High Energy X-ray Timing Experiment Research Articles

A study of the hard X-ray spectral tails in Scorpius X-1 using RXTE observations

ABSTRACT Using all the data of the High Energy X-ray Timing Experiment (HEXTE) on board the Rossi X-ray Timing Explorer for Scorpius X-1 from 1996 February to 2012 January, we systematically search for hard X-ray tails in the X-ray spectra in 20–220 keV and, together with the data of the Proportional Counter Array (PCA), investigate the evolution of the detected hard X-ray tails along the Z-track on its hardness-intensity diagram (HID). The hard X-ray tails are detected in 30 observations and their presence is not confined to a specific position on the HID. Our analysis suggests that from the horizontal branch (HB), through the normal branch (NB), to the flaring branch (FB) on the HID, the hard X-ray tail becomes hard and its flux decreases. Jointly fitting the PCA+HEXTE spectra in 3–220 keV, it is found that the Bulk-Motion Comptonization (BMC) could be an alternative mechanism for producing the hard X-ray tails on the HB and the NB of this source. The temperature of the seed photons for the BMC spans in the range of ∼(2.4–2.6) keV, indicating that the seed photons might come from the surface of the neutron star (NS) or the boundary layer and, therefore, the BMC process could take place around the NS or in the boundary layer. Some possible mechanisms for producing the hard X-ray tails on the FB are given.

Time Evolution of the X-Ray and γ-Ray Fluxes of the Crab Pulsar

Abstract We studied the evolution of the X-ray and γ-ray spectra of the Crab pulsar utilizing the 11-year observations from the Rossi X-ray Timing Explorer (RXTE) and 9-year observations from the Fermi Gamma-ray Space Telescope (FGST). By fitting the spectrum of each observation, we obtained the corresponding flux and then analyzed the long-term evolution of the X-ray (or γ-ray) luminosities as well as their correlations with the spin-down power of the pulsar. The X-ray flux in 5–60 keV obtained by the Proportional Counter Array of RXTE decreases with a rate of (−2.4 ± 0.4) × 10−14 erg cm−2 s−1 per day. The X-ray flux in 15–250 keV obtained by the High Energy X-ray Timing Experiment of RXTE and the γ-ray flux in 0.1–300 GeV by the Large Area Telescope onboard FGST show similar decreasing trends but are insignificant statistically. The 5–60 keV X-ray luminosity L X is correlated with the spin-down power L sd by , which is similar to the statistical results for young pulsars.

Reflection Spectroscopy of the Black Hole Binary XTE J1752−223 in Its Long-stable Hard State

Abstract We present a detailed spectral analysis of the black hole binary XTE J1752−223 in the hard state of its 2009 outburst. Regular monitoring of this source by the Rossi X-ray Timing Explorer mission provided high signal-to-noise spectra along the outburst rise and decay. During one full month this source stalled at ∼30% of its peak count rate at a constant hardness and intensity. By combining all the data in this exceptionally stable hard state, we obtained an aggregate proportional counter array spectrum (3–45 keV) with 100 million counts, and a corresponding high energy X-ray timing experiment spectrum (20–140 keV) with 5.8 million counts. Implementing a version of our reflection code with a physical model for Comptonization, we obtain tight constraints on important physical parameters for this system. In particular, the inner accretion disk is measured very close in, at R in = 1.7 ± 0.4 R g . Assuming R in = R ISCO, we find a relatively high black hole spin (a * = 0.92 ± 0.06). Imposing a lamppost geometry, we obtain a low inclination (i = 35° ± 4°), which agrees with the upper limit found in the radio (i < 49°). However, we note that this model cannot be statistically distinguished from a non-lamppost model with a free emissivity index, for which the inclination is markedly higher. Additionally, we find a relatively cool corona (57–70 keV) and large iron abundance (3.3–3.7 solar). We further find that properly accounting for Comptonization of the reflection emission improves the fit significantly and causes an otherwise low reflection fraction (∼0.2–0.3) to increase by an order of magnitude, in line with geometrical expectations for a lamppost corona. We compare these results with similar investigations reported for GX 339−4 in its bright hard state.

Discovery and modelling of a flattening of the positive cyclotron line/luminosity relation in GX 304−1 with RXTE

The Rossi X-ray Timing Explorer (RXTE) observed four outbursts of the accreting X-ray binary transient source, GX 304−1 in 2010 and 2011. We present results of detailed 3–100 keV spectral analysis of 69 separate observations, and report a positive correlation between cyclotron line parameters, as well as other spectral parameters, with power-law flux. The cyclotron line energy, width and depth versus flux, and thus luminosity, correlations show a flattening of the relationships with increasing luminosity, which are well described by quasi-spherical or disc accretion that yield the surface magnetic field to be ∼5 × 10^(12) Gauss. Since HEXTE (High Energy X-ray Timing Experiment) cluster A was fixed aligned with the Proportional Counter Array field of view and cluster B was fixed viewing a background region 1°.5 off of the source direction during these observations near the end of the RXTE mission, the cluster A background was estimated from cluster B events using HEXTEBACKEST. This made possible the detection of the ∼55 keV cyclotron line and an accurate measurement of the continuum. Correlations of all spectral parameters with the primary 2–10 keV power-law flux reveal it to be the primary driver of the spectral shape. The accretion is found to be in the collisionless shock braking regime.

AN EMPIRICAL METHOD FOR IMPROVING THE QUALITY OF RXTE HEXTE SPECTRA

ABSTRACT We have developed a correction tool to improve the quality of Rossi X-ray Timing Explorer (RXTE) High Energy X-ray Timing Experiment (HEXTE) spectra by employing the same method we used earlier to improve the quality of RXTE Proportional Counter Array (PCA) spectra. We fit all of the hundreds of HEXTE spectra of the Crab individually to a simple power-law model, some 37 million counts in total for Cluster A and 39 million counts for Cluster B, and we create for each cluster a combined spectrum of residuals. We find that the residual spectrum of Cluster A is free of instrumental artifacts while that of Cluster B contains significant features with amplitudes ∼1%; the most prominent is in the energy range 30–50 keV, which coincides with the iodine K edge. Starting with the residual spectrum for Cluster B, via an iterative procedure we created the calibration tool hexBcorr for correcting any Cluster B spectrum of interest. We demonstrate the efficacy of the tool by applying it to Cluster B spectra of two bright black holes, which contain several million counts apiece. For these spectra, application of the tool significantly improves the goodness of fit, while affecting only slightly the broadband fit parameters. The tool may be important for the study of spectral features, such as cyclotron lines, a topic that is beyond the scope of this paper.

Hard X-ray Emission along the Z Track in GX 17 + 2

Using the data from the Proportional Counter Array (PCA) and the High-Energy X-ray Timing Experiment (HEXTE) on board Rossi X-Ray Timing Explorer for Z source GX 17 + 2, we investigate the evolution of its PCA spectra and HEXTE spectra along a ‘Z’ track on its hardness-intensity diagram. A hard X-ray tail is detected in the HEXTE spectra. The detected hard X-ray tails are discontinuously scattered throughout the Z track. The found hard X-ray tail hardens from the horizontal branch, through the normal branch, to the flaring branch in principle and it contributes ∼(20–50)% of the total flux in 20–200 keV. Our joint fitting results of the PCA + HEXTE spectra in 3–200 keV show that the portion of Comptonization in the Bulk-Motion Comptonization (BMC) model accounts for the hard X-ray tail, which indicates that the BMC process could be responsible for the detected hard tail. The temperature of the seed photons for BMC is ∼2.7 keV, implying that these seed photons might be emitted from the surface of the neutron star (NS) or the boundary layer between the NS and the disk and, therefore, this process could take place around the NS or in the boundary layer.

SPECTRAL SURVEY OF X-RAY BRIGHT ACTIVE GALACTIC NUCLEI FROM THE ROSSI X-RAY TIMING EXPLORER

Using long-term monitoring data from the Rossi X-ray Timing Explorer (RXTE), we have selected 23 active galactic nuclei (AGN) with sufficient brightness and overall observation time to derive broadband X-ray spectra from 3 to >100 keV. Our sample includes mainly radio-quiet Seyferts, as well as seven radio-loud sources. Given the longevity of the RXTE mission, the greater part of our data is spread out over more than a decade, providing truly long-term average spectra and eliminating inconsistencies arising from variability. We present long-term average values of absorption, Fe line parameters, Compton reflection strengths and photon indices, as well as fluxes and luminosities for the hard and very hard energy bands, 2-10 keV and 20-100 keV respectively. We find tentative evidence for high-energy rollovers in three of our objects. We improve upon previous surveys of the very hard X-ray energy band in terms of accuracy and sensitivity, particularly with respect to confirming and quantifying the Compton reflection component. This survey is meant to provide a baseline for future analysis with respect to the long-term averages for these sources and to cement the legacy of RXTE, and especially its High Energy X-ray Timing Experiment, as a contributor to AGN spectral science.

Signature of long-term class evolution in GRS 1915+105 at a high accretion rate

We find long-term evolution of the ω class from the study of X-ray timing and spectral analysis of the Galactic microquasar GRS 1915+105 during two outburst activities, observed by the proportional counter array (PCA) and the High Energy X-ray Timing Experiment on-board Rossi X-ray Timing Explorer. The class is characterized by unusual periodic-like variation in intensity. With the passage of time, the variability gradually disappears and the class stabilizes at higher intensity. Low-frequency (0.1–10 Hz) power density spectral evolution and energy spectral evolution show that the class initially occurs as an unstable and oscillatory stage between the high-soft state and the low-soft/hard-intermediate state and finally settles in the high-soft state. This modulation process gives a clear signature of the adjustment of the accretion disc during an increasing accretion rate. Spectral analysis of this class at different stages shows a distinct feature of a single thermal Comptonization component along with a strong hard X-ray tail. We try to fit different observational characteristics found during the timing and spectral evolution with different disc accretion models and we find that the presence of an extended, optically thin corona coupled with an accretion disc can explain our results adequately. The thermal electron cloud covers a large fraction of the inner disc region, and it is powered by the accretion energy of the accretion disc. The change in the coronal temperature and optical depth between dip and non-dip regions of the light curve shows the change in the underneath disc emissivity at nearly constant disc temperature. When a high state is achieved, the corona becomes cool and optically thick and emission is dominated by a disc blackbody component. The strong support for the presence of an accretion-powered Comptonizing medium is the detection of delay in arriving time between soft and hard photons during this class. Both All-Sky Monitor and PCA flaring intensity and the nature of evolution are found to be the same for both X-ray outbursts at different time. This indicates that this process has fundamental contribution to the overall disc accretion mechanism during the outburst and is hence important in understanding the possible ways for utilizing the total accretion energy budget.

The disc–jet coupling in the neutron star X-ray binary Aquila X-1

We study the accretion/ejection processes (i.e. disc/jet coupling) in the neutron star X-ray binary Aquila X-1 via a multiwavelength approach. We use in the radio band the publicly available Very Large Array archive containing observations of the object between 1986 and 2005, in the X-ray band the archival Rossi X-ray Timing Explorer data (Proportional Counter Array and High Energy X-ray Timing Experiment) between 1997 and 2008, and in optical (R band) observations with the Small and Moderate Aperture Research Telescope System recorded between 1998 and 2007. In the combined data set, we find three outbursts for which quasi-simultaneous radio, optical (R band) and X-ray data exist and focus on them to some extent. We provide evidence that the disc/jet coupling in Aquila X-1 is similar to what has been observed in black hole X-ray binaries, at least from the point of view of the behaviour in the hardness-intensity diagrams (the hysteresis effect included), when the phenomenology of the jet is taken into account. Although based on a very small number of observations, a radio/X-ray correlation seems to exist for this system, with a slope of α= 0.40 ± 0.07 (Fradio∝FαX), which is different than the slope of α= 1.40 ± 0.25 found for another atoll source, 4U 1728−34, but interestingly enough is relatively close to the values obtained for several black hole X-ray binaries. No significant correlation is found between the radio and optical (R-band) emissions. We also report a significant drop in the radio flux from Aquila X-1 above an X-ray flux of ∼5 × 10−9 erg cm−2 s−1. This behaviour, also reported in the neutron star X-ray binary 4U 1728−34, may be analogous to the suppression of radio emission in black hole X-ray binaries in bright, soft X-ray states. It suggests that from this point of view neutron star X-ray binaries can mimic the behaviour of black hole X-ray binaries in suppressing the jet in soft/disc-dominated X-ray states.

Temporal variations of strength and location of the South Atlantic Anomaly as measured by RXTE

The evolution of the particle background at an altitude of ~ 540 km during the time interval between 1996 and 2007 is studied using the particle monitor of the High Energy X-ray Timing Experiment on board NASA's Rossi X-ray Timing Explorer. A special emphasis of this study is the location and strength of the South Atlantic Anomaly (SAA). The size and strength of the SAA are anti-correlated with the 10.7 cm radio flux of the Sun, which leads the SAA strength by ~ 1 year reflecting variations in solar heating of the upper atmosphere. The location of the SAA is also found to drift westwards with an average drift rate of about 0.3°/yr following the drift of the geomagnetic-field configuration. Superimposed to this drift rate are irregularities, where the SAA suddenly moves eastwards and where furthermore the speed of the drift changes. The most prominent of these irregularities is found in the second quarter of 2003 and another event took place in 1999. We suggest that these events are previously unrecognized manifestations of the geomagnetic jerks of the Earth's magnetic field.

Evolution of Hard X‐Ray Spectra along the Orbital Phase in Circinus X‐1

Using the data from the Proportional Counter Array and the High-Energy X-Ray Timing Experiment on board the Rossi X-Ray Timing Explorer satellite, we investigate the evolution of the 3-200 keV spectrum of the peculiar low-mass X-ray binary Circinus X-1 along its orbital phase. A power-law hard component, dominating above ~30 keV as well as contributing between 0.9% and 3.5% of the total flux, and the iron emission line are detected in the spectrum. It is found that the derived photon index (Γ) of the power-law hard component and the characteristics of the iron emission line are modulated by the orbital phase: the hardest hard tail, the lowest line energy, and the largest line width of the iron emission line are detected at the periastron. However, the softest hard tail, the highest line energy, and the lowest line width of the iron emission line are detected after the apastron. At the periastron the source evolves along a complete track in the color-color diagram. These behaviors of orbital modulation are discussed. A possible origin of the power-law hard component is also discussed.

INTEGRAL/RXTE high-energy observation of a state transition of GX 339-4

On 2004 August 15, we observed a fast (shorter than 10 h) state transition in the bright black hole transient GX 339-4 simultaneously with Rossi X-Ray Timing Explorer (RXTE) and INTEGRAL. This transition was evident both in timing and spectral properties. Combining the data from the Proportional Counter Array (PCA), the High-Energy X-ray Timing Experiment (HEXTE) and the Imager on Board the INTEGRAL Satellite (IBIS), we obtained good quality broad-band (3-200 keV) energy spectra before and after the transition. These spectra indicate that the hard component steepened. Also, the high-energy cut-off that was present at ∼70 keV before the transition was not detected after the transition. This is the first time that an accurate determination of the broad-band spectrum across such a transition has been measured on a short time-scale. It shows that, although some spectral parameters do not change abruptly through the transition, the high-energy cut-off increases/disappears rather fast. These results constitute a benchmark on which to test theoretical models for the production of the hard component in these systems.

X-ray spectral evolution of Her X-1 in a low state and the following short high state

We analysed spectral variations of ∼8.5-d long Rossi X-ray Timing Explorer (RXTE) monitoring observations of Her X-1 in 2001 December. This set of observations enables, for the first time, frequent continuous monitoring (111 pointings in ∼8.5 d) of the source with RXTE including a ∼1.7-d-long low state part and the following ∼6.8-d-long short high state part. We used an absorbed power-law model with an iron line energy complex modelled as a Gaussian to fit both the 3–60 keV Proportional Counter Array—High Energy X-ray Timing Experiment (PCA—HEXTE) overall short high state spectrum and 3–20 keV individual PCA spectra. An additional partial cold absorber model was used for both cases. Using 3–20 keV individual PCA spectra, absorption in anomalous dips (ADs) and pre-eclipse dips (PDs) in the short high state was compared. Decreasing the ratio of unabsorbed to absorbed flux with increasing unabsorbed flux in ADs and PDs was interpreted as evidence of the fact that the regions causing opaque obscuration and soft absorption are not geometrically far away from each other. Higher iron line peak energies in the low state and the short high state (∼6.6–6.9 keV) were interpreted as a clue of the presence of iron line components other than the Kα emission line.

X1908+075: A Pulsar Orbiting in the Stellar Wind of a Massive Companion

We have observed the persistent but optically unidentified X-ray source X1908+075 with the Proportional Counter Array and High-Energy X-Ray Timing Experiment instruments on the Rossi X-Ray Timing Explorer (RXTE). The binary nature of this source was recently established by Wen et al., who found a 4.4 day orbital period in results from the RXTE All-Sky Monitor. We report the discovery of 605 s pulsations in the X-ray flux. The Doppler delay curve is measured and provides a mass function of 6.1 M☉, which is a lower limit to the mass of the binary companion of the neutron star. The degree of attenuation of the low-energy end of the spectrum is found to be a strong function of orbital phase. A simple model of absorption in a stellar wind from the companion star fits the orbital phase dependence reasonably well and limits the orbital inclination angle to the range 38°-72°. These measured parameters lead to an orbital separation of ~60-80 lt-s, a mass for the companion star in the range 9-31 M☉, and an upper limit to the size of the companion of ~22 R☉. From our analysis, we also infer a wind mass-loss rate from the companion star of ≳1.3 × 10-6 M☉ yr-1 and, when the properties of the companion star and the effects of photoionization are considered, likely ≳4 × 10-6 M☉ yr-1. Such a high rate is inconsistent with the allowed masses and radii that we find for a main-sequence or modestly evolved star unless the mass-loss rate is enhanced in the binary system relative to that of an isolated star. We discuss the possibility that the companion might be a Wolf-Rayet star that could evolve to become a black hole in 104 to 105 yr. If so, this would be the first identified progenitor of a neutron star-black hole binary.

Spectral Analysis ofRossi X‐Ray Timing ExplorerObservations of A3667

X-ray emission from the cluster of galaxies A3667 was measured by the Proportional Counter Array and High-Energy X-Ray Timing Experiment experiments on board the Rossi X-Ray Timing Explorer (RXTE) satellite during the period 2001 December-2002 July. Analysis of the ~141 ks RXTE observation and lower energy ASCA Gas Imaging Spectrometer data yields only marginal evidence for a secondary power-law emission component in the spectrum. The 90% confidence upper limit on nonthermal emission in the 15-35 keV band is determined to be 2.6 × 10-12 ergs cm-2 s-1. When combined with the measured radio flux and spectral index of the dominant region of extended radio emission, this upper limit implies a lower limit of ~0.4 μG on the mean, volume-averaged, intracluster magnetic field in A3667.

Discovery of Hard Nonthermal Pulsed X‐Ray Emission from the Anomalous X‐Ray Pulsar 1E 1841−045

We report the discovery of nonthermal pulsed X-ray/soft gamma-ray emission up to ~150 keV from the anomalous 11.8 s X-ray pulsar AXP 1E 1841-045 located near the center of supernova remnant Kes 73 using Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array and High Energy X-Ray Timing Experiment (HEXTE) data. The morphology of the double-peaked pulse profile changes rapidly with energy from 2 keV up to ~8 keV, above which the pulse shape remains more or less stable. The pulsed spectrum is very hard; its shape above 10 keV can be described well by a power law with a photon index of 0.94+/-0.16. 1E 1841-045 is the first AXP for which such very hard pulsed emission has been detected, which points to an origin in the magnetosphere of a magnetar. We have also derived the total emission spectrum from the Kes 73/1E 1841-045 complex for the ~1-270 keV energy range using RXTE HEXTE and XMM-Newton pn data. A comparison of the total emission from the complex with the pulsed+DC emission from 1E 1841-045 as derived from Chandra ACIS CC-mode data (Morii et al. 2003) leaves little room for emission from Kes 73 at energies near 7 keV or above. This suggests that the HEXTE spectrum above ~15 keV, satisfactorily described by a power law with index 1.47+/-0.05, is dominated by emission from 1E 1841-045. In that case the pulsed fraction for energies above 10 keV would increase from about 25% near 10 keV to 100% near 100 keV. The origin of the DC-component extending up to ~100 keV is probably magnetospheric and could be a manifestation of pulsed emission that is ``on'' for all phases.

Long term studies of Z sources with HEXTE/RXTE

We have analyzed the long pointed observations of the Z sources in the Rossi X-Ray Timing Explorer (RXTE) public archive to study the high energy emission in those sources. Our analysis is concentrated on the High Energy X-Ray Timing Experiment (HEXTE) waveband, since we are primarily interested in studying the hard X-ray (i.e., E>20 keV) production in those sources. We give here the preliminary results of this ongoing study. We have found no hard X-ray tails (besides ScoX-1) in our database from any of the Z sources, i.e., GX 349+2 (<7.9 × 10 −5 photons cm −2 s −1, 3 σ, 50–150 keV), Cyg X-2 (<8.4 × 10 −5 photons cm −2 s −1, 3 σ, 50–150 keV), GX 17+2 (<4.2 × 10 −5 photons cm −2 s −1, 3 σ, 50–150 keV), GX5-1 (<2.1 × 10 −5 photons cm −2 s −1, 3 σ, 50–150 keV), and GX 340+0 (<6.0 × 10 −5 photons cm −2 s −1 3 σ, 50–150 keV). From the point of view of HEXTE/ RXTE observations shown here, the production of hard X-ray tails in Z sources is a process triggered when special conditions are fulfilled. One of these conditions, as derived from our analysis, is a threshold of ∼4 × 10 36 erg s −1 for the luminosity of the source’s thermal component.

Evolution of Hard X-Ray Spectra along the Branches in Circinus X-1

Using the data from the Proportional Counter Array and the High-Energy X-Ray Timing Experiment on board the Rossi X-Ray Timing Explorer satellite, we investigate the evolution of the 3-200 keV spectra of the peculiar low-mass X-ray binary Circinus X-1 along the branches on its hardness-intensity diagram (HID) from the vertical horizontal branch, through the horizontal horizontal branch and normal branch, to the flaring branch. We detect a power-law hard component in the spectra. It is found that the derived photon indices (Γ) of the power-law hard component are correlated with the position on the HID. The power-law component dominates the X-ray emission of Cir X-1 in the energy band higher than ~20 keV. The fluxes of the power-law component are compared with those of the bremsstrahlung component in the spectra. A possible origin of the power-law hard component is discussed.

RXTE monitoring of Centaurus A

We report on the analysis from 110 ks of X-ray observations of Centaurus A carried out with the Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE) instruments on Rossi X-ray Timing Explorer (RXTE) during three monitoring campaigns over the last 4 years (10 ks in 1996, 74 ks in 1998, and 25 ks in 2000). The joint PCA/HEXTE X-ray spectrum can be well described by a heavily absorbed power law with photon index 1.8 and a narrow iron line due to fluorescence of cold matter. The measured column depth decreased by about 30% between 1996 and 2000, while the detected 2{10 keV continuum flux remained constant between 1996 and 1998, but increased by 60% in 2000. Since in all three observations the iron line flux did not vary, a corresponding decrease in equivalent width was noted. No appreciable evidence for a reflection continuum in the spectrum was detected. We present the interpretation of the iron line strength through Monte Carlo computations of various geometries. No signicant temporal variability was found in Cen A at time scales from days to tens of minutes.

High-Energy X-Ray Timing Experiment Detections of Hard X-Ray Tails in Scorpius X-1

We report the detection of a nonthermal hard X-ray component from Sco X-1 based on the analysis of 20-220 keV spectra obtained with the High-Energy X-Ray Timing Experiment on board the Rossi X-Ray Timing Explorer satellite. We find that the addition of a power-law component to a thermal bremsstrahlung model is required to achieve a good fit in five of 16 observations analyzed. Using Proportional Counter Array data, we were able to track the movement of the source along the Z diagram, and we found that the presence of the hard X-ray tail is not confined to a specific Z position. However, we do observe an indication that the power-law index hardens with increasing , as indicated from the position on the Z diagram. We find that the derived nonthermal luminosities are ~10% of that derived for the brightest of the atoll sources.