arXiv subjects

Thomas J. Maccarone

Publications and source records attributed to Thomas J. Maccarone.

195 records · Page 11Linked to original sources

arXiv2004

An explanation for metallicity effects on X-ray Binary properties

We show that irradiation induced stellar winds can explain two important metallicity effects in X-ray binaries - the higher numbers and the softer spectra of the X-ray binaries in metal rich globular clusters compared to the metal poor ones. As has been previously noted by Iben, Tutukov and Fedorova, the winds should be stronger at lower metallicity due to less efficient line cooling. This will speed up the evolution of the LMXBs in metal poor clusters, hence reducing their numbers. These winds can also provide extra material near the accreting object which may create an intrinsic absorber to harden the X-ray spectra of the metal poor cluster systems relative to the metal rich ones, as suggested by observations. We outline some additional observational predictions of the model.

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arXiv2003

An X-ray Selected AGN at z=4.6 Discovered by the CYDER Survey

We present the discovery of a high redshift, X-ray selected AGN by the Calan-Yale Deep Extragalactic Research (CYDER) survey: CXOCY J033716.7-050153, located at $z=4.61$, the second high redshift AGN discovered by this survey. Here, we present its optical, near-IR and X-ray properties and compare it with other optical and X-ray selected high redshift AGN. The optical luminosity of this object is significantly lower than most optically selected high redshift quasars. It also has a lower rest frame UV to X-ray emission ratio than most known quasars at this redshift. This mild deviation can be explained either by dust obscuring the UV radiation of a normal radio quiet AGN emitting at 10% of its Eddington luminosity or because this is intrinsically a low luminosity radio loud AGN, with a super-massive Black Hole of $\sim 10^8M_\sun$ emitting at 1% of its Eddington luminosity. Deep radio observations can discriminate between these two hypotheses.

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arXiv2003

Do X-ray Binary Spectral State Transition Luminosities Vary?

We tabulate the luminosities of the soft-to-hard state transitions of all X-ray binaries for which there exist good X-ray flux measurements at the time of the transition, good distance estimates, and good mass estimates for the compact star. We show that the state transition luminosities are at about 1-4% of the Eddington rate, markedly smaller than those typically quoted in the literature, with a mean value of 2%. Only the black hole candidate GRO J~1655-40 and the neutron star systems Aql X-1 and 4U 1728-34 have measured state transition luminosities inconsistent with this value at the 1$σ$ level. GRO J~1655-40, in particular, shows a state transition luminosity below the mean value for the other sources at the $4σ$ level. This result, combined with the known inner disk inclination angle (the disk is nearly parallel to the line of sight) from GRO J~1655-40's relativistic jets suggest that the hard X-ray emitting region in GRO J~1655-40 can have a velocity of no more than about $β=0.68$, with a most likely value of about $β=0.52$, and a minimum speed of $β=0.45$, assuming that the variations in state transition luminosities are solely due to relativistic beaming effects. The variance in the state transition luminosities suggests an emission region with a velocity of $\sim0.2c$. The results are discussed in terms of different emission models for the low/hard state. We also discuss the implications for measuring the dimensionless viscosity parameter $α$. We also find that if its state transitions occur at typical luminosities, then GX 339-4 is likely to be at a distance of at least 7.6 kpc, much further than typically quoted estimates.

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arXiv2003

A New Simple Model for High Frequency Quasi Periodic Oscillations in Black Hole Candidates

Observations of X-ray emissions from binary systems have long since been considered important tools to test General Relativity in strong-field regimes. The high frequency quasi-periodic oscillations (HFQPOs) observed in binaries containing a black hole candidate, in particular, have been proposed as a means to measure more directly the black hole properties such as its mass and spin. Numerous models have been suggested to explain the HFQPOs and the rich phenomenology accompanying them. Many of these models rest on a number of assumptions and are at times in conflict with the most recent observations. We here propose a new, simple model in which the HFQPOs result from basic $p$-mode oscillations of a small accretion torus orbiting close to the black hole. We show that within this model the key properties of the HFQPOs can be explained simply, given a single reasonable assumption. We also discuss observational tests that can falsify the model.

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arXiv2003

Some Constraints On the Effects of Age and Metallicity on the Low Mass X-ray Binary Formation Rate

We have studied the low mass X-ray binary (LMXB) populations within and outside globular clusters (GC) in NGC 4365 and NGC 3115. Using published age and metallicity constraints from optical and IR observations of their GCs, we do not find any evidence for an increase in the LMXB formation rate in the intermediate age cluster population of NGC 4365, as has been proposed in some scenarios of dynamical LMXB formation in GCs. The old, metal-rich, red population of GCs in NGC 3115 on the other hand is {\it at least} three times as efficient at creating LMXBs as the old, metal-poor, blue clusters. These data suggest that the higher formation efficiency of LMXBs in the red GC subsystems of many galaxies is largely a consequence of their higher metallicity. A comparison of the densities of field LMXBs in different galaxies does not reveal an obvious correlation with the age of the field stars as predicted by models in which the LMXB formation rate in the field drops monotonically with time after an initial burst. This suggests that either a significant fraction of the field LMXBs are created in GCs and subsequently injected into the field, or the LMXB formation rate has a more complex time evolution pattern.

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arXiv2003

Spectral Fits to the 1999 Aql X-1 Outburst Data

We present analysis and spectral fits of the RXTE data from the May/June 1999 outburst of Aql X-1. These data include observations in the rising portion of the hard state, in the soft state, and in the falling portion of the hard state. We show that the data can be fit by a purely thermal Comptonization model for all the observations, but that more complicated models cannot be ruled out. Up to 60% of the corona's power in the soft state may be injected into non-thermal electrons. The soft state data show approximately constant optical depth and coronal temperatures over a range of $\sim10$ in luminosity, while they show evidence for a reduction of seed photon temperature with reduced luminosity and indicate that the characteristic size of the seed photon emitting region is roughly constant throughout the soft state. The hard state before the soft state shows a higher luminosity, higher optical depth, and lower electron temperature than the hard state after the soft state. We find a reduction of the hard (30-60 keV) X-ray flux during a type I burst and show that it requires a total corona energy reservoir of less than $\sim 10^{38}$ ergs.

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arXiv2003

Jet-Induced Nucleosynthesis in Misaligned Microquasars

The jet axes and the orbital planes of microquasar systems are usually assumed to be approximately perpendicular, eventhough this is not currently an observational requirement. On the contrary, in one of the few systems where the relative orientations are well-constrained, V4641Sgr, the jet axis is known to lie not more than ~36 degrees from the binary plane. Such a jet, lying close to the binary plane, and traveling at a significant fraction of the speed of light may periodically impact the secondary star initiating nuclear reactions on its surface. The integrated yield of such nuclear reactions over the age of the binary system (less the radiative mass loss) will detectably alter the elemental abundances of the companion star. This scenario may explain the anomalously high Li enhancements (roughly ~20-200 times the sun's photospheric value; or, equivalently, 0.1-1 times the average solar system value) seen in the companions of some black-hole X-ray binary systems. (Such enhancements are puzzling since Li nuclei are exceedingly fragile - being easily destroyed in the interiors of stars - and Li would be expected to be depleted rather than enhanced there.) Gamma-ray line signatures of the proposed process could include the 2.22 MeV neutron capture line as well as the 0.478 MeV 7Li* de-excitation line, both of which may be discernable with the INTEGRAL satellite if produced in an optically thin region during a large outburst. For very energetic jets, a relatively narrow neutral pion gamma-decay signature at 67.5 MeV could also be measurable with the GLAST satellite. We argue that about 10-20% of all microquasar systems ought to be sufficiently misaligned as to be undergoing the proposed jet-secondary impacts.

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arXiv2002

The Low Mass X-ray Binary-Globular Cluster Connection II: NGC 4472 X-ray Source Properties and Source Catalogs

We present the results of a Chandra/HST study of the point sources of the NGC 4472. We identify 144 X-ray sources, 72 with HST matches. The optical data show 1102 sources, 829 with globular cluster colors. Thirty matches are found - likely to be low mass X-ray binaries in globular clusters, while 42 have no optical counterparts to V~25 and I~24 - likely predominantly LMXBs in the field star population. Thus approximately 40% of the X-ray sources are in globular clusters and ~4% of the globular clusters contain X-ray sources. The blue GC sources may have harder X-ray spectra than the red GC sources. No significant differences are found between the X-ray properties of the field sources and of the GC sources. This study, along with our previous result from Paper I in this series on the similarity of the spatial profile of the field LMXBs, globular cluster LMXBs, and the globular clusters themselves suggest that a significant fraction of the observed low mass X-ray binaries in the field may be created in a globular cluster then ejected into the field by stellar interaction s; however, by comparing the results for NGC 4472 with those in several other galaxies, we find tentative evidence for a correlation be tween the globular cluster specific frequency and the fraction of LMXBs in globular clusters, a correlation which would be most easily explained if some of the field sources were generated in situ. We show that isolated accreting very massive black holes are unlikely to be observable with current X-ray instrumentation and that these sources hence do not contaminate the LMXB population. We discuss the possibility that several equatorial point sources may indicate the presence of a disk wind responsible for the low radiative efficiency observed in the nucleus of this source. (abridged)

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arXiv2002

Hysteresis in the Light Curves of Soft X-ray Transients

Using PCA data from the Rossi X-Ray Timing Explorer (RXTE), we track the spectral states of the neutron star transient system Aql X-1 through a complete outburst cycle. We find a hard-to-soft state transition during the very early, rising phase of the outburst and show that there is a hysteresis effect such that the transition back to the hard state occurs at a luminosity $\sim $ 5 times {\it lower} than the hard-to-soft transition. This hysteresis effect rules out the propeller mechanism as the sole cause of state transitions in Aql X-1. Assuming the propeller mechanism only operates at a luminosity equal to or below that of the observed soft-to-hard transition requires that the magnetic field of Aql X-1 be less than $7\times 10^7$ Gauss, the lowest neutron star field known to date. To compare the state transition behavior of Aql X-1 with that found in transient black hole systems, we use RXTE ASM data to compute hardness-intensity diagrams for four black hole candidate transients where the ASM data should also give us state information throughout much of the outburst cycles. In all four systems, we find evidence for a hard-to-soft state transition during the rising outburst phase and for the source staying in a soft state down to much lower luminosities during the declining phase, i.e., a hysteresis effect. This similarity suggests a common origin for state transitions in low magnetic field neutron star and black hole systems, and the hysteresis effect rules out the ``Strong ADAF Principle'' for determining the state of an accretion disc. We discuss the general implications of these observations for current models of state transitions. We note the contrast to previous observations of the non-transient systems Cygnus X-1 and X-3 which do not show a hysteresis effect.

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arXiv2002

On the Misalignment of Jets in Microquasars

We discuss the timescales for alignment of black hole and accretion disc spins in the context of binary systems. We show that for black holes that are formed with substantial angular momentum, the alignment timescales are likely to be at least a substantial fraction of the systems' lifetimes. This result explains the observed misalignment of the disc and the jet in the microquasar GRO J 1655-40 and in SAX J 1819-2525 as being likely due to the Bardeen-Petterson effect. We discuss the implications of these results on the mass estimate for GRS 1915+105, which has assumed the jet is perpendicular to the orbital plane of the system and may hence be an underestimate. We show that the timescales for the spin alignment in Cygnus X-3 are consistent with the likely misalignment of disc and jet in that system, and that this is suggested by the observational data.

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arXiv2002

A Test of Photometric Redshifts for X-ray Selected Sources

We test the effectiveness of photometric redshifts based upon galaxy spectral template fitting for X-ray luminous objects, using a sample of 65 sources detected by Chandra in the field of the Caltech Faint Galaxy Redshift Survey (CFGRS). We find that sources with quasar-dominated spectra (for which galaxy spectral templates are not appropriate) are easily identified, and that photometric redshifts are robust for the rest of the sources in our sample. Specifically, for the 59 sources that are not quasar-dominated at optical wavelengths, we find that the photometric redshift estimates have scatter comparable to the field galaxy population in this region. There is no evidence for a trend of increasing dispersion with X-ray luminosity over the range L_X = 10^39 - 5x10^43 erg/s, nor is there a trend with the ratio of X-ray to optical flux, f_X/f_R. The practical implication of this work is that photometric redshifts should be robust for the majority (~90%) of the X-ray sources down to f_X ~ 10^-16 erg/s/cm^2 that have optical counterparts brighter than R ~ 24. Furthermore, the same photometry can be easily used to identify the sources for which the photometric redshifts are likely to fail. Photometric redshift estimation can thus be utilized as an efficient tool in analyzing the statistical properties of upcoming large Chandra and XMM-Newton data sets and identifying interesting subsamples for further study.

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arXiv2002

Short Timescale Correlations Between Line and Continuum Fluxes in Cygnus X-1

We present the results of 16-s time scale spectral fits for Cygnus X-1 in the soft state and in the transition state, using a Comptonised blackbody plus an iron line. On these timescales, we find that that the continuum source flux can vary by factors 2-3 and that the iron line intensity appears to track these changes well, i.e., the inferred equivalent width of the line remains constant to within the errors. We also find no significant changes in the seed (blackbody) photon temperature, while the properties of the Comptonising corona clearly do vary, with the spectral hardness and flux generally being correlated. The corona therefore seems to be the overall driver for the rapid timescale variability observed in the soft and transition states. These results are consistent with the Fourier resolved spectroscopy results of Gilfanov et al. (2000) that indicate the iron line shows rapid flux variations while the blackbody component does not and suggest that the iron line flux in fact tracks continuum changes down to very short timescales. We extend this work by showing that not only the variability amplitudes, but also the {\it phases} of the iron line and continuum components are identical. We note that the short timescale variability properties of the soft and transition states are actually not very different from those of the hard state, suggesting that the corona is the main cause of rapid variability in that state too, and hence that the mechanism responsible for the corona is similar in all three states.

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arXiv2002

Higher Order Variability Properties of Accreting Black Holes

To better constrain the emission mechanism underlying the hard state of galactic black hole candidates, we use high-time resolution RXTE lightcurves for Cyg X-1 and GX 399-4 to compute two higher order variability statistics for these objects, the skewness and the Fourier bispectrum. Similar analyses, in particular using the skewness measure, have been attempted before, but the photon collection area of RXTE allows us to present results of much greater statistical significance. The results for the two objects are qualitatively similar, reinforcing the idea that the same basic mechanisms are at work in both. We find a significantly positive skewness for variability timescales less than $\sim 1 $ second, and a {\it negative} skewness for timescale $1- 5 $ sec. Such a skewness pattern cannot be reproduced by the simplest shot variability models where individual shots have a fixed profile and intensity and are uncorrelated in time. Further evidence against simple shot models comes from the significant detection of a non-zero bicoherence for Fourier periods $\sim 0.1-10$ sec, implying that significant coupling does exist between variations on these timescales. We discuss how current popular models for variability in black hole systems can be modified to match these observations. Using simulated light curves, we suggest that the most likely way to reproduce this observed behavior is to have the variability come in groups of many shots, with the number of shots per unit time fitting an envelope function which has a rapid rise and slow decay, while the individual shots have a slow rise and a rapid decay. Invoking a finite energy reservoir that is depleted by each shot is a natural way of producing the required shot correlations.

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arXiv2002

RXTE/OSSE Fits to the Hard State Spectrum of Cygnus X-1

We present three spectra of Cygnus X-1 in the low/hard state from 3-1000 keV produced with RXTE and OSSE. We demonstrate that a pure thermal Comptonization model, with Compton reflection fits the data well, with coronal temperatures of about 90 keV, optical depths of about 1.3, and reflection fractions of about 0.25 $\times$ 2$π$. We find that no additional components are necessary to fit the data, although the presence of an additional non-thermal component does result in a marginal improvement to the goodness of the fit. Two of the observations are fit better by an almost purely non-thermal Comptonization model with $Γ_{inj}$ of $\sim$ 3.2 for the electrons and the other parameters essentially the same as for the thermal model, while the third observation is also consistent with this model. Observations that could break this degeneracy are discussed. We also demonstrate that the low reflection parameter (i.e. $R/2π<1$) is due to a lack of strong curvature in the 30-50 keV range of the spectrum and not due to ionization effects. The spectrum changes very little over the three weeks of the observations, in accordance with a three week correlation timescale found in the All Sky Monitor data. We show that for a purely thermal corona, the radius of the corona must be between $3\times 10^6$ and $1.5\times 10^{10}$ cm, while the radius of a hybrid corona or a purely non-thermal corona is constrained to be between $10^8$ and $10^9$ cm.

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arXiv2000

Time Domain Analysis of Variability in Cygnus X-1: Constraints on the Emission Models

We use time domain analysis techniques to investigate the rapid variability of Cygnus X-1. We show that the cross-correlation functions between hard and soft energy bands reach values very close to unity and peak at a lag of less than 2 millisecond for energies separated by a factor of 10. This confirms that the process that produces X-ray photons at different energies is extremely coherent on short time scales and strongly constrains emission models proposed to explain Fourier-frequency-dependent time lags. We present autocorrelation functions at different energies, and note their widths decrease with increasing energy. We show that the extended Compton corona model produces auto-correlation functions whose widths increase with increasing energy, that the model of cylindrical waves moving inward through a transition disk has too large a peak lag in the cross-correlation function. Models of magnetic flaring and of drifting blobs in a hot corona can qualitatively fit the observations.

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