arXiv subjects

Michael A. Nowak

Publications and source records attributed to Michael A. Nowak.

At least 73 records · Page 4Linked to original sources

arXiv2008

Suzaku Observations of Cyg X-1

We present highlights from a series of four simultaneous Suzaku/RXTE observations of the black hole candidate Cyg X-1. We briefly summarize several key results from our decade long RXTE monitoring campaign. We then comment on challenges of analyzing the Suzaku data, i.e., improving the aspect correction beyond that of the existing tools, and quantitatively assessing pileup. All of our Suzaku observations (one, by design) occurred at or very near orbital phase 0 (superior conjunction), and hence show evolution in color-color diagrams due to X-ray absorption by material from the wind of the secondary. We present simple partial absorption models for this evolution. We then compare the Suzaku and RXTE data, and explicitly divide the Fe line region into narrow and broad components. Both are required for the Suzaku data, and are seen to be consistent with the RXTE data. These Suzaku observations occurred near historically hard, low flux states. We present fits of the broad band spectra with a simple phenomenological broken powerlaw model, as well as a more physically motivated Comptonization model. Whereas the former class of models described nearly all of the RXTE campaign better than any physical model, here the latter model is slightly more successful. The Comptonization model, however, exhibits little evidence for a soft disk component, which formally corresponds to a small, inner disk radius. Whether this is physical, due to unmodeled absorption, or is a calibration issue, remains an open question.

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arXiv2008

Disk Dominated States of 4U 1957+11: Chandra, XMM, and RXTE Observations of Ostensibly the Most Rapidly Spinning Galactic Black Hole

We present simultaneous Chandra-HETG and RXTE observations of a moderate flux `soft state' of the black hole candidate 4U1957+11. These spectra, having a minimally discernible hard X-ray excess, are an excellent test of modern disk atmosphere models that include the effects of black hole spin. The HETG data show that the soft disk spectrum is only very mildly absorbed with N_H =1-2 X 10^{21} cm^-2. These data additionally reveal 13.449 A NeIX absorption consistent with the warm/hot phase of the interstellar medium. The fitted disk model implies a highly inclined disk around a low mass black hole rapidly rotating with normalized spin a*~1. We show, however, that pure Schwarzschild black hole models describe the data extremely well, albeit with large disk atmosphere ``color-correction'' factors. Standard color-correction factors can be attained if one additionally incorporates mild Comptonization. We find that the Chandra observations do not uniquely determine spin. Similarly, XMM/RXTE observations, taken only six weeks later, are equally unconstraining. This lack of constraint is partly driven by the unknown mass and unknown distance of 4U1957+11; however, it is also driven by the limited bandpass of Chandra and XMM. We therefore present a series of 48 RXTE observations taken over the span of several years and at different brightness/hardness levels. These data prefer a spin of a*~1, even when including a mild Comptonization component; however, they also show evolution of the disk atmosphere color-correction factors. If the rapid spin models with standard atmosphere color-correction factors of h_d=1.7 are to be believed, then the RXTE observations predict that 4U1957+11 can range from a 3 M_sun black hole at 10 kpc with a*~0.83 to a 16 M_sun black hole at 22 kpc with a* ~ 1, with the latter being statistically preferred.

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arXiv2008

Beyond XSPEC: Towards Highly Configurable Analysis

We present a quantitative comparison between software features of the defacto standard X-ray spectral analysis tool, XSPEC, and ISIS, the Interactive Spectral Interpretation System. Our emphasis is on customized analysis, with ISIS offered as a strong example of configurable software. While noting that XSPEC has been of immense value to astronomers, and that its scientific core is moderately extensible--most commonly via the inclusion of user contributed "local models"--we identify a series of limitations with its use beyond conventional spectral modeling. We argue that from the viewpoint of the astronomical user, the XSPEC internal structure presents a Black Box Problem, with many of its important features hidden from the top-level interface, thus discouraging user customization. Drawing from examples in custom modeling, numerical analysis, parallel computation, visualization, data management, and automated code generation, we show how a numerically scriptable, modular, and extensible analysis platform such as ISIS facilitates many forms of advanced astrophysical inquiry.

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arXiv2007

Limits on Hot Galactic Halo Gas from X-ray Absorption Lines

Although the existence of large-scale hot gaseous halos around massive disk galaxies have been theorized for a long time, there is yet very little observational evidence. We report the Chandra and XMM-Newton grating spectral detection of OVII and NeIX Kalpha absorption lines along the sight-line of 4U 1957+11. The line absorption is consistent with the interstellar medium in origin. Attributing these line absorptions to the hot gas associated with the Galactic disk, we search for the gaseous halo around the Milky Way by comparing this sight-line with more distant ones (toward X-ray binary LMC X-3 and the AGN Mrk 421). We find that all the line absorptions along the LMC X-3 and Mrk 421 sight-lines are attributable to the hot gas in a thick Galactic disk, as traced by the absorption lines in the spectra of 4U~1957+11 after a Galactic latitude dependent correction. We constrain the OVII column density through the halo to be N(OVII) < 5E15 cm^{-2} (95% confidence limit), and conclude that the hot gas contribution to the metal line absorptions, if existing, is negligible.

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arXiv2007

Tracking the Orbital and Super-orbital Periods of SMC X-1

The High Mass X-ray Binary (HMXB) SMC X-1 demonstrates an orbital variation of 3.89 days and a super-orbital variation with an average length of 55 days. As we show here, however, the length of the super-orbital cycle varies by almost a factor of two, even across adjacent cycles. To study both the orbital and super-orbital variation we utilize lightcurves from the Rossi X-ray Timing Explorer All Sky Monitor (RXTE-ASM). We employ the orbital ephemeris from Wojdowski et al. (1998) to obtain the average orbital profile, and we show that this profile exhibits complex modulation during non-eclipse phases. Additionally, a very interesting ``bounceback'' in X-ray count rate is seen during mid-orbital eclipse phases, with a softening of the emission during these periods. This bounceback has not been previously identified in pointed observations. We then define a super-orbital ephemeris (the phase of the super-orbital cycle as a function of date) based on the ASM lightcurve and analyze the trend and distribution of super-orbital cycle lengths. SMC X-1 exhibits a bimodal distribution of these lengths, similar to what has been observed in other systems (e.g., Her X-1), but with more dramatic changes in cycle length. There is some hint, but not conclusive evidence, for a dependence of the super-orbital cycle length upon the underlying orbital period, as has been observed previously for Her X-1 and Cyg X-2. Using our super-orbital ephemeris we are also able to create an average super-orbital profile over the 71 observed cycles, for which we witness overall hardening of the spectrum during low count rate times. We combine the orbital and super-orbital ephemerides to study the correlation between the orbital and super-orbital variations in the system.

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arXiv2007

Correlated radio--X-ray variability of Galactic Black Holes: A radio--X-ray flare in Cygnus X-1

We report on the first detection of a quasi-simultaneous radio-X-ray flare of Cygnus X-1. The detection was made on 2005 April 16 with pointed observations by the Rossi X-ray Timing Explorer and the Ryle telescope, during a phase where the black hole candidate was close to a transition from the its soft into its hard state. The radio flare lagged the X-rays by approximately 7 minutes, peaking at 3:20 hours barycentric time (TDB 2453476.63864). We discuss this lag in the context of models explaining such flaring events as the ejection of electron bubbles emitting synchrotron radiation.

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arXiv2007

Using the X-ray Dust Scattering Halo of 4U 1624-490 to determine distance and dust distributions

We present X-ray dust scattering halo results based on our 76 ks {\it Chandra} ACIS-S/HETGS observation of the LMXB dipping source 4U 1624-490. Through analysis of the halo light curves with 2-6 keV spectra over the persistent and dipping periods, we estimate a geometric distance of $\sim$15 kpc to 4U 1624-490. We also fit halo radial profiles with different ISM dust grain models to assess the location, uniformity, and density of the halo. Our analysis shows that the dust spatial distribution is not uniform along the line-of-sight; rather, it is consistent with the spiral arm structure mapped in {\sc Hii}. The large difference between the absorption Hydrogen column ($N_{\rm H}^{abs} \sim 8 \times10^{22} \rm cm^{-2}$; probes all gas along the line-of-sight) derived from broadband spectral fitting, and the scattering Hydrogen column ($N_{\rm H}^{sca} \sim 4 \times10^{22} \rm cm^{-2}$; probes only Galactic gas) derived from our studies of the 4U 1624-490 X-ray halo suggests that a large fraction of the column is local to the X-ray binary. We also present (and apply) a new method for assessing the {\it Chandra} point spread function at large ($> 50''$) angles, through use of the time delays from the observed dips.

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arXiv2006

The Galactic Black Holes Cyg X-1, GX 339-4, and 4U 1957+11: In Transition and at High Resolution

I review here some of the open questions regarding the geometry and emission mechanisms of galactic black hole candidates. For hard states, I concentrate on the perspective of ``disk+Compton coronae'' models (for discussions of jet models, see the papers by Sera Markoff). Specifically, I discuss the implications from our 10 year long RXTE monitoring campaign of Cyg X-1. I then present simultaneous RXTE/Chandra observations of the ``soft state'' black hole candidate 4U 1957+11, and discuss to what extent it does or does not allow one to test ``relativistic disk models''. The use of such models has been claimed to measure black hole spin parameters. I then briefly present a particularly freaky-weird observation of GX 339-4, where the source ``fell off'' the usual radio/X-ray correlation in the low/hard state. Questions addressed by the above observations include: are the Compton corona models unique fits to the data? (No. Jets work equally well, and simple broken power laws work better still. We argue that the latter models indicate multiple, broad-band continuum components.) Is there good evidence for a receding disk as sources transit into the hard state? (The jury is still out.) What does the relativistically broadened Fe line tell us? (Sometimes the disk, even into quiescence, stays very close to the central object, in contrast to expectations of ADAF models.) How much better/more necessary are recently discussed relativistic disk models? (I am very doubtful that such models will ever usefully measure black hole spin.)

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arXiv2004

Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/5: Tidal ULXs, Emergent Wind, and Resolved HII Regions

We present Chandra X-ray images for the interacting galaxy pair NGC 7714/5. In addition to the unresolved starburst nucleus, a variable point source with L(X) ~ 10^40 erg/s was detected 1.5" (270 pc) northwest of the nucleus, coincident with a blue, extremely optically-luminous (M(V) ~ -14.1) point source on HST images. Eleven more candidate point-like ultraluminous X-ray sources (ULXs) are seen, two >= 10^40 erg/s. Ten of these are associated with interaction-induced features but only two with star formation. We found diffuse emission with L(X) ~ 3 X 10^40 erg/s extending 11" (1.9 kpc) to the north of the nucleus. Its spectrum can be fit with a 2-temperature Mekal function (0.6/8 keV) or a 0.6 keV Mekal function plus a power law. The hard component may be due to high mass X-ray binaries (HMXBs) with contributions from inverse Compton radiation, while the soft component is likely from a superwind. We also detected extended X-ray emission from four extra-nuclear HII region complexes. This emission may be due to HMXBs or to diffuse gas heated by winds from supernovae, if the X-ray production efficiency L(X)/L(mech) is high (~5%). To estimate L(X)/L(mech), we collected published data for well-studied HII regions and superbubbles in nearby galaxies. For young HII regions (<3.5 Myrs), the median L(X)/L(mech) ~ 0.02%, while for older regions, L(X)/L(mech) ~ 0.2-7%. Thus gas heating by supernovae may be sufficient to account for the HII region emission. In galaxies much more distant than NGC 7714, for example, the Cartwheel galaxy, HII region complexes similar to those in NGC 7714 will be unresolved by Chandra and will mimick ULXs.

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arXiv2004

X-ray and Radio Monitoring of GX 339-4 and Cyg X-1

Previous work by Motch et al. (1985) suggested that in the low/hard state of GX339-4, the soft X-ray power-law extrapolated backward in energy agrees with the IR flux level. Corbel and Fender (2002) later showed that the typical hard state radio power-law extrapolated forward in energy meets the backward extrapolated X-ray power-law at an IR spectral break, which was explicitly observed twice in GX339-4. This has been cited as further evidence that jet synchrotron radiation might make a significant contribution to the observed X-rays in the hard state. We explore this hypothesis with a series of simultaneous radio/X-ray hard state observations of GX339-4. We fit these spectra with a simple, but remarkably successful, doubly broken power-law model that indeed requires a spectral break in the IR. For most of these observations, the break position as a function of X-ray flux agrees with the jet model predictions. We then examine the radio flux/X-ray flux correlation in Cyg X-1 through the use of 15 GHz radio data, obtained with the Ryle radio telescope, and Rossi X-ray Timing Explorer data, from the All Sky Monitor and pointed observations. We find evidence of `parallel tracks' in the radio/X-ray correlation which are associated with `failed transitions' to, or the beginning of a transition to, the soft state. We also find that for Cyg X-1 the radio flux is more fundamentally correlated with the hard, rather than the soft, X-ray flux.

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arXiv2004

Constraining X-ray Binary Jet Models via Reflection

Although thermal disk emission is suppressed or absent in the hard state of X-ray binaries, the presence of a cold, thin disk can be inferred from signatures of reprocessing in the ~2-50 keV band. The strength of this signature is dependent on the source spectrum and flux impinging on the disk surface, and is thus very sensitive to the system geometry. The general weakness of this feature in the hard state has been attributed to either a truncation of the thin disk, large ionization, or beaming of the corona region away from the disk with β~0.3. This latter velocity is comparable to jet nozzle velocities, so we explore whether a jet can account for the observed reflection fractions. It has been suggested that jets may contribute to the high-energy spectra of X-ray binaries, via either synchrotron from around 100-1000 r_g along the jet axis or from inverse Compton (synchrotron self-Compton and/or external Compton) from near the base. Here we calculate the reflection fraction from jet models wherein either synchrotron or Compton processes dominate the emission. Using as a guide a data set for GX 339-4, where the reflection fraction previously has been estimated as ~10%, we study the results for a jet model. We find that the synchrotron case gives < 2% reflection, while a model with predominantly synchrotron self-Compton in the base gives ~10-18%. This shows for the first time that an X-ray binary jet is capable of significant reflection fractions, and that extreme values of the reflection may be used as a way of discerning the dominant contributions to the X-ray spectrum.

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arXiv2003

The Future of X-ray Spectroscopy of Galactic Black Hole Binaries

There are four major X-ray satellites currently in operation, with two more shortly to follow, and several very ambitious observatories in various stages of planning. This very rich period of X-ray observation is leading to great advances in our understanding of the accretion flow onto the black hole, although we are quickly learning (or perhaps better put, remembering) exactly how complicated this flow can be. This review was meant to assess future prospects for X-ray spectroscopy of black hole binaries; however, I first look backward to the observations and theories that helped us arrive at our current `paradigm'. I then discuss current and near-future spectroscopic studies, which increasingly (and very fruitfully) treat X-ray spectroscopy as part of a larger, intimately connected picture along with radio, optical, and gamma-ray spectroscopy. Equally importantly, and in large part thanks to the success of RXTE, there is now a strong realization that spectral-temporal correlations, even across wavelength bands, are crucial to our understanding of the physics of these systems. Going forward, we are well-poised to continue to advance our knowledge via X-ray spectroscopy, both with existing satellites that have a long lifetime ahead of them, and with the next generation of instruments. If there is any `hole' in this bright future, it is the potential loss of RXTE, with no designated follow-up mission. Studies of multi-wavelength spectral-temporal correlations will become more difficult due to the loss of two important attributes of RXTE: its fast timing capabilities and its extremely flexible scheduling which has made many of these studies possible.

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arXiv2003

Fluorescent iron lines as a probe of astrophysical black hole systems

(abridged) With most physicists and astrophysicists in agreement that black holes do indeed exist, the focus of astrophysical black hole research has shifted to the detailed properties of these systems. Nature has provided us with an extremely useful probe of the region very close to an accreting black hole - X-ray irradiation of relatively cold material in the vicinity of the black hole can imprint characteristic features into the X-ray spectra of black hole systems, most notably the Kalpha fluorescent line of iron. Detailed X-ray spectroscopy of these features can be used to study Doppler and gravitational redshifts, thereby providing key information on the location and kinematics of the cold material. This is a powerful tool that allows us to probe within a few gravitational radii, or less, of the event horizon. Here, we present a comprehensive review of relativistic iron line studies for both accreting stellar mass black holes (i.e., Galactic Black Hole Candidate systems; GBHCs), and accreting supermassive black holes (i.e., active galactic nuclei; AGN). After a pedagogical introduction to black hole astrophysics, we discuss studies of relativistic iron lines in the AGN context and show how differences between classes of AGN can be diagnosed using X-ray spectroscopy. Furthermore, through a detailed discussion of one particular object (MCG--6-30-15), we illustrate how the exotic physics of black hole spin are now open to observational study. We proceed to discuss GBHCs, which turn out to possess rather more complicated X-ray spectra, making robust conclusions more difficult to draw. However, even in these cases, modern X-ray observatories are now providing convincing evidence for relativistic effects. We conclude by discussing the science that can be addressed by future X-ray observatories.

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arXiv2002

State Transitions in Black Hole Candidates

As described in the review by Marek Abramowicz (these proceedings), accretion theory is at something of a crossroads. Many theoretical descriptions of observations have centered on the ``hydrodynamic'' approach of the ``standard'' Shakura & Sunyaev alpha-disk; however, recent MHD simulations of (non-radiative) accretion flow have called into question the validity of this approach. There has been a great deal of optimism that these simulations give direct insight into current observations as similar phenomena appear in both; e.g., jets, rapid high-amplitude fluctuations, etc. In comparison to real data from black hole candidates (BHC), however, these similarities are in many ways only superficial. In some aspects, simple theories agree with the observations quite well. The observed rapid variability is much more highly structured than found in simulations, and shows interesting correlations with spectra that have been interpreted in terms of simple models. On the other hand, debates have arisen over even the most basic phenomenological issues concerning, e.g., the geometry and dominant radiation mechanisms of the accretion flow. In this article I present my views of those observational properties of BHC states that most urgently need to be addressed, and I briefly discuss some of the models currently being debated.

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arXiv2002

Coronal--Temporal Correlations in GX339-4: Hysteresis, Possible Reflection Changes, and Implications for ADAFs

We present spectral fits and timing analysis of Rossi X-ray Timing Explorer observations of GX339-4. These observations were carried out over a span of more than two years and encompassed both the soft/high and hard/low states. Hysteresis in the soft state/hard state transition is observed. The hard state exhibits a possible anti-correlation between coronal compactness (i.e., spectral hardness) and the covering fraction of cold, reflecting material. The correlation between `reflection fraction' and soft X-ray flux, however, appears to be more universal. Furthermore, low flux, hard state observations - taken over a decline into quiescence- show that the Fe line, independent of `reflection fraction', remains broad and at a roughly constant equivalent width, counter to expectations from ADAF models. All power spectral densities (PSD) of the hard state X-ray lightcurves are describable as the sum of just a few broad, quasi-periodic features with frequencies that roughly scale as coronal compactness to the -3/2 power. Similar to observations of Cyg X-1, time lags between soft and hard variability anti-correlate with coronal compactness. A stronger correlation is seen between the time lags and the `reflection fraction'.

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arXiv2000

Are There Three Peaks in the Power Spectra of GX 339-4 and Cyg X-1?

Among the variability behaviour exhibited by neutron star systems are the so-called ``horizontal branch oscillations'' (HBO, with frequencies ~50 Hz), the ``lower-frequency kHz quasi-periodic oscillation'' (QPO) and the ``upper-frequency kHz QPO'', with the latter two features being separated in frequency by an amount comparable to, but varying slightly from, the suspected spin-frequency of the neutron star. Recently, Psaltis, Belloni, & van der Klis (1999) have suggested that there exists a correlation between these three frequencies that, when certain identifications of variability features are made, even encompasses black hole sources. We consider this hypothesis by reanalyzing a set of GX 339-4 observations. The power spectral density (PSD) constructed from a composite of 7 separate, but very similar, observations shows evidence for three broad peaks in the PSD. If the peak frequencies of these features are identified with ``QPO'', then their frequencies approximately fit the correlations suggested by Psaltis, Belloni, & van der Klis (1999). We also reanalyze a Cyg X-1 observation and show that the suggested QPO correlation may also hold, but that complications arise when the ``QPOs'' (which, in reality, are fairly broad features) are considered as a function of energy band. These fits suggest the existence of at least three separate, independent physical processes in the accretion flow, a hypothesis that is also supported by consideration of the Fourier frequency-dependent time lags and coherence function between variability in different energy bands.

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arXiv2000

Implications of the X-ray Variability for the Mass of MCG-6-30-15

The bright Seyfert 1 galaxy \mcg shows large variability on a variety of time scales. We study the $\aproxlt 3$ day time scale variability using a set of simultaneous archival observations that were obtained from \rxte and the {\it Advanced Satellite for Cosmology and Astrophysics} (\asca). The \rxte\ observations span nearly $10^6$ sec and indicate that the X-ray Fourier Power Spectral Density has an rms variability of 16%, is flat from approximately 10^{-6} - 10^{-5} Hz, and then steepens into a power law $\propto f^{-α}$ with $α\aproxgt 1$. A further steepening to $α\approx 2$ occurs between 10^{-4}-10^{-3} Hz. The shape and rms amplitude are comparable to what has been observed in \ngc and \cyg, albeit with break frequencies that differ by a factor of 10^{-2} and 10^{4}, respectively. If the break frequencies are indicative of the central black hole mass, then this mass may be as low as $10^6 {\rm M}_\odot$. An upper limit of $\sim 2$ ks for the relative lag between the 0.5-2 keV \asca band compared to the 8-15 keV \rxte band was also found. Again by analogy with \ngc and \cyg, this limit is consistent with a relatively low central black hole mass.

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arXiv1999

On the enigmatic X-ray Source V1408 Aql (=4U 1957+11)

Models for the characteristically soft X-ray spectrum of the compact X-ray source V1498 Aql (=4U 1957+11) have ranged from optically thick Comptonization to multicolor accretion disk models. We critically examine the X-ray spectrum of V1408 Aql via archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data, archival Roentgensatellit (ROSAT) data, and recent Rossi X-Ray Timing Explorer (RXTE) data. Although we are able to fit a variety of X-ray spectral models to these data, we favor an interpretation of the X-ray spectrum as being due to an accretion disk viewed at large inclination angles. Evidence for this hypothesis includes long term (117 day, 235 day, 352 day) periodicities seen by the RXTE All Sky Monitor (ASM), which we interpret as being due to a warped precessing disk, and a 1 keV feature in the ASCA data, which we interpret as being the blend of L fluorescence features from a disk atmosphere or wind. We also present timing analysis of the RXTE data and find upper limits of 4% for the root mean square (rms) variability between f=0.001-16 Hz. The situation of whether the compact object is a black hole or neutron star is still ambiguous; however, it now seems more likely that an X-ray emitting, warped accretion disk is an important component of this system.

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