arXiv subjects

Maureen van den Berg

Publications and source records attributed to Maureen van den Berg.

32 records · Page 2Linked to original sources

arXiv2009

ChaMPlane Deep Galactic Bulge Survey. I. Faint accretion-driven binaries in the Limiting Window

[Abridged] We have carried out a deep X-ray and optical survey with Chandra and HST of low-extinction regions in the Galactic bulge. Here we present the results of a search for low-luminosity (L_X <~ 1e34 ergs/s) accreting binaries in the region closest to the Galactic Center, at an angular offset of 1.4deg, that we have named the Limiting Window. Based on their blue optical colors, excess Halpha fluxes, and high X-ray--to--optical flux ratios, we identify three likely cataclysmic variables (CVs). Distance estimates put these systems farther than >~2 kpc. Based on their Halpha-excess fluxes and/or high X-ray--to--optical flux ratios, we find 22 candidate accreting binaries; however, the properties of some can also be explained if they are dMe stars or active galaxies. We investigate the CV number density towards the bulge and find that the number of observed candidate CVs is consistent with or lower than the number expected for a constant CV-to-star ratio that is fixed to the local value. The X-ray properties of two likely CVs are similar to those of the faint, hard X-ray sources in the Galactic-Center region that have been explained by (mainly) magnetic CVs. If our candidates belong to the same population, they would be the first members to be optically identified; optical or infrared identification of their Galactic-Center analogs would be impossible due to the higher obscuration. We speculate that all Galactic hard X-ray sources in our field can be explained by magnetic CVs.

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arXiv2009

Chandra Discovery of an Intermediate Polar in Baade's Window

(Abridged) We have discovered an intermediate polar (IP) in the 100 ks Chandra observation of Baade's Window (BW), a low extinction region at ~ 4 deg south of the Galactic Center. The source exhibits large X-ray modulations at a period of 1028.4 s in the 0.3 - 8 keV band. The X-ray spectral fit with a power law model shows the integrated spectrum is intrinsically hard (photon index ~ 0.44) and moderately absorbed (NH22 ~ 0.15). Quantile analysis reveals that the modulations in the X-ray flux strongly correlate with spectral changes that are dominated by varying internal absorption. The X-ray spectrum of the source is heavily absorbed (NH22 > 1) during the faint phases, while the absorption is consistent with the field value during the bright phases. These X-ray properties are typical signatures of IPs. Images taken with the IMACS camera on the Magellan 6.5m telescope show a faint (V ~ 22), relatively blue object within the 2 sigma error circle of the Chandra source. If we assume a nominal range of absolute V magnitude for a cataclysmic variable (MV ~ 5.5-10.5) and the known reddening in the region, the source would likely be at a distance of 2-10 kpc. The corresponding average Lx would be $6 \times 10^{31} - 10^{33}$ erg s$^{-1}$ in the 2-8 keV band. Assuming the space density of IPs follows the stellar distribution, which is highly concentrated in the Galactic Bulge, the source is probably a relatively bright IP belonging to the Galactic Bulge X-ray population, the majority of which is now believed to be magnetic cataclysmic variables.

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arXiv2008

A variable near-infrared counterpart to the neutron-star low-mass X-ray binary 4U 1705-440

We report the discovery of a near-infrared (nIR) counterpart to the persistent neutron-star low-mass X-ray binary 4U 1705-440, at a location consistent with its recently determined Chandra X-ray position. The nIR source is highly variable, with K_s-band magnitudes varying between 15.2 and 17.3 and additional J- and H-band observations revealing color variations. A comparison with contemporaneous X-ray monitoring observations shows that the nIR brightness correlates well with X-ray flux and X-ray spectral state. We also find possible indications for a change in the slope of the nIR/X-ray flux relation between different X-ray states. We discuss and test various proposed mechanisms for the nIR emission from neutron-star low-mass X-ray binaries and conclude that the nIR emission in 4U 1705-440 is most likely dominated by X-ray heating of the outer accretion disk and the secondary star.

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arXiv2008

Investigating ChaMPlane X-ray sources in the Galactic Bulge with Magellan LDSS2 spectra

We have carried out optical and X-ray spectral analyses on a sample of 136 candidate optical counterparts of X-ray sources found in five Galactic-bulge fields included in our Chandra Multi-wavelength Plane Survey. We use a combination of optical spectral fitting and quantile X-ray analysis to obtain the hydrogen column density towards each object, and a three-dimensional dust model of the Galaxy to estimate the most probable distance in each case. We present the discovery of a population of stellar coronal emission sources, likely consisting of pre-main sequence, young main sequence and main sequence stars, as well as a component of active binaries of RS CVn or BY Dra type. We identify one candidate quiescent low-mass X-ray binary with a sub-giant companion; we note that this object may also be an RS CVn system. We report the discovery of three new X-ray detected cataclysmic variables (CVs) in the direction of the Galactic Center (at distances ~2kpc). This number is in excess of predictions made with a simple CV model based on a local CV space density of <~ 10^-5 pc^-3, and a scale height ~200pc. We discuss several possible reasons for this observed excess.

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arXiv2008

The Brightest Serendipitous X-ray Sources in ChaMPlane

The Chandra Multiwavelength Plane (ChaMPlane) Survey is a comprehensive effort to constrain the population of accretion-powered and coronal low-luminosity X-ray sources (L_X < 10^33 erg s^-1) in the Galaxy. ChaMPlane incorporates X-ray, optical, and infrared observations of fields in the Galactic Plane imaged with Chandra in the past six years. We present the results of a population study of the brightest X-ray sources in ChaMPlane. We use X-ray spectral fitting, X-ray lightcurve analysis, and optical photometry of candidate counterparts to determine the properties of 21 sources. Our sample includes a previously unreported quiescent low-mass X-ray binary or cataclysmic variable (R = 20.9) and ten stellar sources (12.5 < R < 15), including one flare star (R = 17.3). We find that quantile analysis, a new technique developed for constraining the X-ray spectral properties of low-count sources, is largely consistent with spectral fitting.

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arXiv2008

A Massive Neutron Star in the Globular Cluster M5

We report the results of 19 years of Arecibo timing for two pulsars in the globular cluster NGC 5904 (M5), PSR B1516+02A (M5A) and PSR B1516+02B (M5B). This has resulted in the measurement of the proper motions of these pulsars and, by extension, that of the cluster itself. M5B is a 7.95-ms pulsar in a binary system with a > 0.13 solar mass companion and an orbital period of 6.86 days. In deep HST images, no optical counterpart is detected within ~2.5 sigma of the position of the pulsar, implying that the companion is either a white dwarf or a low-mass main-sequence star. The eccentricity of the orbit (e = 0.14) has allowed a measurement of the rate of advance of periastron: (0.0142 +/-0.0007) degrees per year. We argue that it is very likely that this periastron advance is due to the effects of general relativity, the total mass of the binary system then being 2.29 +/-0.17 solar masses. The small measured mass function implies, in a statistical sense, that a very large fraction of this total mass is contained in the pulsar: 2.08 +/- 0.19 solar masses (1 sigma); there is a 5% probability that the mass of this object is < 1.72 solar masses and a 0.77% probability that is is between 1.2 and 1.44 solar masses. Confirmation of the median mass for this neutron star would exclude most ``soft'' equations of state for dense neutron matter. Millisecond pulsars (MSPs) appear to have a much wider mass distribution than is found in double neutron star systems; about half of these objects are significantly more massive than 1.44 solar masses. A possible cause is the much longer episode of mass accretion necessary to recycle a MSP, which in some cases corresponds to a much larger mass transfer.

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arXiv2005

X-ray Processing of ChaMPlane Fields: Methods and Initial Results for Selected Anti-Galactic Center Fields

We describe the X-ray analysis procedure of the on-going Chandra Multiwavelength Plane (ChaMPlane) survey and report the initial results from the analysis of 15 selected anti-Galactic center observations (90 deg < l < 270 deg). We describe the X-ray analysis procedures for ChaMPlane using custom-developed analysis tools appropriate for Galactic sources but also of general use: optimum photometry in crowded fields using advanced techniques for overlapping sources, rigorous astrometry and 95% error circles for combining X-ray images or matching to optical/IR images, and application of quantile analysis for spectral analysis of faint sources. We apply these techniques to 15 anti-Galactic center observations (of 14 distinct fields) in which we have detected 921 X-ray point sources. We present logN-logS distributions and quantile analysis to show that in the hard band (2 - 8 keV) active galactic nuclei dominate the sources. Complete analysis of all ChaMPlane anti-Galactic center fields will be given in a subsequent paper, followed by papers on sources in the Galactic center and Bulge regions.

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arXiv2005

ChaMPlane Optical Survey: Mosaic Photometry

The ChaMPlane survey to identify and analyze the serendipitous X-ray sources in deep Galactic plane fields incorporates the ChaMPlane Optical Survey, which is one of NOAO's Long-term Survey Programs. We started this optical imaging survey in March 2000 and completed it in June 2005. Using the NOAO 4-m telescopes with the Mosaic cameras at CTIO and KPNO, deep images of the ChaMPlane fields are obtained in V, R, I and H-alpha bands. This paper describes the process of observation, data reduction and analysis of fields included in the ChaMPlane Optical Survey, and describes the search for H-alpha emission objects and Chandra optical counterparts. We illustrate these procedures using the ChaMPlane field for the black hole X-ray binary GRO J0422+32 as an example.

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arXiv2005

An X-ray Variable Millisecond Pulsar in the Globular Cluster 47 Tucanae: Closing the Link to Low Mass X-ray Binaries

We report the discovery of peculiar X-ray spectral variability in the binary radio millisecond pulsar PSR J0024--7204W in the globular cluster 47 Tucanae. The observed emission consists of a dominant non-thermal component, which is eclipsed for a portion of the orbit, and a thermal component, which appears to be persistent. We propose that the non-thermal X-rays originate in a relativistic intrabinary shock, formed due to interaction between the relativistic particle wind from the pulsar and matter from the main-sequence companion star, while the thermal photons are from the heated magnetic polar caps of the millisecond pulsar. At optical wavelengths, the emission exhibits large-amplitude variations at the orbital period, which can be attributed to heating of one side of the tidally-locked secondary star by the pulsar wind. The observed X-ray and optical properties of PSR J0024--7204W are remarkably similar to those of the low mass X-ray binary and X-ray millisecond pulsar SAX J1808.4--3658 in quiescence. This supports the conjecture that the non-thermal X-ray emission and optical modulations seen in the SAX J1808.4--3658 system in a quiescent state are due to interaction between the wind from a reactivated rotation-powered pulsar and matter from the companion star. The striking similarities between the two systems provide support for the long-sought connection between millisecond radio pulsars and accreting neutron star systems.

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arXiv2003

A Chandra observation of the old open cluster M67

We present the results of a 47-ks Chandra-ACIS observation of the old open cluster M67. We detected 25 proper-motion cluster members (including ten new sources) and 12 sources (all new) that we suspect to be members from their locations close to the main sequence (1 < B-V < 1.7). Of the detected members, 23 are binaries. Among the new sources that are members and probable members are four spectroscopic binaries with P_orb < 12 d, two contact binaries and two periodic photometric variables with P_ph < 8.4 d. Their X-rays are likely the result of coronal activity enhanced by tidally locked rapid rotation. The X-rays of the new source S997, a blue straggler in a wide eccentric orbit, are puzzling. Spectral fits show that the X-rays of the brightest sources S1063 (a binary with a sub-subgiant), S1082 (a triple blue straggler with a close binary) and S1040 (a circular binary of a giant and a cool white dwarf), are consistent with coronal emission. We detected a new bright source that must have brightened at least about ten times since the time of the ROSAT observations. It is not clear whether its faint blue optical counterpart belongs to M67. We discuss the possibility that this source is a low-mass X-ray binary in quiescence, which would be the first of its kind in an open cluster. In addition to cluster members, we detected about 100 background sources, many of which we identify with faint objects in the ESO Imaging Survey.

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arXiv2003

XMM-Newton light curves of the low-mass X-ray binary EXO 0748--676: dips, eclipses, and bursts

We present an analysis of XMM-Newton light curves of the dipping, bursting, and eclipsing low-mass X-ray binary EXO 0748-676, focusing on the variability on time scales of seconds to hours. The observed variability can be roughly divided in three types: dips, eclipses and bursts. We find that the appearance of the latter two, depends strongly on the strength of the first. We show that the absorption dips change from spectrally hard to spectrally soft as they become deeper, which supports suggestions that the source is composed of a spectrally hard compact source and a spectrally soft extended source. The fast variability in the soft light curve indicates that the large structures causing the dips are made up of smaller absorption cores. We present the first clear detection with XMM-Newton of eclipses below 2 keV in this source, and show that dipping activity is apparently unrelated to the source luminosity. We also test several proposed models for the spectral evolution during dips and confirm the presence of a scatter/reflection component in the eclipse spectrum.

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arXiv2001

Photometric variability in the open cluster M67 I. Cluster members detected in X-rays

We study photometric variability among the optical counterparts of X-ray sources in the old open cluster M67. The two puzzling binaries below the giant branch are both variables: for S1113 the photometric period is compatible with the orbital period, S1063 either varies on a period longer than the orbital period, or does not vary periodically. For the spectroscopic binaries S999, S1070 and S1077 the photometric and orbital periods are similar. Another new periodic variable is the main-sequence star S1112, not known to be a binary. An increase of the photometric period in the WUMa system S1282 (AHCnc) is in agreement with a previously reported trend. Six of the eight variables we detected are binaries with orbital periods of 10 days or less and equal photometric and orbital periods. This confirms the interpretation that their X-ray emission arises in the coronae of tidally locked magnetically active stars. No variability was found for the binaries with orbital periods longer than 40 days; their X-ray emission remains to be explained.

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arXiv2001

The blue straggler S1082: a triple system in the old open cluster M67

We present a photometric and spectroscopic study of the blue straggler S1082 in the open cluster M67. Our observations confirm the previously reported 1.07 day eclipse light curve and the absence of large radial-velocity variations of the narrow-lined star. However, we find two more spectral components which do vary on the 1.07 day period. We conclude that the system is triple. We fit the light and radial-velocity curves and find that the total mass of the inner binary is more than twice the turnoff mass and that the outer companion to the binary is a blue straggler on its own account. We briefly discuss formation scenarios for this multiple system.

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arXiv1999

Optical spectroscopy of X-ray sources in the old open cluster M67

We have obtained optical spectra of seven stars in the old galactic cluster M67 that are unusual sources of X-rays, and investigate whether the X-ray emission is due to magnetic activity or to mass transfer. The two binaries below the giant branch S1063 and S1113, the giant with the white dwarf companion S1040 and the eccentric binary on the subgiant branch S1242 show magnetic activity in the form of Ca II H&K emission and H-alpha emission, suggesting that their X-rays are coronal. The reason for the enhanced activity level in S1040 is not clear. The two wide, eccentric binaries S1072 and S1237 and the blue straggler S1082 do not show evidence for Ca II H&K emission. A second spectral component is found in the spectrum of S1082, most clearly in the variable H-alpha absorption profile. We interpret this as a signature of the proposed hot subluminous companion.

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