arXiv subjects

D. A. Leahy

Publications and source records attributed to D. A. Leahy.

At least 37 records · Page 2Linked to original sources

arXiv2016

Neutron Star Mass-Radius Constraints using Evolutionary Optimization

The equation of state of cold supra-nuclear-density matter, such as in neutron stars, is an open question in astrophysics. A promising method for constraining the neutron star equation of state is modelling pulse profiles of thermonuclear X-ray burst oscillations from hotspots on accreting neutron stars. The pulse profiles, constructed using spherical and oblate neutron star models, are comparable to what would be observed by a next-generation X-ray timing instrument like ASTROSAT, NICER, or LOFT. In this paper, we showcase the use of an evolutionary optimization algorithm to fit pulse profiles to determine the best-fit masses and radii. By fitting synthetic data, we assess how well the optimization algorithm can recover the input parameters. Multiple Poisson realizations of the synthetic pulse profiles, constructed with 1.6 million counts and no background, were fitted with the Ferret algorithm to analyze both statistical and degeneracy-related uncertainty, and to explore how the goodness-of-fit depends on the input parameters. For the regions of parameter space sampled by our tests, the best-determined parameter is the projected velocity of the spot along the observer's line-of-sight, with an accuracy of $\le3$% compared to the true value and with $\le5$\% statistical uncertainty. The next best-determined are the mass and radius; for a neutron star with a spin frequency of 600\,Hz, the best-fit mass and radius are accurate to $\le5$%, with respective uncertainties of $\le7$% and $\le10$%. The accuracy and precision depend on the observer inclination and spot co-latitude, with values of $\sim1$ % achievable in mass and radius if both the inclination and co-latitude are > 60 degrees.

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arXiv2016

Kinematic Distance of Galactic Planetary Nebulae

We construct \HI~absorption spectra for 18 planetary nebulae (PNe) and their background sources using the data from the International Galactic Plane Survey. We estimate the kinematic distances of these PNe, among which 15 objects' kinematic distances are obtained for the first time. The distance uncertainties of 13 PNe range from 10% to 50%, which is a significant improvement with uncertainties of a factor two or three smaller than most of previous distance measurements. We confirm that PN G030.2-00.1 is not a PN because of its large distance found here.

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arXiv2015

Spectral Signature of Atmospheric Reflection in Hercules X-1/HZ Hercules During Low and Short-High States

We analyse RXTE/PCA X-ray spectra of the binary X-ray pulsar Her X-1/HZ Her during short high state and one binary orbit in the preceding low state, just before short high turn-on. The spectrum is well described by two continuum components (absorbed and unabsorbed). The resulting spectral parameters are modulated with orbital phase. During low state a significant component of the flux, and its spectrum, is consistent with X-ray reflection off the face of the companion star HZ Her. This component has a significantly harder X-ray spectrum than the rest of the flux from the Her X-1 system. A second component in low state is consistent with emission from the accretion disk corona. During short high a third strong component is present with a softer spectrum, which is associated with the neutron star and accretion disk. Due to this direct emission from the neutron star and accretion disk, the reflected emission is less clear, however parameters and fluxes modulations during short high state indicate its presence. In low state, the hard X-ray flux ($h ν$ $>$ $10 keV$) peaks at orbital phase $ϕ_{orb}\simeq$ 0.55, which is expected from a simple model of atmospheric reflection from the companion star. The offset indicates an asymmetry in the X-ray illumination of the companion, which could be due to shadowing of the the inner face of HZ Her by the accretion disk and/or stream.

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arXiv2014

HZ Her: Stellar Radius from X-ray Eclipse Observations, Evolutionary State and a New Distance

Observations of HZ Her/Her X-1 by the Rossi X-ray Timing Explorer (RXTE) covering high state eclipses of the neutron star are analyzed here. Models of the eclipse are used to measure the radius and atmospheric scale height of HZ Her, the stellar companion to the neutron star. The radius is 2.58 to 3.01 $\times10^{11}$ cm, depending on system inclination and mass ratio(q), with accuracy of $\sim$1 part in 1000 for given inclination and $q$. We fit Kurucz model stellar atmosphere models to archival optical observations. The resulting effective temperature ($T_{eff}$) of the unheated face of HZ Her is determined to be in the 2$σ$ range 7720K to 7865K, and metallicity ($log(Z/Z_{\odot})$) in the range -0.27 to +.03. The model atmosphere surface flux and new radius yield a new distance to HZ Her/ Her X-1, depending on system inclination and $q$: a best-fit value of 6.1 kpc with upper and lower limits of 5.7 kpc and 7.0 kpc. We calculate stellar evolution models for the range of allowed masses (from orbital parameters), and allowed metallicities (from optical spectrum fits). The stellar models agree with $T_{eff}$ and radius of HZ Her for two narrow ranges of mass: 2.15 to 2.20 $M_{\odot}$ and 2.35 to 2.45 $M_{\odot}$ This lower mass range implies a low neutron star mass (1.3 $M_{\odot}$), whereas the higher mass range implies a high neutron star mass (1.5-1.7 $M_{\odot}$).

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arXiv2013

X-ray and Radio Observations of the gamma Cygni Supernova Remnant G78.2+2.1

We analyse \textit{ROSAT} and \textit{Chandra} ACIS X-ray observations and HI absorption spectra of the $γ$ Cygni supernova remnant (G78.2+2.1, DR4). The \textit{ROSAT} All-Sky-Survey image shows G78.2+2.1 has an adjacent limb-brightened shell north of it. A new \textit{ROSAT} mosaic shows details of the X-ray emission over the entire face of G78.2+2.1. We also create \textit{Chandra} mosaics which cover much of the northern rim and central regions of G78.2+2.1. HI absorption spectra result in association of G78.2+2.1 with the $γ$ Cygni nebula, with distance 1.7 to 2.6 kpc. Chandra spectra for G78.2+2.1 give an X-ray temperature of 0.6-1.2 keV (90% error), and that a Sedov model has age of 6800-10000 yr. A compact power-law X-ray source in G78.2+2.1 is consistent with the same distance as G78.2+2.1. The northern X-ray shell is identified with a B3 star at distance of 980 pc and is proposed as a stellar wind bubble.

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arXiv2013

High velocity HI is not associated with TeV supernova remnant W51C

The recently-detected TeV gamma-ray source HESS J1923+141 coincides with Supernova Remnant (SNR) W51C and the star forming region W51B of the W51 complex. We construct HI absorption spectra to SNR W51C, HII regions G49.2-0.35 and G49.1-0.38 in W51B, and a nearby compact extragalactic source. Our study detects high-velocity (HV) HI clouds (above 83 km/s) which coincide with W51B, but finds that the clouds are behind W51B. Both W51C and G49.2-0.35 have have similar highest-velocity absorption features at ~70 km/s. The HII region G49.1-0.38 is behind the SNR because its HI absorption spectrum has a feature at 83 km/s. These new results argue against previous claims that the SNR has shocked the HV HI clouds. Therefore the TeV emission from the complex should not be associated with the HV HI clouds. W51C has a distance of about 4.3 kpc, smaller than the tangent point distance of 5.5 kpc in that direction, but still in the Sagittarius spiral arm.

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arXiv2010

X-ray emission from HESS J1731-347/SNR G353.6-0.7 and Central Compact Source XMMS J173203-344518

We present new results of the HESS J1731-347/SNR G353.6-0.7 system from XMM-NEWTOM and Suzaku X-ray observations, and Delinha CO observations. We discover extended hard X-rays coincident with the bright, extended TeV source HESS J1731-347 and the shell of the radio SNR. We find that spatially-resolved X-ray spectra can generally be characterized by an absorbed power-law model, with photon-index of ~ 2, typical of non-thermal emission. A bright X-ray compact source, XMMS J173203-344518, is also detected near the center of the SNR. We find no evidence of a radio counterpart or an extended X-ray morphology for this source, making it unlikely to be a pulsar wind nebular (PWN). The spectrum of the source can be well fitted by an absorbed blackbody with a temperature of ~ 0.5 keV plus a power-law tail with a photon-index of ~ 5, reminiscent of the X-ray emission of a magnetar. CO observations toward the inner part of the HESS source reveal a bright cloud component at -20+/-4 km s^{-1}, which is likely located at the same distance of ~ 3.2 kpc as the SNR. Based on the probable association between the X-ray and $γ$-ray emissions and likely association between the CO cloud and the SNR, we argue that the extended TeV emission originates from the interaction between the SNR shock and the adjacent CO clouds rather than from a PWN.

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arXiv2008

Distance Determination to the Crab-like PWN G54.1+0.3 and Search for its Supernova Remnant Shell

We discover a large-scale shell G53.9+0.2 around the Crab-like pulsar wind nebula (PWN) G54.1+0.3 with 1420 MHz continuum VLA observations. This is confirmed by a new infrared image at 8 μm from the GLIMPSE Legacy Project, which reveals an intriguing infrared shell just surrounding the large radio shell. We analyze the 21 cm HI absorption spectra and 13CO emission spectra towards PWN G54.1+0.3 and bright sources on both radio and IR shells. Continuous HI absorption up to the tangent point and absence of negative HI absorption features imply that PWN G54.1+0.3 has a distance beyond the tangent point but within the Solar circle, i.e. 4.5 to 9 kpc. G54.1+0.3 is likely at distance of \simeq 6.2 kpc due to the morphological association between the PWN and a CO molecular cloud at velocity of \simeq 53 km/s, as revealed by high-resolution 13CO images. Based on the HI absorption spectrum and recombination line velocity (\simeq 40 km/s) of the bright HII region G54.09-0.06, which is on the IR shell, the IR shell is likely located at a distance of \simeq 7.3 kpc, which is also the distance of the associated large-scale radio shell. At this distance, the radio shell has a radius of ~ 30 pc. The radio shell may be thermal and lack IR emission due to dust destruction, or it may be nonthermal and part of a newly found old SNR. In either case it is located at a different distance than PWN G54.1+0.3.

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arXiv2008

Discovery of the radio and X-ray counterpart of TeV Gamma-ray source HESS J1731-347

We discover a faint shell-type radio and X-ray source, G353.6-0.7, associated with HESS J1731-347. G353.6-0.7 is likely an old supernova remnant (SNR), based on radio (0.8 GHz, 1.4 GHz and 5 GHz), infrared (8 $μ$m from the GLIMPSE Legacy Project and 21 $μ$m from the Midcourse Space Experiment), and X-ray (0.1 keV - 2.4 keV from the ROSAT survey and 5 - 20 keV from the INTEGRAL survey) data. The SNR, centered at ({\sl l}, {\sl b})=(353.55, -0.65) with a radius of $\sim$ 0.25$^{\circ}$, closely matches the outline of the recently discovered extended TeV source HESS J1731-347, which has no previously identified counterpart. A diffuse X-ray enhancement detected in the ROSAT all-sky survey is coincident with lower half shell of the SNR. Therefore the SNR is the best radio counterpart of both the HESS source and the diffuse X-ray enhancement. G353.6-0.7 has an age of $\sim$ 27000 yrs. Altogether, the new discovery provides the best case that an old SNR emits TeV $γ$-rays.

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arXiv2007

Stellar Wind Accretion in GX301-2: Evidence for a High-density Stream

The X-ray binary system GX301-2 consists of a neutron star in an eccentric orbit accreting from the massive early-type star WRAY 977. It has previously been shown that the X-ray orbital light curve is consistent with existence of a gas stream flowing out from Wray 977 in addition to its strong stellar wind. Here, X-ray monitoring observations by the Rossi X-ray Timing Explorer (RXTE)/ All-Sky-Monitor (ASM) and pointed observations by the RXTE/ Proportional Counter Array (PCA) over the past decade are analyzed. We analyze both the flux and column density dependence on orbital phase. The wind and stream dynamics are calculated for various system inclinations, companion rotation rates and wind velocities, as well as parametrized by the stream width and density. These calculations are used as inputs to determine both the expected accretion luminosity and the column density along the line-of-sight to the neutron star. The model luminosity and column density are compared to observed flux and column density vs. orbital phase, to constrain the properties of the stellar wind and the gas stream. We find that the change between bright and medium intensity levels is primarily due to decreased mass loss in the stellar wind, but the change between medium and dim intensity levels is primarily due to decreased stream density. The mass-loss rate in the stream exceeds that in the stellar wind by a factor of 2.5. The quality of the model fits is significantly better for lower inclinations, favoring a mass for WRAY 977 of 53 to 62 Msun.

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arXiv2007

Radio and X-ray images of SNR G18.8+0.3 interacting with molecular clouds

New HI images from the VLA Galactic Plane Survey (VGPS) show clear absorption features associated with the supernova remnant (SNR) G18.8+0.3. High-resolution $^{13}$CO images reveal that molecular clouds overlap the radio filaments of G18.8+0.3. The $^{13}$CO emission spectrum over the full velocity range in the direction of G18.8+0.3 shows two molecular components with high brightness-temperatureand three molecular components with low brightness-temperature, all with respective HI absorption. This implies that these clouds are in front of G18.8+0.3. In the HI images, the highest velocity absorption feature seen against the continuum image of G18.8+0.3 is at 129 km s$^{-1}$, which corresponds to the tangent point in this direction. This yields a lower distance limit of 6.9 kpc for G18.8+0.3. Absence of absorption at negative velocities gives an upper distance limit of 15 kpc. The broadened profile at 20$\pm$5 km s$^{-1}$ in the $^{13}$CO emission spectra is a strong indicator of a possible SNR/CO cloud interaction. Thus, G18.8+0.3 is likely to be at the distance of about 12 kpc. The upper mass limit and mean density of the giant CO cloud at 20$\pm$5 km s$^{-1}$ are \sim 6 \times 10^{5} M_{\odot} and \sim 2 \times 10^{2} cm^{-3}. We find an atomic hydrogen column density in front of G18.8+0.3 of $N_{HI}$ \sim 2 \times 10^{22} cm^{-2}. The ROSAT PSPC observations show a diffuse X-ray enhancement apparently associated with part of the radio shell of G18.8+0.3. Assuming an association, the unabsorbed flux is 2.2 \times10^{-12} erg cm^{-2} s^{-1}, suggesting an intrinsic luminosity of 3.6 \times 10^{34} erg s^{-1} for G18.8+0.3.

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arXiv2007

Discovery of A New Faint Radio SNR G108.2-0.6

A new faint and large shell-type radio Supernova Remnant (SNR) G108.2-0.6 has been discovered in the Canadian Galactic Plane Survey (CGPS). The SNR shows an elliptical shell-type structure at 1420 MHz, and has a 408-1420 MHz TT-plot spectral index of $α$=-0.5$\pm$0.1 (S$_ν$$\propto$$ν$$^α$), typical of a shell-type SNR. The remnant's flux density at 1420 MHz is 6.6$\pm$0.7 Jy, and at 408 MHz is 11.5$\pm$1.2 Jy. Both of these are corrected for compact sources. An integrated spectral index of $-0.45\pm$0.13 is determined. This new SNR has among the lowest surface brightness of any known remnant ($Σ$$_{1 GHz}$=2.4$\times10^{-22}$ W m$^{-2}$ Hz$^{-1}$ sr$^{-1}$). 21 cm Stokes Q and U CGPS data (plus preliminary Effelsberg Q and U maps) show some suggestive features that correlate with total power. \ion{H}{i} observations show structures associated with G108.2$-$0.6 in the radial velocity range $-$53 to $-$58 km s$^{-1}$, and indicate it is located in the Perseus arm shock at a distance of 3.2$\pm$0.6 kpc. At this distance the diameter of G108.2$-$0.6 is 58 pc. IRAS maps (12, 25, 60 and 100μm) of the new SNR show rich infrared emission surrounding G108.2$-$0.6.

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arXiv2007

VLA and XMM-Newton observations of the SNR W41/TeV Gamma-ray source HESS J1834-087

The recently discovered extended TeV source HESS J1834-087 is associated with both a diffuse X-ray enhancement and a molecular cloud, projected at the center of an old radio supernova remnant G23.3-0.3 (SNR W41). New HI observations from the VLA Galactic Plane Survey (VGPS) show unambiguous structures associated with W41 in the radial velocity range of 53 to 63 km/s, so we obtain for W41 a distance of 4+-0.2 kpc. A new higher sensitivity VGPS continuum image of W41 at 1420 MHz shows faint emission in its eastern part not detected by previous observations, so we give a new angular size of 36'x30' in b-l direction (average radius of 19 pc). We estimate for W41 a Sedov age of ~10e5 yr. New XMM-Newton observation reveals diffuse X-ray emission within the HESS source and suggests an association between the X-ray and Gamma-ray emission. The high-resolution ^{13}CO images of W41 further reveal a giant molecular cloud (GMC) located at the center of W41, likely associated with W41 in the the radial velocity range of 61 to 66 km/s. Altogether, the new observations can be interpreted as providing the first evidence that an old SNR encounters a GMC to emit TeV gamma-rays in the cloud material.

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arXiv2006

Radio Spectrum and Distance of the SNR HB9

New images are presented of the supernova remnant (SNR) HB9 based on 408 MHz and 1420 MHz continuum emission and HI-line emission data of the Canadian Galactic Plane Survey (CGPS) by the Dominion Radio Astrophysical Observatory (DRAO). Two methods of spectral index analysis for HB9 are presented and compared: one removes compact sources at both frequencies but is limited to the resolution of the 408 MHz image; the other removes compact sources only in the 1420 MHz image so is effective at higher spatial resolution. The second allows more detailed spectral index variation studies than the first. The two T-T plot methods and new integrated flux densities give spectral index (S_ν\proptoν^{-alpha}) for the whole of HB9 of 0.48+-0.03; and 0.47+-0.06, respectively. These are lower than previous spectral index for HB9 (alpha=0.61). Spatial variations of spectral index are derived using the second method and yield a steeper spectral index for interior regions than for the rim. This can be explained by a standard curved interstellar electron energy spectrum combined with lower interior magnetic field compared to that near the outer shock, which results in a larger proportion of steep spectrum emission for lines-of-sight through the central body of the SNR. HI observations show structures probably associated with the SNR in the radial velocity range -3 to -9 km/s and suggest a distance of 0.8+-0.4 kpc for the SNR. This is consistent with the distance to the radio pulsar 0458+46, offset from the center of HB9 by 23'. However the pulsar spindown and kinematic ages are significantly greater than estimates of the SNR age: the Sedov age for HB9 is 6600 yr and the evaporative cloud model yields ages of 4000-7,000 yr.

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arXiv2006

The radio SNR G65.1+0.6 and its associated pulsar J1957+2831

New images of the radio Supernova Remnant (SNR) G65.1+0.6 are presented, based on the 408 MHz and 1420 MHz continuum emission and the HI-line emission data of the Canadian Galactic Plane Survey (CGPS). A large shell-like structure seen in the 2695 MHz Effelsberg map appears to have nonthermal spectral index. HI observations show structures associated with the SNR G65.1+0.6 in the radial velocity range of -20 to -26 km$/$s and suggest a distance of 9.2 kpc for the SNR. The estimated Sedov age for G65.1+0.6 is 4 - 14 x10E4 yr. The pulsar (PSR) J1957+2831 is possibly associated with G65.1+0.6, with consistent distance and kinematic age estimate, but different characteristic age than the SNR. The EGRET source 3EG J1958+2909 and gamma-ray source 2CG 065+00 are also near the eastern edge of the SNR but do not agree in position with the pulsar and are likely not associated with the SNR. The SNR's flux densities at 408 MHz (8.6+-0.8 Jy), 1420 MHz (4.9+-0.5 Jy) and 2695 MHz (3.3+-0.5 Jy) have been corrected for flux densities from compact sources within the SNR. The integrated flux density based spectral index between 1420 MHz and 408 MHz is 0.45+-0.11 and agrees with the T-T plot spectral index of 0.34+-0.20. The nearby SNR DA495 has a T-T plot spectral index of 0.50+-0.01.

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arXiv2000

EUVE Observations of Hercules X-1 During a Short High State Turn-On

Observations of Hercules X-1 by the Extreme Ultraviolet Explorer (EUVE) covering low state and the early part of the Short High state are reported here. This is the first EUV observation of this part of the 35-day cycle of Her X-1. The low state portion of the EUV light curve (prior to the start of the Short High state) has similar properties as that following the end of the Short High state (Leahy and Marshall, 1999). This is evidence that the low state EUV emission is primarily due to EUV reflection from the companion star HZ Her. The EUV lightcurve during the Short High state is pulsed and closely resembles the average 2-12 keV X-ray Short High state lightcurve indicating that the EUV emission, like the X-ray emission, originates near the neutron star. The Short High state EUV spectrum is consistent with a blackbody of temperature 0.13 keV and radius 230 km. The Short High state EUV spectrum and pulse shape are similar to that in the soft X-rays (0.1 - 1 keV). The most likely origin of the EUV emission is reprocessed X-rays from the inner edge of the accretion disk, and the radius of the inner edge of the accretion disk is likely to be small, consistent with that determined from analysis of the X-ray pulse shape evolution (Scott, Leahy and Wilson, 2000).

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arXiv1999

The USA X-ray Timing Experiment

The USA Experiment is a new X-ray timing experiment with large collecting area and microsecond time resolution capable of conducting a broad program of studies of galactic X-ray binaries. USA is one of nine experiments aboard the Advanced Research and Global Observation Satellite which was launched February 23, 1999. USA is a collimated proportional counter X-ray telescope with about 1000 cm$^2$ of effective area per detector with two detectors sensitive to photons in the energy range 1-15 keV. A unique feature of USA is that photon events are time tagged by reference to an onboard GPS receiver allowing precise absolute time and location determination. We will present an overview of the USA instrument, capabilities, and scientific observing plan as well as the current status of the instrument.

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arXiv1999

Simultaneous X-ray and Radio Monitoring of the Unusual Binary LSI+61 303: Measurements of the Lightcurve and High-Energy Spectrum

The binary system, LSI+61 303, is unusual both because of the dramatic, periodic, radio outbursts, and because of its possible association with the 100 MeV gamma-ray source, 2CG135+01. We have performed simultaneous radio and Rossi X-ray Timing Explorer X-ray observations at eleven intervals over the 26.5 day orbit, and in addition searched for variability on timescales ranging from milliseconds to hours. We confirm the modulation of the X-ray emission on orbital timescales originally reported by Taylor et al. (1996), and in addition we find a significant offset between the peak of the X-ray and radio flux. We argue that based on these results, the most likely X-ray emission mechanism is inverse Compton scattering of stellar photons off of electrons accelerated at the shock boundary between the relativistic wind of a young pulsar and the Be star wind. In these observations we also detected 2 -- 150 keV flux from the nearby low-redshift quasar QSO~0241+622. Comparing these measurements to previous hard X-ray and gamma-ray observations of the region containing both LSI+61 303 and QSO~0241+622, it is clear that emission from the QSO dominates.

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