arXiv subjects

Stephen S. Murray

Publications and source records attributed to Stephen S. Murray.

At least 73 records · Page 4Linked to original sources

arXiv2005

A Soft X-ray Transient in the M31 Bulge

We have examined a probable soft X-ray transient source in the M31 bulge at R.A.=0:42:41.814 +/- 0.08", Dec. = 41:16:35.86 +/- 0.07". On the three occasions we observed the source, its spectrum was soft (kT_{in} ~1 keV). The brightest detection of the source was 2004 July 17 with a 0.3-7 keV luminosity of ~5 X 10^{37} erg/s. The only previous detection of the source was in 1979 by the Einstein observatory. The multiple detections over 25 years suggest the duty cycle of the source is in the range 0.02-0.06. Coordinated HST/ACS imaging before, during, and after the outburst revealed no variable optical source within the position errors of the X-ray source. The optical data place a firm upper limit on the brightness of the counterpart of the X-ray outburst of B>24.7, suggesting the binary has a period <5.2 days. The X-ray spectrum and lack of bright stars at the source location indicate the source was a soft transient event occurring in a low-mass X-ray binary, making this source a good black hole candidate in M31.

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arXiv2005

Chandra and Hubble Study of a New Transient X-ray Source in M31

We present X-ray and optical observations of a new transient X-ray source in M31 first detected 23-May-2004 at R.A.=00:43:09.940 +/- 0.65'', Dec.=41:23:32.49 +/- 0.66''. The X-ray lightcurve shows two peaks separated by several months, reminiscent of many Galactic X-ray novae. The location and X-ray spectrum of the source suggest it is a low mass X-ray binary (LMXB). Follow-up HST ACS observations of the location both during and after the outburst provide a high-confidence detection of variability for one star within the X-ray position error ellipse. This star has $Δ$B ~ 1 mag, and there is only a ~1% chance of finding such a variable in the error ellipse. We consider this star a good candidate for the optical counterpart of the X-ray source. The luminosity of this candidate provides a prediction for the orbital period of the system of 2.3$^{+3.7}_{-1.2}$ days.

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arXiv2005

On the discovery of the first 350 micron-selected galaxy

We report the detection of a 3.6sigma 350micron-selected source in the Bootes Deep Field. The source, the first Short-wavelength Submillimeter-selected Galaxy (SSG 1), was discovered as part of a blank field extragalactic survey using the 350micron-optimised Submillimeter High Angular Resolution Camera (SHARC II) at the Caltech Submillimeter Observatory. With multiwavelength photometry from NOAO-NDWFS (R and I band), FLAMEX (J and K_s), Spitzer (IRAC and MIPS) and the Westerbork 1.4GHz Deep Survey (radio upper limit), we are able to constrain the photometric redshift using different methods, all of which suggest a redshift of close to 1. In the absence of long-wavelength submillimeter data we use SED templates to infer that this source is an ultraluminous infrared galaxy (ULIRG) with a dust temperature of 30+/-5 K, occupying a region of luminosity-temperature space shared by modarate redshift ISO-selected ULIRGs (rather than high redshift SCUBA-selected SMGs). SHARC II can thus select SMGs with moderately "warm'' dust that might be missed in submillimeter surveys at longer wavelengths.

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arXiv2005

The XBootes Chandra Survey Paper II: The X-ray Source Catalog

We present results from a Chandra survey of the nine square degree Bootes field of the NOAO Deep Wide-Field Survey (NDWFS). This XBootes survey consists of 126 separate contiguous ACIS-I observations each of approximately 5000 seconds in duration. These unique Chandra observations allow us to search for large scale structure and to calculate X-ray source statistics o ver a wide, contiguous field of view with arcsecond angular resolution and uniform coverage. Optical spectroscopic follow-up observations and the rich NDWFS data set will allow us to identify and classify these X-ray selected sources. Using wavelet decomposition, we detect 4642 point sources with n $\ge$ 2 counts. In order to keep our detections ~99% reliable, we limit our list to sources with n $\ge$ 4 counts. The full 0.5--7 keV band n $\ge$ 4 count list has 3293 point sources. In addition to the point sources, 43 extended sources have been detected consistent, with the depth of these observations and the number counts of clusters. We present here the X-ray catalog for the XBootes survey, including source positions, X-ray fluxes, hardness ratios and their uncertainties. We calculate and present the differential number of sources per flux densit y interval, $N(S)$, for the point sources.

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arXiv2005

XBootes: An X-Ray Survey of the NDWFS Bootes Field - Paper I Overview and Initial Results

We obtained a 5 ksec deep Chandra X-ray Observatory ACIS-I map of the 9.3 square degree Bootes field of the NOAO Deep Wide-Field Survey. Here we describe the data acquisition and analysis strategies leading to a catalog of 4642 (3293) point sources with 2 or more (4 or more) counts, corresponding to a limiting flux of roughly 4(8)x10^{-15} erg cm^{-2}s^{-1} in the 0.5-7 keV band. These Chandra XBootes data are unique in that they consitute the widest contiguous X-ray field yet observed to such a faint flux limit. Because of the extraordinarily low background of the ACIS, we expect only 14% (0.7%) of the sources to be spurious. We also detected 43 extended sources in this survey. The distribution of the point sources among the 126 pointings (ACIS-I has a 16 x 16 arcminute field of view) is consistent with Poisson fluctuations about the mean of 36.8 sources per pointing. While a smoothed image of the point source distribution is clumpy, there is no statistically significant evidence of large scale filamentary structure. We do find however, that for theta>1 arcminute, the angular correlation function of these sources is consistent with previous measurements, following a power law in angle with slope -0.7. In a 1.4 deg^{2} sample of the survey, approximately 87% of the sources with 4 or more counts have an optical counterpart to R ~26 mag. As part of a larger program of optical spectroscopy of the NDWFS Bootes area, spectra have been obtained for \~900 of the X-ray sources, most of which are QSOs or AGN.

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arXiv2005

Optical Constraints on an X-ray Transient Source in M31

We have detected a transient X-ray source in the M31 bulge through a continuing monitoring campaign with the Chandra ACIS-I camera. The source was detected at R.A.=00:42:33.428 +/- 0.11'', Dec.=+41:17:03.37 +/- 0.11'' in only a single observation taken 2004 May 23. Fortuitous optical HST/ACS imaging of the transient location prior to the X-ray outburst, along with follow-up HST/ACS imaging during and after the outburst, reveals no transient optical source brighter than B (equivalent) = 25.5. The location of the source and its X-ray properties suggest it is a low mass X-ray binary (LMXB). Assuming the transient is similar to many Galactic X-ray novae, the X-ray luminosity of (3.9 +/- 0.5) X 10$^{37}$ erg s$^{-1}$ and the upper-limit on the optical luminosity provide a prediction of <1.6 days for the orbital period of the binary system.

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arXiv2005

An X-ray Transient and Optical Counterpart in the M31 Bulge

We have obtained snapshot images of a transient X-ray source in M31 from Chandra ACIS-I and the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The Chandra position of the X-ray nova was R.A.=00:42:56.038 +/-0.08'', Dec.=+41:12:18.50 +/-0.07''. The transient was active for at least 6 months. Previous observations set an upper limit before the X-ray outburst, demonstrating variability by a factor of >100 and confirming the transient nature of the source. For the first 6 months after the initial detection, the X-ray luminosity was ~6 X 10$^{37}$ erg/s; it then decayed to <5 X 10$^{36}$ erg/s over the following 2 months. An HST observation 29 days after the initial X-ray detection revealed a source at R.A.=00:42:56.042, Dec.=+41:12:18.45 that was B=24.52 +/- 0.07. This optical source faded to B=24.95 +/- 0.08$ in 9 months. The HST identification of an optical source at the same position as the X-ray source, fading in concert with the X-ray source, indicates that this optical source is the counterpart of the X-ray transient. The lack of high-mass stars in the region suggests this source is a low-mass X-ray binary, and the X-ray and optical luminosities provide a rough orbital period estimate of 8$^{+12}_{-5}$ days for the system.

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arXiv2005

The Effect of Use and Access on Citations

It has been shown (S. Lawrence, 2001, Nature, 411, 521) that journal articles which have been posted without charge on the internet are more heavily cited than those which have not been. Using data from the NASA Astrophysics Data System (ads.harvard.edu) and from the ArXiv e-print archive at Cornell University (arXiv.org) we examine the causes of this effect.

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arXiv2004

Discovery of an X-ray Nova in M31

We have obtained snapshot images of an X-ray nova in M31 from Chandra ACIS-I and the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The Chandra position of the X-ray nova was R.A.=00:44:06.68 +/- 1.74", Dec.=+41:12:20.0 +/- 2.31". A follow-up HST observation 24 days later revealed a source at R.A.=00:44:06.81, Dec.=+41:12:24.0 that was B=25.75 +/- 0.05. This optical source faded to B=27.1 +/- 0.1 in 3 months. During this time period, the X-ray flux decayed linearly from (3.6 +/- 0.2) X 10^{-4} to <(6.9 +/- 0.09) X 10^{-5} ct cm^{-2} s^{-1}. The HST identification of an optical source in the same region experiencing an obvious drop in brightness in concert with the X-ray nova suggests that this optical source is the counterpart of the X-ray nova. However, the precision of the X-ray position allows the possibility that the optical source is a nearby variable star. We discuss the implications of both possibilities.

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arXiv2004

Mid-Infrared Selection of Active Galaxies

Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ultraviolet to mid-infrared continuum of normal galaxies is dominated by the composite stellar black body curve and peaks at approximately 1.6 microns, the ultraviolet to mid-infrared continuum of active galaxies is dominated by a power law. Consequently, with sufficient wavelength baseline, one can easily distinguish AGN from stellar populations. Mirroring the tendency of AGN to be bluer than galaxies in the ultraviolet, where galaxies (and stars) sample the blue, rising portion of stellar spectra, AGN tend to be redder than galaxies in the mid-infrared, where galaxies sample the red, falling portion of the stellar spectra. We report on Spitzer Space Telescope mid-infrared colors, derived from the IRAC Shallow Survey, of nearly 10,000 spectroscopically identified sources from the AGN and Galaxy Evolution Survey. Based on this spectroscopic sample, we find that simple mid-infrared color criteria provide remarkably robust separation of active galaxies from normal galaxies and Galactic stars, with over 80% completeness and less than 20% contamination. Considering only broad-lined AGN, these mid-infrared color criteria identify over 90% of spectroscopically identified quasars and Seyfert 1s. Applying these color criteria to the full imaging data set, we discuss the implied surface density of AGN and find evidence for a large population of optically obscured active galaxies.

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arXiv2004

A Chandra Observation of the Diffuse Emission Centered on the Low Mass X-ray Binary 4U 1755-33

We present an analysis of a Chandra observation of the field surrounding the low-mass X-ray binary 4U 1755-33, which has been in quiescence since 1996. In 2003, Angelini and White reported the appearance of a narrow 7 arcmin long jetlike feature centered on the position of 4U 1755-33 using the XMM-Newton telescope. Though the source and jet are not visibly apparent in our Chandra/ACIS-S image, there is a significant excess (4-6 sigma) of counts in a region that encloses the jet when compared to adjacent regions. We examined a knot of emission in the jet that was detected by XMM-Newton but not by Chandra and calculated that if the knot flux observed by XMM-Newton was concentrated in a point source, Chandra would have easily detected it; we therefore conclude that this knot of emission is diffuse. In summary, we suggest that the jetlike feature found previously in the XMM-Newton data is quite diffuse and likely a true jet, and is not due to a chance alignment of discrete point sources or point-like regions of emission associated with internal shocks.

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arXiv2004

New Constraints on the Structure and Evolution of the Pulsar Wind Nebula 3C 58

We present an investigation of the spectral and spatial structure of the X-ray emission from 3C 58 based on a 350 ks observation with the Chandra X-ray Observatory. This deep image, obtained as part of the Chandra Large Project program, reveals new information on nearly all spatial scales in the pulsar wind nebula (PWN). On the smallest scales we derive an improved limit of T < 1.02 X 10^6 K for blackbody emission from the entire surface of the central neutron star (NS), confirming the need for rapid, nonstandard cooling in the stellar interior. Furthermore, we show that the data are consistent with emission from a light element atmosphere with a similar temperature. Surrounding the NS, a toroidal structure with a jet is resolved, consistent with earlier measurements and indicative of an east-west orientation for the projected rotation axis of the pulsar. A complex of loop-like X-ray filaments fills the nebula interior, and corresponds well with structures seen in the radio band. Several of the structures coincide with optical filaments as well. The emission from the interior of the PWN, including the pulsar, jet, and filaments, is primarily nonthermal in nature. The power law index steepens with radius, but appears to also show small azimuthal variations. The outermost regions of the nebula require a thermal emission component, confirming the presence of an ejecta-rich swept up shell.

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arXiv2004

A Synoptic X-ray Study of M31 with the Chandra-HRC

We have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot images, each covering most of the M31 disk. The data cover a total baseline of 2.5 years and contain a mean effective exposure of 17 ks. We have measured the mean fluxes and long-term lightcurves for 166 objects detected in these data. At least 25% of the sources show significant variability. The cumulative luminosity function (CLF) of the disk sources is well-fit by a power-law with a slope comparable to those observed in typical elliptical galaxies. The CLF of the bulge is a broken power law similar to measurements made by previous surveys. We note several sources in the southwestern disk with L_X > 10^{37} erg/s . We cross-correlate all of our sources with published optical and radio catalogs, as well as new optical data, finding counterpart candidates for 55 sources. In addition, 17 sources are likely X-ray transients. We analyze follow-up HST WFPC2 data of two X-ray transients, finding F336W (U-band equivalent) counterparts. In both cases, the counterparts are variable. In one case, the optical counterpart is transient with F336W = 22.3 +/- 0.1 mag. The X-ray and optical properties of this object are consistent with a ~10 solar mass black hole X-ray nova with an orbital period of ~20 days. In the other case, the optical counterpart varies between F336W = 20.82 +/- 0.06 mag and F336W = 21.11 +/- 0.02 mag. Ground-based and HST observations show this object is bright (V = 18.8 +/- 0.1) and slightly extended. Finally, the frequency of bright X-ray transients in the M31 bulge suggests that the ratio of neutron star to black hole primaries in low-mass X-ray binaries (NS/BH) is ~1.

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arXiv2003

Micro-second Timing of PSR B1821-24 with Chandra/HRC-S

We perform absolute timing of PSR B1821-24 in M28, using a 50 ksec observation with Chandra/HRC-S. We have obtained the highest signal-to-noise X-ray pulsed lightcurve of this source to date, detecting two X-ray pulses, as well as significant non-pulsed emission -- a persistent X-ray flux which comprises 15+/-3% of the total X-ray flux of the pulsar. The Gaussian width of the sharp X-ray peak is 34+/-3 micro-sec in time, implying a size of the X-ray beam as it crosses the line of sight of 4.0+/-0.4 deg. We find evidence for a significant trailing component in both X-ray peaks of the pulse profile. Including three RXTE/PCA observations in our analysis, and tying the phases together using a radio ephemeris obtained at Nancay, we find the absolute phases in the X-ray wander with respect to this radio ephemeris by up to 60 micro-sec, likely due to the variable dispersion measure, which changes the pulse arrival time in the radio band but not the X-ray band. The present analysis makes clear that study of pulsar timing noise properties in millisecond pulsars such as PSR B1821-24 -- hitherto only studied at radio wavelengths, where variable dispersion measure requires a significant correction -- can be studied at X-ray wavelengths, where the effect of variable dispersion measure is negligible. We also examine the known uncertainties in the absolute Chandra/HRC-S timing accuracy, which amount to +/-12 microsec. We limit the amount of linear drift in the relative timing accuracy of HRC-S to <3e-10 s s-1.

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arXiv2003

Chandra Discovery of an X-ray jet and Extended X-ray Structure in z=0.63 quasar, B2 0738+313

We have made a 30 ksec Chandra observation of the redshift z=0.63 GPS quasar B2 0738+313. We detected X-ray emission from the core and have discovered a 200 kpc (projected on the sky) X-ray jet. The X-ray jet is narrow and curves, following the extended radio structure to the south of the quasar, and ending with a hot spot at the southernmost part of the radio lobe. The jet has a knot at ~13 arcsec away from the core. The knot emission is consistent with the X-rays being created by the inverse Compton scattering of the cosmic microwave background (CMB) photons and requires jet bulk Lorentz factors of a few (Gamma_{bulk} ~ 5-7). We discuss the emission mechanisms that may be responsible for the jet emission. We present new VLA data of the core and jet, and discuss the relation between the extended radio and X-ray emission. Extended emission observed in several GPS sources has been interpreted as a signature of the source past activity, while the GPS source is young and newly expanded. We argue that B2~0738+313 may be an example of a new class of radio sources similar to the FRII radio galaxies in their high jet bulk velocities, but with the powerful GPS-like nucleus. B2 0738+313 also has two damped Lyman-alpha systems along the line of sight, at z_{abs} = 0.0912 and 0.2212. We discuss the possible connection between the X-ray absorption (7.2+/-0.9 e20 cm(-2)) detected in the ACIS spectrum and these two intervening absorbers. We also investigate an extended structure within the central 10 arcsec of the core in the relation to structure seen in the optical.

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arXiv2003

Multiwavelength Spectrum of the Black Hole XTE J1118+480 in Quiescence

We present an X-ray/UV/optical spectrum of the black hole primary in the X-ray nova XTE J1118+480 in quiescence at L_x = 4 x 10^{-9} of the Eddington luminosity. The Chandra, HST and MMT spectroscopic observations were performed simultaneously on 2002 January 12 UT. Because this 4.1-hr binary is located at b = 62 deg, the transmission of the ISM is very high (e.g., 70% at 0.3 keV). We present many new results for the quiescent state, such as the first far-UV spectrum and evidence for an 0.35 mag orbital modulation in the near-UV flux. However, the centerpiece of our work is the multiwavelength spectrum of XTE J1118+480, which we argue represents the canonical spectrum of a stellar-mass black hole radiating at L_x = 4 x 10^{-8.5} of the Eddington luminosity. This spectrum is comprised of two apparently disjoint components: a hard X-ray spectrum with a photon index Gamma = 2.02 +/- 0.16, and an optical/UV continuum that resembles a 13,000 K disk blackbody spectrum punctuated by several strong emission lines. We present a model of the source in which the accretion flow has two components: (1) an X-ray-emitting interior region where the flow is advection-dominated, and (2) a thin, exterior accretion disk with a truncated inner edge (R_tr ~ 10^4 Schwarzschild radii) that is responsible for the optical/UV spectrum. For D = 1.8 kpc, the luminosity of the X-ray component is L_x = 3.5 x 10^{30} erg/s (0.3-7 keV); the bolometric luminosity of the optical/UV component is 20 times greater.

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arXiv2002

New Constraints on Neutron Star Cooling from Chandra Observations of 3C58

3C58 is a young Crab-like supernova remnant. Historical evidence strongly suggests an association of the remnant with supernova SN 1181, which would make 3C58 younger than the Crab Nebula. Recent Chandra observations have identified the young 65 ms pulsar J0205+6449 at its center, embedded in a compact nebula which, we show here, appears to be confined by the pulsar wind termination shock. We present new Chandra observations of this compact nebula and embedded pulsar which set strong upper limits on thermal emission originating from the neutron star surface. These limits fall far below predictions of standard neutron star cooling, requiring the presence of exotic cooling processes in the neutron star core.

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arXiv2001

Discovery of X-ray Pulsations from the Compact Central Source in the Supernova Remnant 3C 58

We report on high time resolution observations of the SNR 3C 58 using the High Resolution Camera on the Chandra X-ray Observatory. These data show a point-like central source, from which we detect 65.68 ms pulsations at 6.7 sigma significance. We interpret these pulsations as corresponding to a young rotation-powered pulsar (PSR J-205+6449) which is associated with, and powers, 3C 58. Analysis of archival RXTE data from three years earlier confirms these pulsations, and allows us to determine a spin-down rate of P-dot=1.93x10^-13 s/s. Assuming a magnetic dipole model for PSR J0205+6449, we infer a surface magnetic field of 3.6x10^12 G. The characteristic age for this pulsar is 5400 yr, indicating either that 3C 58 was not the supernova of 1181 CE, or that the pulsar's initial spin period was ~60 ms.

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