the SNR is low. Comment: 5 pages, 3 figures; accepted for publication in ApJL10/2002; -
Article: Chandra HRC and HST observations of NGC 6240: resolving the active galactic nucleus and starburst
[show abstract] [hide abstract]
ABSTRACT: We present high spatial resolution X-ray Chandra HRC and HST WFPC2 Hα observations of the prototypical infrared-luminous galaxy NGC 6240. The central region of this system shows a remarkably complex morphology, with filaments and loops observed in the optical and X-rays. The total X-ray luminosity is dominated by the extended emission. Both nuclei are clearly detected in the HRC image and both appear to be extended. The energetics of the nuclei imply that the southern nucleus is the more plausible counterpart to the obscured active galactic nucleus. The overall spectral energy distribution of the galaxy is in good agreement with a blend of starburst and AGN components that have similar bolometric luminosities, Lbol∼5×1045 erg s-1, with the starburst dominating the observed continuum in the near-infrared (K band), optical and soft X-ray bands.Monthly Notices of the Royal Astronomical Society 06/2002; 333(3):709 - 714. · 4.90 Impact Factor -
Article: X-ray Point Sources in The Central Region of M31 as seen by Chandra
[show abstract] [hide abstract]
ABSTRACT: We report on \chandra observations of the central region of M31. By combining eight \chandra ACIS-I observations taken between 1999 and 2001, we have identified 204 X-ray sources within the central $\sim 17'\times17'$ region of M31, with a detection limit of $\sim 2\times10^{35}$\lum. Of these 204 sources, 22 are identified with globular clusters, 2 with supernova remnants, 9 with planetary nebula, and 9 as supersoft sources. By comparing individual images, about 50% of the sources are variable on time scales of months. We also found 13 transients, with light curves showing a variety of shapes. We also extracted the energy spectra of the 20 brightest sources; they can be well fit by a single power-law with a mean photon index of 1.8. The spectral shapes of 12 sources are shown to be variable, suggesting that they went through state changes. The luminosity function of all the point sources is consistent with previous observations (a broken power-law with a luminosity break at $1.7\times10^{37}$\lum). However, when the X-ray sources in different regions are considered separately, different luminosity functions are obtained. This indicates that the star-formation history might be different in different regions.04/2002; -
Article: The X-ray Source Population of "The Antennae" Galaxies: X-ray Properties from Chandra and Multiwavelength Associations
[show abstract] [hide abstract]
ABSTRACT: [abridged] We investigate the nature of the luminous X-ray source population detected in a (72 ks) Chandra ACIS-S observation of NGC 4038/39, the Antennae galaxies. We derive the average X-ray spectral properties of sources in different luminosity ranges, and we correlate the X-ray positions with radio, IR, and optical HST data. The X-ray sources are predominantly associated with young stellar clusters, indicating that they belong to the young stellar population. Based on both their co-added X-ray spectrum, and on the lack of associated radio emission, we conclude that the Ultra Luminous X-ray sources (ULXs), with Lx>10^{39} erg\s, are not young compact Supernova Remnants (SNR), but accretion binaries. While their spectrum is consistent with those of ULXs studied in nearby galaxies, and interpreted as the counterparts of intermediate mass black-holes (M >10-1000 \msun), comparison with the position of star-clusters suggests that some of the ULXs may be runaway binaries, thus suggesting lower-mass binary systems. The co-added spectrum of the sources in the 3x10^{38}-10^{39} erg\s luminosity range is consistent with those of Galactic black-hole candidates. These sources are also on average displaced from neighboring star clusters. The softer spectrum of the less luminous sources suggests the presence of SNRs or of hot interstellar medium (ISM) in the Chandra source extraction area. Comparison with HI and CO observations shows that most sources are detected in the outskirts of large concentrations of gas. The absorbing columns inferred from these observations would indeed absorb X-rays up to 5 keV, so there may be several hidden X-ray sources. Associated with these obscured regions we find 6 sources with heavily absorbed X-ray spectra and absorption-corrected luminosities in the ULX range.04/2002; -
Article: Chandra Observations of "The Antennae" Galaxies (NGC 4038/39): II. Detection and Analysis of Galaxian X-ray source
[show abstract] [hide abstract]
ABSTRACT: We report the detailed analysis of the X-ray properties of the discrete sources detected in a long (72 ks) \chandra ACIS-S observation of the Antennae galaxies. We detect 49 sources, down to a detection limit of ~ 10^{38} erg\s; 18 sources have Lx >10^{39} erg\s (Ultra Luminous X-ray sources; ULXs). Six of the 49 sources have an extended component. Two sources show evidence for variability during this observation, and three sources exhibit long term variability in timescales of a few years. The spectra of the discrete sources are diverse, suggesting different emission mechanisms, broadly correlated with the source luminosity: the most luminous sources exhibit harder emission, while the spectra of fainter sources appear softer. Spectra and variability suggest that the ULXs may be binary accretion sources; Supernova remnants or hot ISM in the Chandra beam may be responsible for some of the softer sources. Comment: 27 pages, 9 figures, submitted to ApJS03/2002; -
Article: Two Years of the X-ray Sky in M31
[show abstract] [hide abstract]
ABSTRACT: We have been monitoring M31 with HRC and ACIS onboard \chandra regularly in the past two years. By combining eight \chandra ACIS-I observations taken between 1999 to 2001, we have identified 204 X-ray sources within the central $\sim 17\arcmin\times17\arcmin$ region of M31, with a detection limit of $\sim 1.6\times10^{35}$\lum. Of these 204 sources, 21 are identified with globular clusters, 2 with supernova remnants (one of them is spatially resolved with \chandra), and 8 with planetary nebula. By comparing individual images, about 50% of the sources are variable in time scales of months. We also found 14 transients. Combining all the available transients found in literatures, there are 25 transients in M31 and M32 detected by \chandra and \xmm; we present some of the long-term lightcurves by using the HRC, ACIS and \xmm data. The spectral shape of 12 sources is shown to be variable, suggesting that they went through state changes. The luminosity function of all the point sources is consistent with previous observations (a broken power-law with a luminosity break at $1.7\times10^{37}$\lum). However, when the X-ray sources in different regions are considered separately, different luminosity functions are obtained. This indicates that the star formation history might be different in different regions.03/2002; -
Article: Chandra HRC and HST observations of NGC6240: resolving the AGN and Starburst
[show abstract] [hide abstract]
ABSTRACT: We present high spatial resolution X-ray Chandra HRC and HST WFPC2 Halpha observations of the prototypical infrared luminous galaxy NGC6240. The central region of this system shows a remarkably complex morphology, with filaments and loops observed in the optical and X-rays. The total X-ray luminosity is dominated by the extended emission. Both nuclei are clearly detected in the HRC image and both appear to be extended. The energetics of the nuclei imply that the southern nucleus is the more plausible counterpart to the obscured AGN. The overall SED of the galaxy is in good agreement with a blend of starburst and AGN components which have similar bolometric luminosities, L_bol ~ 5e45 ergs/s, with the starburst dominating the observed continuum in the near-IR (K-band), optical and soft X-ray bands. Comment: Minor changes made, references added. 5 pages, 3 figures, accepted for publication in MNRAS02/2002; -
Article: The X-ray Spectra of Black Hole X-ray Novae in Quiescence as Measured by Chandra
[show abstract] [hide abstract]
ABSTRACT: We present Chandra observations of black hole X-ray novae V404 Cyg, A0620-00, GRO J1655-40 and XTE J1550-564 in quiescence. Their quiescent spectra can be well fitted by a power-law model with slope $\alpha \sim 2$. While a coronal (Raymond-Smith) model is also a statistically acceptable representation of the spectra, the best fit temperatures of these models is $\sim 5$ times higher than that seen in active stellar coronae. These four spectra of quiescent X-ray novae are all consistent with that expected for accretion via an advection-dominated accretion flow (ADAF) and inconsistent with that expected from a stellar corona. This evidence for continued accretion in quiescence further strengthens the case for the existence of event horizons in black holes. Both A0620-00 and GRO J1655-40 were fainter than in previous observations, while V404 Cyg was more luminous and varied by a factor of 2 in a few ksec. A reanalysis of the X-ray data for XTE J1550-564 shows that (like V404 Cyg and A0620-00) its luminosity exceeds the maximum prediction of the coronal model by a large factor. The 0.3-7 keV luminosity of the four sources studied ranges from $\sim 10^{30}-10^{33}$ erg/s. Comment: 9 pages, 6 figures, accepted for publication in ApJ11/2001; -
Article: Bright X-Ray Sources in M31 Globular Clusters
[show abstract] [hide abstract]
ABSTRACT: We have conducted Chandra observations of ~2560 square arcmin (~131 kpc^2) of of M31, and find that the most luminous X-ray sources in our fields are in globular clusters (GCs). Of the 28 GC X-ray sources in our fields, 17 are newly discovered. Approximately 1/3 of all the sources have L_x([0.5-7] keV) > 10^{37} ergs/s; approximately 1/10 of all the sources have L_x([0.5-7] keV) close to or above 10^{38} erg/s. The most luminous source, in the globular cluster Bo 375, is consistently observed to have L_x greater than 2 x 10^{38} ergs/s. (1) We present data on the spectra and/or light curves of the 5 most luminous M31 GC sources. (2)We explore possible explanations for the high X-ray luminosities of the brightest sources. These include that the X-ray sources may be composites, the radiation we receive may be beamed, metallicity effects could be at work, or the sources may be accreting black holes. We weigh each of these possibilities against the data. In addition, we introduce a neutron star model in which mass transfer proceeds on the thermal time scale of the donor star. Our model can produce luminosities of several times 10^{38} ergs/s, and leads to a set of well-defined predictions. (3) We compute the X-ray luminosity function and the distribution of counts in wavebands that span the range of energies to which Chandra is sensitive. We find the peak X-ray luminosity is higher and that systems with L_x > 10^{37} ergs/s constitute a larger fraction of all GC sources than in the MW. (4) We study the possible reasons for this difference between M31 and Galactic GC X-ray sources and identify three promising explanations. Comment: 21 pages, 12 postscript figures, use emulateapj.sty. Accepted for publication in the Astrophysical Journal06/2001; -
Article: Chandra Observations of "The Antennae" Galaxies (NGC 4038/39)
[show abstract] [hide abstract]
ABSTRACT: We report the results of a deep Chandra ACIS pointing at the merging system NGC 4038/39. We detect an extraordinarily luminous population of X-ray sources, with luminosity well above that of XRBs in M31 and the Milky Way. If these sources are unbeamed XRBs, our observations may point to them being 10-100Mo black hole counterparts. We detect an X-ray bright hot ISM, with features including bright superbubbles associated with the actively star-forming knots, regions where hot and warm (H$\alpha$) ISM intermingle, and a large-scale outflow. Comment: 7 pages, 10 figures, Accepted for publication in ApJ02/2001; -
Article: Chandra Observations of M31
[show abstract] [hide abstract]
ABSTRACT: As part of the Chandra GTO program we are monitoring and surveying M31 using the HRC and ACIS cameras. These observations have resolved the nuclear X-ray source into five separate sources, one of which is very soft and may (or may not!) be associated with the central super-massive black hole. In addition, the superb spatial resolution and low scattering of the Chandra telescope allows us to unambiguously resolve the diffuse emission from the point sources. This emission is clearly softer than the point sources, and also increases with temperature radially. The monitoring nature of the observations allows detailed study of the variability of the point sources.01/2001;
Top co-authors
Top Journals
Institutions
-
2002–2008
-
Harvard-Smithsonian Center for Astrophysics
Cambridge, MA, USA -
University of Santiago, Chile
- Departamento de Economía
Santiago, Region Metropolitana de Santiago, Chile
-
