arXiv subjects

Jon M. Miller

Publications and source records attributed to Jon M. Miller.

At least 163 records · Page 9Linked to original sources

arXiv2011

A strong emission line near 24.8 angstrom in the X-ray binary system MAXI J0556--332: gravitational redshift or unusual donor?

We report the discovery of a strong emission line near 24.8 angstrom (0.5 keV) in the newly discovered X-ray binary system MAXI J0556-332 with the reflection grating spectrometer onboard the XMM-Newton observatory. The X-ray light curve morphology during these observations is complex and shows occasional dipping behavior. Here we present time- and rate-selected spectra from the RGS and show that this strong emission line is unambiguously present in all the XMM observations. The measured line center is consistent with the Ly-alpha transition of N VII in the rest frame. While the spectra contain imprints of absorption lines and edges, there appear to be no other significantly prominent narrow line due to the source itself, thus making the identification of the 24.8 angstrom line uncertain. We discuss possible physical scenarios, including a gravitationally redshifted O VIII Ly-alpha line originating at the surface of a neutron star or an unusual donor with an extremely high N/O abundance (>57) relative to solar, that may have produced this comparatively strong emission line.

astro-ph.HE↗

arXiv2011

A Jet Model for the Broadband Spectrum of the Seyfert-1 Galaxy NGC 4051

Recent radio VLBI observations of the ~parsec-scale nuclear region of the narrow line Seyfert 1 galaxy NGC 4051 hint toward the presence of outflowing plasma. From available literature we have collected high-quality, high-resolution broadband spectral energy distribution data of the nuclear region of NGC 4051 spanning from radio through X-rays, to test whether the broadband SED can be explained within the framework of a relativistically outflowing jet model. We show that once the contribution from the host galaxy is taken into account, the broadband emission from the active galactic nucleus of NGC 4051 can be well described by the jet model. Contributions from dust and ongoing star-formation in the nuclear region tend to dominate the IR emission even at the highest resolutions. In the framework of the jet model, the correlated high variability of the extreme ultraviolet and X-rays compared to other wavelengths suggests that the emission at these wavelengths is optically thin synchrotron originating in the particle acceleration site(s) in the jet very close (few $r_g=GM_{BH}/c^2$) to the central supermassive black hole of mass M_{BH}. Our conclusions support the hypothesis that narrow line Seyfert 1 galaxies (which NGC 4051 is a member of) harbor a "jetted" outflow with properties similar to what has already been seen in low-luminosity AGNs and stellar mass black holes in hard X-ray state.

astro-ph.HE↗

arXiv2011

A Fast X-ray Disk Wind in the Transient Pulsar IGR J17480-2446 in Terzan 5

Accretion disk winds are revealed in Chandra gratings spectra of black holes. The winds are hot and highly ionized (typically composed of He-like and H-like charge states), and show modest blue-shifts. Similar line spectra are sometimes seen in "dipping" low-mass X-ray binaries, which are likely viewed edge-on; however, that absorption is tied to structures in the outer disk, and blue-shifts are not typically observed. Here we report the detection of blue-shifted He-like Fe XXV (3100 +/- 400 km/s) and H-like Fe XXVI (1000 +/- 200 km/s) absorption lines in a Chandra/HETG spectrum of the transient pulsar and low-mass X-ray binary IGR J17480-2446 in Terzan 5. These features indicate a disk wind with at least superficial similarities to those observed in stellar-mass black holes. The wind does not vary strongly with numerous weak X-ray bursts or flares. A broad Fe K emission line is detected in the spectrum, and fits with different line models suggest that the inner accretion disk in this system may be truncated. If the stellar magnetic field truncates the disk, a field strength of B = 0.7-4.0 E+9 Gauss is implied, which is in line with estimates based on X-ray timing techniques. We discuss our findings in the context of accretion flows onto neutron stars and stellar-mass black holes.

astro-ph.HE↗

arXiv2011

A Distinctive Disk-Jet Coupling in the Seyfert-1 AGN NGC 4051

We report on the results of a simultaneous monitoring campaign employing eight Chandra X-ray (0.5-10 keV) and six VLA/EVLA (8.4 GHz) radio observations of NGC 4051 over seven months. Evidence for compact jets is observed in the 8.4 GHz radio band; This builds on mounting evidence that jet production may be prevalent even in radio-quiet Seyferts. Assuming comparatively negligible local diffuse emission in the nucleus, the results also demonstrate an inverse correlation of L_radio proportional to L_X-ray ^(-0.72+/-0.04) . Current research linking the mass of supermassive black holes and stellar-mass black holes in the "low/hard" state to X-ray luminosities and radio luminosities suggest a "fundamental plane of accretion onto black holes" that has a positive correlation of L_radio proportional to L_X-ray^(0.67+/-0.12) . Our simultaneous results differ from this relation by more than 11 sigma, indicating that a separate mode of accretion and ejection may operate in this system. A review of the literature shows that the inverse correlation seen in NGC 4051 is seen in three other black hole systems, all of which accrete at near 10% of their Eddington luminosity, perhaps suggesting a distinct mode of disk-jet coupling at high Eddington fractions. We discuss our results in the context of disk and jets in black holes and accretion across the black hole mass scale.

astro-ph.HE↗

arXiv2011

An Improved Dynamical Model for the Microquasar XTE J1550-564

We present an improved dynamical model of the X-ray binary and microquasar XTE J1550-564 based on new moderate-resolution optical spectroscopy and near-infrared photometry. By combining our new radial velocity measurements with previous measurements obtained 2001 May at the 8.2m VLT and with light curves, we find an orbital period of P=1.5420333 +/- 0.0000024 days and a radial velocity semiamplitude of K_2=363.14 +/- 5.97$ km/sec, which together imply an optical mass function of f(M)=7.65 +/- 0.38 solar masses. We find that the projected rotational velocity of the secondary star is 55 +/- 5 km/sec, which implies a very extreme mass ratio of Q=M/M_2=30. Using a model of a Roche lobe-filling star and an azimuthally symmetric accretion disk, we fit simultaneously optical light curves from 2001, near-infrared light curves from 2008 and all of the radial velocity measurements to derive system parameters. We find an inclination of 74.7 +/- 3.8 deg and component masses of M_2=0.30 +/- 0.07 solar masses and M=9.10 +/- 0.61 solar masses for the secondary star and black hole, respectively. The radius of the secondary star for the adopted model is 1.75 +/- 0.12 solar radii. Using this radius, the average K_S magnitude, and an extinction of A_K=0.507 +/- 0.050 mag, we find a distance of 4.38^{+0.58}_{-0.41} kpc, which is in good agreement with a recent distance estimate based on HI absorption lines (abstract shortened).

astro-ph.SR↗

arXiv2011

A Census of Baryons and Dark Matter in an Isolated, Milky Way-sized Elliptical Galaxy

We present a study of the dark and luminous matter in the isolated elliptical galaxy NGC720, based on deep X-ray observations made with Chandra and Suzaku. The gas is reliably measured to ~R2500, allowing us to place good constraints on the enclosed mass and baryon fraction (fb) within this radius (M2500=1.6e12+/-0.2e12 Msun, fb(2500)=0.10+/-0.01; systematic errors are <~20%). The data indicate that the hot gas is close to hydrostatic, which is supported by good agreement with a kinematical analysis of the dwarf satellite galaxies. We confirm a dark matter (DM) halo at ~20-sigma. Assuming an NFW DM profile, our physical model for the gas distribution enables us to obtain meaningful constraints at scales larger than R2500, revealing that most of the baryons are in the hot gas. We find that fb within Rvir is consistent with the Cosmological value, confirming theoretical predictions that a ~Milky Way-mass (Mvir=3.1e12+/-0.4e12 Msun) galaxy can sustain a massive, quasi-hydrostatic gas halo. While fb is higher than the cold baryon fraction typically measured in similar-mass spiral galaxies, both the gas fraction (fg) and fb in NGC720 are consistent with an extrapolation of the trends with mass seen in massive galaxy groups and clusters. After correcting for fg, the entropy profile is close to the self-similar prediction of gravitational structure formation simulations, as observed in galaxy clusters. Finally, we find a strong heavy metal abundance gradient in the ISM similar to those observed in massive galaxy groups.

astro-ph.CO↗

arXiv2010

Continued cooling of the crust in the neutron star low-mass X-ray binary KS 1731-260

Some neutron star low-mass X-ray binaries have very long outbursts (lasting several years) which can generate a significant amount of heat in the neutron star crust. After the system has returned to quiescence, the crust then thermally relaxes. This provides a rare opportunity to study the thermal properties of neutron star crusts, putting constraints on the thermal conductivity and hence the structure and composition of the crust. KS 1731-260 is one of only four systems where this crustal cooling has been observed. Here, we present a new Chandra observation of this source approximately 8 years after the end of the last outburst, and 4 years since the last observation. We find that the source has continued to cool, with the cooling curve displaying a simple power-law decay. This suggests that the crust has not fully thermally relaxed yet, and may continue to cool further. A simple power law decay is in contrast to theoretical cooling models of the crust, which predict that the crust should now have cooled to the same temperature as the neutron star core.

astro-ph.HE↗

arXiv2010

Chandra grating spectroscopy of the Be/X-ray binary 1A 0535+262

We present Chandra HETGS spectroscopy of the Be/X-ray binary 1A 0535+262 obtained during the 2009/2010 giant outburst. These are the first CCD grating spectra of this type of system during a giant outburst. Our spectra reveal a number of lines including a narrow Fe K_alpha emission line with a FWHM of ~ 5000 km s^-1. For the first time, we detect the presence of a highly ionized outflow in a Be/X-ray binary. Assuming that the line is He-like Fe XXV, fits with a simple Gaussian imply an outflow velocity of ~ 1500 km s^-1. However, self-consistent photoionization modeling with XSTAR suggests that Fe XXIII-XXIV must also contribute. In this case, an outflow velocity of ~ 3000 km s^-1 is implied. These results are discussed in the context of the accretion flow in Be-star, neutron star, and black hole X-ray binaries.

astro-ph.HE↗

arXiv2010

Quiescent X-ray emission from Cen X-4: a variable thermal component

The nearby neutron star low-mass X-ray binary, Cen X-4, has been in a quiescent state since its last outburst in 1979. Typically, quiescent emission from these objects consists of thermal emission (presumably from the neutron star surface) with an additional hard power-law tail of unknown nature. Variability has been observed during quiescence in Cen X-4 on both timescales as short as hundreds of seconds and as long as years. However, the nature of this variability is still unknown. Early observations seemed to show it was all due to a variable hard X-ray tail. Here, we present new and archival observations that contradict this. The most recent Suzaku observation of Cen X-4 finds it in a historically low state, a factor of 4.4 fainter than the brightest quiescent observation. As the spectrum during the brightest observation was comprised of approximately 60% from the thermal component and 40% from the power-law component, such a large change cannot be explained by just power-law variability. Spectral fits with a variable thermal component fit the data well, while spectral fits allowing both the column density and the power-law to vary do not, leading to the conclusion that the thermal component must be variable. Interestingly, we also find that the thermal fraction remains consistent between all epochs, implying that the thermal and power-law fluxes vary by approximately the same amount. If the emitting area remains unchanged between observations, then the effective surface temperature must change. Alternatively, if the temperature remains constant, then the emitting area must change. The nature of this thermal variability is unclear, but may be explained by variable low-level accretion.

astro-ph.HE↗

arXiv2009

The Fundamental Plane of Accretion Onto Black Holes with Dynamical Masses

Black hole accretion and jet production are areas of intensive study in astrophysics. Recent work has found a relation between radio luminosity, X-ray luminosity, and black hole mass. With the assumption that radio and X-ray luminosity are suitable proxies for jet power and accretion power, respectively, a broad fundamental connection between accretion and jet production is implied. In an effort to refine these links and enhance their power, we have explored the above relations exclusively among black holes with direct, dynamical mass-measurements. This approach not only eliminates systematic errors incurred through the use of secondary mass measurements, but also effectively restricts the range of distances considered to a volume-limited sample. Further, we have exclusively used archival data from the Chandra X-ray Observatory to best isolate nuclear sources. We find log(L_R) = (4.80 +/- 0.24) + (0.78 +/- 0.27) log(M_BH) + (0.67 +/- 0.12) log(L_X), in broad agreement with prior efforts. Owing to the nature of our sample, the plane can be turned into an effective mass predictor. When the full sample is considered, masses are predicted less accurately than with the well-known M-sigma relation. If obscured AGN are excluded, the plane is potentially a better predictor than other scaling measures.

astro-ph.HE↗

arXiv2009

Spectrum Synthesis Modeling of the X-ray Spectrum of GRO J1655-40 Taken During the 2005 Outburst

The spectrum from the black hole X-ray transient GRO J1655-40. obtained using the $Chandra$ High Energy Transmission Grating (HETG) in 2005 is notable as a laboratory for the study of warm absorbers, and for the presence of many lines from odd-$Z$ elements between Na and Co (and Ti and Cr) not previously observed in X-rays. We present synthetic spectral models which can be used to constrain these element abundances and other parameters describing the outflow from the warm absorber in this object. We present results of fitting to the spectrum using various tools and techniques, including automated line fitting, phenomenological models, and photoionization modeling. We show that the behavior of the curves of growth of lines from H-like and Li-like ions indicate that the lines are either saturated or affected by filling-in from scattered or a partially covered continuum source. We confirm the conclusion of previous work by \cite{Mill06} and \cite{Mill08} which shows that the ionization conditions are not consistent with wind driving due to thermal expansion. The spectrum provides the opportunity to measure abundances for several elements not typically observable in the X-ray band. These show a pattern of enhancement for iron peak elements, and solar or sub-solar values for elements lighter than calcium. Models show that this is consistent with enrichment by a core-collapse supernova. We discuss the implications these values for the evolutionary history of this system.

astro-ph.HE↗

arXiv2008

Cooling of the crust in the neutron star low-mass X-ray binary MXB 1659-29

In quasi-persistent neutron star transients, long outbursts cause the neutron star crust to be heated out of thermal equilibrium with the rest of the star. During quiescence, the crust then cools back down. Such crustal cooling has been observed in two quasi-persistent sources: KS 1731-260 and MXB 1659-29. Here we present an additional Chandra observation of MXB 1659-29 in quiescence, which extends the baseline of monitoring to 6.6 yr after the end of the outburst. This new observation strongly suggests that the crust has thermally relaxed, with the temperature remaining consistent over 1000 days. Fitting the temperature cooling curve with an exponential plus constant model we determine an e-folding timescale of 465 +/- 25 days, with the crust cooling to a constant surface temperature of kT = 54 +/- 2 eV (assuming D=10 kpc). From this, we infer a core temperature in the range 3.5E7-8.3E7 K (assuming D=10 kpc), with the uncertainty due to the surface composition. Importantly, we tested two neutron star atmosphere models as well as a blackbody model, and found that the thermal relaxation time of the crust is independent of the chosen model and the assumed distance.

astro-ph↗

arXiv2008

Fe K alpha and hydrodynamic loop model diagnostics for a large flare on II Peg

The observation by the Swift X-ray Telescope of the Fe K alpha_1, alpha_2 doublet during a large flare on the RS CVn binary system II Peg represents one of only two firm detections to date of photospheric Fe K alpha from a star other than our Sun. We present models of the Fe K alpha equivalent widths reported in the literature for the II Peg observations and show that they are most probably due to fluorescence following inner shell photoionisation of quasi-neutral Fe by the flare X-rays. Our models constrain the maximum height of flare the to 0.15 R_* assuming solar abundances for the photospheric material, and 0.1 R_* and 0.06 R_* assuming depleted photospheric abundances ([M/H]=-0.2 and [M/H]=-0.4, respectively). Accounting for an extended loop geometry has the effect of increasing the estimated flare heights by a factor of ~3. These predictions are consistent with those derived using results of flaring loop models, which are also used to estimate the flaring loop properties and energetics. From loop models we estimate a flare loop height of 0.13 R_*, plasma density of ~4 * 10^12 cm^-3 and emitting volume of ~6 * 10^30 cm^3. Our estimates for the flare dimensions and density allow us to estimate the conductive energy losses to E_cond <= 2 * 10^36 erg, consistent with upper limits previously obtained in the literature. Finally, we estimate the average energy output of this large flare to be ~10^33 erg sec^-1, or 1/10th of the stellar bolometric luminosity.

astro-ph↗

arXiv2007

Relativistic iron emission lines in neutron star low-mass X-ray binaries as probes of neutron star radii

Using Suzaku observations of three neutron star low-mass X-ray binaries (Ser X-1, 4U 1820-30 and GX 349+2) we have found broad, asymmetric, relativistic Fe K emission lines in all three objects. These Fe K lines can be well fit by a model for lines from a relativistic accretion disk ('diskline'), allowing a measurement of the inner radius of the accretion disk, and hence an upper limit on the neutron star radius. These upper limits correspond to 14.5 - 16.5 km for a 1.4 M(solar) neutron star. The inner disk radii we measure with Fe K lines are in good agreement with the inner disk radii implied by kHz QPOs observed in both 4U 1820-30 and GX 349+2, supporting the inner disk nature of kHz QPOs. Additionally, the Fe K lines observed in these neutron stars are narrower than those in the black holes that are thought to be close to maximally spinning, as one would expect if inferences for spin are robust.

astro-ph↗

arXiv2007

Non-Detection of Gravitationally Redshifted Absorption Lines in the X-ray Burst Spectra of GS 1826-24

During a 200 ks observation with the XMM-Newton Reflection Grating Spectrometer, we detected 16 type-I X-ray bursts from GS 1826-24. We combined the burst spectra in an attempt to measure the gravitational redshifts from the surface of the neutron star. We divided the composite GS 1826-24 burst spectrum into three groups based on the blackbody temperature during the bursts. The spectra do not show any obvious discrete absorption lines. We compare our observations with those of EXO 0748-676.

astro-ph↗

arXiv2006

Cooling of the quasi-persistent neutron star X-ray transients KS 1731-260 and MXB 1659-29

We present Chandra and XMM-Newton X-ray observations that monitor the neutron star cooling of the quasi-persistent neutron star X-ray transients KS 1731-260 and MXB 1659-29 for approximately 4 years after these sources returned to quiescence from prolonged outbursts. In both sources the outbursts were long enough to significantly heat the neutron star crust out of thermal equilibrium with the core. We analyse the X-ray spectra by fitting absorbed neutron star atmosphere models to the observations. The results of our analysis strengthen the preliminary findings of Wijnands et al. that in both sources the neutron star crust cools down very rapidly suggesting it has a high heat conductivity and that the neutron star core requires enhanced core cooling processes. Importantly, we now detect the flattening of the cooling in both sources as the crust returns to thermal equilibrium with the core. We measure the thermal equilbrium flux and temperature in both sources by fitting a curve that decays exponentially to a constant level. The cooling curves cannot be fit with just a simple exponential decay without the constant offset. We find the constant bolometric flux and effective temperature components to be (9.2 +/- 0.9)E-14 ergs/cm^2/s and 70.0 +/- 1.6 eV in KS 1731-260 and (1.7 +/- 0.3)E-14 ergs/cm^2/s and 51.6 +/- 1.4 eV in MXB 1659-29. In addition, we find that the crust of KS 1731-260 cools faster than that of MXB 1659-29 by a factor of ~2, likely due to different crustal properties. This is the first time that the cooling of a neutron star crust into thermal equilibrium with the core has been observed in such detail.

astro-ph↗

arXiv2005

A Precessing Ring Model for Low-Frequency Quasi-periodic Oscillations

We develop a simple physical model to describe the most common type of low-frequency quasi-periodic oscillations (QPOs) seen in a number of accreting black hole systems, as well as the shape of the relativistically broadened iron emission lines that often appear simultaneously in such sources. The model is based on an inclined ring of hot gas that orbits the black hole along geodesic trajectories. For spinning black holes, this ring will precess around the spin axis of the black hole at the Lense-Thirring (``frame-dragging'') frequency. Using a relativistic ray-tracing code, we calculate X-ray light curves and observed energy spectra as a function of the radius and tilt angle of the ring, the spin magnitude, and the inclination of the black hole. The model predicts higher-amplitude QPOs for systems with high inclinations, as seen in a growing number of black hole binary systems. We find that the Rossi X-ray Timing Explorer observations of low-frequency QPOs in GRS 1915+105 are consistent with a ring of radius R ~ 10M orbiting a black hole with spin a/M ~0.5 and inclination angle of i ~ 70 deg. Finally, we describe how future X-ray missions may be able to use simultaneous timing and spectroscopic observations to measure the black hole spin and probe the inner-most regions of the accretion disk.

astro-ph↗

arXiv2005

XMM-Newton observations of the black hole X-ray transient XTE J1650-500 in quiescence

We report the result of an XMM-Newton observation of the black-hole X-ray transient XTE J1650-500 in quiescence. The source was not detected and we set upper limits on the 0.5-10 keV luminosity of 0.9e31-1.0e31 erg/s (for a newly derived distance of 2.6 kpc). These limits are in line with the quiescent luminosities of black-hole X-ray binaries with similar orbital periods (~7-8 hr)

astro-ph↗