Hard Tail Research Articles

Disk‐Jet Coupling in the Low‐Mass X‐Ray Binary 4U 1636−53 fromINTEGRALObservations

We report on the spectral analysis of the neutron star (atoll-type) low-mass X-ray binary 4U 1636-53 as observed by the INTEGRAL and BeppoSAX satellites. The spectral behavior in three different epochs, corresponding to three different spectral states, has been investigated in depth. Spectra from two data sets show a continuum that is well described by one or two soft blackbodies plus a Comptonized component, with changes in the Comptonizing electrons and blackbody temperature and accretion rate that are typical of the spectral transition from high to low state. On one occasion the INTEGRAL spectrum shows, for first time in this source, a hard tail dominating the emission above 30 keV. The total spectrum is fitted as the sum of a Comptonized component similar to the soft state and a power-law component (Γ = 2.76), indicating the presence of a nonthermal distribution of electron velocities. In this case, a comparison with the hard tails detected in soft states from neutron star systems and some black hole binaries suggests that a similar mechanism could give rise to these components in both cases.

Average hard X-ray emission from NS LMXBs: observational evidence of different spectral states in NS LMXBs

Aims. We studied and compared the long-term average hard X-ray (>20 keV) spectra of a sample of twelve bright low-mass X-ray binaries hosting a neutron star (NS). Our sample comprises the six well studied Galactic Z sources and six Atoll sources, four of which are bright (“GX”) bulge sources while two are weaker ones in the 2–10 keV range (H 1750–440 and H 1608–55). Methods. For all the sources of our sample, we analysed available public data and extracted average spectra from the IBIS/ISGRI detector on board INTEGRAL . Results. We can describe all the spectral states in terms of the bulk motion Comptonisation scenario. We find evidence that bulk motion is always present, its strength is related to the accretion rate and it is suppressed only in the presence of high local luminosity. The two low-dim Atoll source spectra are dominated by photons up-scattered presumably due to dynamical and thermal Comptonisation in an optically thin, hot plasma. For the first time, we extend the detection of H 1750–440 up to 150 keV. The Z and bright “GX” Atoll source spectra are very similar and are dominated by Comptonised blackbody radiation of seed photons, presumably coming from the accretion disc and NS surface, in an optically thick cloud with plasma temperature in the range of 2.5–3 keV. Six sources show a hard tail in their average spectrum: Cyg X-2 (Z), GX 340+0 (Z), GX 17+2 (Z), GX 5–1 (Z), Sco X–1 (Z) and GX 13+1 (Atoll). This is the first detection of a hard tail in the X-ray spectrum of the peculiar GX 13+1. Using radio data from the literature we find, in all Z sources and bright “GX” Atolls, a systematic positive correlation between the X-ray hard tail (40–100 keV) and the radio luminosity. This suggests that hard tails and energetic electrons causing the radio emission may have the same origin, most likely the Compton cloud located inside the NS magnetosphere.

Five years of SGR 1900+14 observations withBeppoSAX

We present a systematic analysis of all the BeppoSAX data of the soft gamma-ray repeater SGR 1900+14: these observations allowed us to study the long term properties of the source quiescent emission. In the observation carried out before the 1998 giant flare the spectrum in the 0.8-10 keV energy range was harder and there was evidence for a 20-150 keV emission, possibly associated with SGR 1900+14. This possible hard tail, if compared with the recent INTEGRAL detection of SGR 1900+14, has a harder spectrum (power-law photon index ~1.6 versus ~3) and a 20-100 keV flux ~4 times larger. In the last BeppoSAX observation (April 2002), while the source was entering the long quiescent period that lasted until 2006, the 2-10 keV flux was ~25% below the historical level. We also studied in detail the spectral evolution during the 2001 flare afterglow. This was characterized by a softening that can be interpreted in terms of a cooling blackbody-like component.

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.

Discovery of Luminous Pulsed Hard X‐Ray Emission from Anomalous X‐Ray Pulsars 1RXS J1708−4009, 4U 0142+61, and 1E 2259+586 byINTEGRALandRXTE

We investigated the time-averaged high-energy spectral characteristics of the persistent anomalous X-ray pulsars (AXPs) 1RXS J1708-4009, 4U 0142+61, 1E 2259+586, and 1E 1048.1-5937, using RXTE PCA (2-60 keV), RXTE HEXTE (15-250 keV), and INTEGRAL IBIS ISGRI (20-300 keV) data. We discovered hard spectral tails for energies above 10 keV in the total and pulsed spectra of AXPs 1RXS J1708-4009, 4U 0142+4009, and 1E 2259+586, but 1E 1048.1-5937 appeared to be too weak to be detected. Improved hard X-ray spectral information for AXP 1E 1841-045 is also presented. The pulsed and total spectra above 10 keV have power-law shapes, and there is so far no significant evidence for spectral breaks or bends up to ~150 keV. The pulsed spectra above 10 keV are exceptionally hard with indices for four AXPs in the range -1.0 to 1.0. Below 10 keV these indices were in the range 2.0-4.3, indicating the very drastic spectral changes in a narrow energy interval around 10 keV. The best-fit power-law models to the total spectra between ~10 and 150 keV are significantly softer, with indices measured for 1E 1841-045, 1RXS J1708-4009, and 4U 0142+61 in the range 1.0-1.4. For the latter AXPs the pulsed fractions are consistent with 100% around 100 keV but are different at 10 keV: ~10% for 4U 0142+61, ~25% for 1E 1841-045, and consistent with 100% for 1RXS J1708-4009. The luminosities of these total and pulsed spectral tails (10-150 keV) largely exceed the total available spin-down powers by factors ranging from ~100 to ~600. We also reanalyzed archival CGRO COMPTEL (0.75-30 MeV) data: no AXP detections can be claimed, and the obtained upper limits indicate for 1RXS J1708-4009, 4U 0142+61, and 1E 1841-045 that strong breaks or bends must occur somewhere between ~150 and 750 keV. We discuss predictions from first attempts to model our hard X-ray and soft gamma-ray spectra in the context of the magnetar model.

Magnetars as persistent hard X-ray sources: INTEGRAL discovery of a hard tail in SGR 1900+14

Using 2.5 Ms of data obtained by the INTEGRAL satellite in 2003-2004, we discovered persistent hard X-ray emission from the soft gamma-ray repeater SGR 1900+14. Its 20-100 keV spectrum is well described by a steep power law with photon index r = 3.1 ± 0.5 and flux 1.5 x 10 -11 erg cm -2 s -1 . Contrary to SGR 1806-20, the only other soft gamma-ray repeater for which persistent emission above 20 keV was reported, SGR 1900+14 has been detected in the hard X-ray range while it was in a quiescent state (the last bursts from this source were observed in 2002). By comparing the broad band spectra (1-100 keV) of all the magnetars detected by INTEGRAL (the two SGRs and three anomalous X-ray pulsars) we find evidence for a different spectral behaviour of these two classes of sources.

New soft gamma-ray bursts in the BATSE records and spectral properties of X-ray rich bursts

A population of X-ray dominated gamma-ray bursts (GRBs) observed by Ginga, BeppoSAX and the High Energy Transient Explorer (HETE-2) should be represented in the Burst and Transient Source Experiment (BATSE) data as presumably soft bursts. We have performed a search for soft GRBs in the BATSE records in the 25-100 keV energy band. The softness of a burst spectrum could explain why it has been missed by the on-board procedure and by the previous searches for untriggered GRBs tuned to the 50-300 keV range. We have found a surprisingly small number (∼ 20 yr -1 with fluxes down to 0.1 photon cm -2 s -1 ) of soft GRBs where the count rate is dominated by the 25-50 keV energy channel. This fact, as well as the analysis of HETE-2 and common BeppoSAX/BATSE GRBs, indicates that the majority of GRBs with a low E peak have a relatively hard tail with a high-energy power-law photon index β > -3. An exponential cutoff in GRB spectra below 10-15 keV may be a distinguishing feature of non-GRB events.

Effect of suspension systems on the physiological and psychological responses to sub-maximal biking on simulated smoothand bumpy tracks

The aim of this study was to compare the physiological and psychological responses of cyclists riding on a hard tail bicycle and on a full suspension bicycle. Twenty males participated in two series of tests. A test rig held the front axle of the bicycle steady while the rear wheel rotated against a heavy roller with bumps (or no bumps) on its surface. In the first series of tests, eight participants (age 19 – 27 years, body mass 65 – 82 kg) were tested on both the full suspension and hard tail bicycles with and without bumps fitted to the roller. The second series of test repeated the bump tests with a further six participants (age 22 – 31 years, body mass 74 – 94 kg) and also involved an investigation of familiarization effects with the final six participants (age 21 – 30 years, body mass 64 – 80 kg). Heart rate, oxygen consumption ([Vdot]O2), rating of perceived exertion (RPE) and comfort were recorded during 10 min sub-maximal tests. Combined data for the bumps tests show that the full suspension bicycle was significantly different (P < 0.001) from the hard tail bicycle on all four measures. Oxygen consumption, heart rate and RPE were lower on average by 8.7 (s = 3.6) ml · kg−1 · min−1, 32.1 (s = 12.1) beats · min−1 and 2.6 (s = 2.0) units, respectively. Comfort scores were higher (better) on average by 1.9 (s = 0.8) units. For the no bumps tests, the only statistically significant difference (P = 0.008) was in [Vdot]O2, which was lower for the hard tail bicycle by 2.2 (s = 1.7) ml · kg−1 · min−1. The results indicate that the full suspension bicycle provides a physiological and psychological advantage over the hard tail bicycle during simulated sub-maximal exercise on bumps.

Broad-band spectral properties of accreting X-ray binary pulsars

Broad-band spectra of accreting X-ray binary pulsars can be fitted by a phenomenological model composed of a power law with a high energy rollover above 10 keV, plus a blackbody component with a temperature of few hundred eV. While, at least qualitatively, the hard tail can be explained in terms of (inverse) Compton scattering, the origin of the soft component cannot find a unique explanation. Recently, a qualitative picture able to explain the overall broad-band spectrum of luminous X-ray pulsars was carried out by taking into account the effect of bulk Comptonization in the accretion column. After a review of these recent theoretical developments, I will present a case study of how different modeling of the continuum affect broad features, in particular the cyclotron resonance features in Vela X-1.

Hard X-ray component in Sco X-1 spectrum: Synchrotron emission from a nano quasar

Sco X-1 is a low mass X-ray binary system and with the recent observations of a resolved radio jet, the source has been included in the list of galactic microquasars. The observed spectral data in the 2–20 keV energy band fits a thermal emission. Above 20 keV, a hard tail has been reported on occasions. During our continuing balloon borne X-ray survey in the 20–200 keV region using high sensitivity Large Area Scintillation counter Experiment, Sco X-1 was observed on two different occasions. Even though the total X-ray luminosity of the source was different, the spectral nature of the source did not show any variation. The presence of hard X-ray flux is unmistakable. We present the spectral and temporal data in the hard X-ray band and discuss the results in terms of geometrical characteristics of X-ray source and its observed temporal properties. We note that the jet activity is similar to the microquasars, however, the absence of the large magnitude abrupt changes in X-ray light curve compared to GRS1915 + 105 suggest that the quasar-like behaviour is at a nano scale.

3–200 keV Spectral States and Variability of theINTEGRALBlack Hole Binary IGR J17464−3213

On March 2003, IBIS, the gamma-ray imager on board the INTEGRAL satellite, detected an outburst from a new source, IGR J17464-3213, that turned out to be a HEAO 1 transient, H1743-322. In this paper we report on the high-energy behavior of this black hole candidate (BHC) studied with the three main instruments on board INTEGRAL. The data, collected with unprecedented sensitivity in the hard X-ray range, show a quite hard Comptonized emission from 3 up to 150 keV during the rising part of the source outburst, with no thermal emission detectable. A few days later, a prominent soft-disk multicolor component appears, with the hard tail luminosity almost unchanged: ~5 × 10-9 ergs cm-2 s-1. Two months later, during a second monitoring campaign near the end of the outburst, the observed disk component was unchanged. Conversely, the Comptonized emission from the central hot part of the disk reduced by a factor of ~10. We present here its long-term behavior in different energy ranges and the combined JEM-X, SPI, and IBIS wideband spectral evolution of this source.

The transient hard X-ray tail of GX 17+2: New BeppoSAX results

We report on results of two BeppoSAX observations of the Z source GX 17+2. In both cases the source is in the horizontal branch of the colour-intensity diagram. The persistent continuum can be fit by two-component models consisting of a blackbody plus a Comptonization spectrum. With one of these models, two solutions for the blackbody temperature of both the observed and seed photons for Comptonization are equally accepted by the data. In the first observation, when the source is on the left part of the horizontal branch, we observe a hard tail extending up to 120 keV, while in the second observation, when the source moves towards right in the same branch, the tail is no longer detected. The hard (30 keV) X-ray emission can be modeled either by a simple power-law with photon index , or assuming Comptonization of ~1 keV soft photons off a hybrid thermal plus non-thermal electron plasma. The spectral index of the non-thermal injected electrons is . The observation of hard X-ray emission only in the left part of the horizontal branch could be indicative of the presence of a threshold in the accretion rate above which the hard tail disappears. An emission line at 6.7 keV with equivalent width eV is also found in both observations. We discuss these results and their physical implications.

Cluster mergers and non-thermal phenomena: a statistical magneto-turbulent model

There is now firm evidence that the intracluster medium (ICM) consists of a mixture of hot plasma, magnetic fields and relativistic particles. The most important evidence for non-thermal phenomena in galaxy clusters comes from the spectacular synchrotron radio emission diffused over Mpc scales observed in a growing number of massive clusters and, more recently, in the hard X-ray tails detected in a few cases in excess of the thermal bremsstrahlung spectrum. A promising possibility to explain giant radio haloes is given by the presence of relativistic electrons reaccelerated by some kind of turbulence generated in the cluster volume during merger events. With the aim of investigating the connection between thermal and non-thermal properties of the ICM, in this paper we develop a statistical magneto-turbulent model which describes in a self-consistent way the evolution of the thermal ICM and that of the non-thermal emission from clusters. Making use of the extended Press-Schechter formalism, we follow cluster mergers and estimate the injection rate of the fluid turbulence generated during these energetic events. We then calculate the evolution of the spectrum of the relativistic electrons in the ICM during the cluster life by taking into account both the electron acceleration due to the merger-driven turbulence and the relevant energy losses of the electrons. We end up with a synthetic population of galaxy clusters for which the evolution of the ICM and of the non-thermal spectrum emitted by the accelerated electrons is calculated. The generation of detectable non-thermal radio and hard X-ray emission in the simulated clusters is found to be possible during major merger events for reliable values of the model parameters. In addition the occurrence of radio haloes as a function of the mass of the parent clusters is calculated and compared with observations. In this case it is found that the model expectations are in good agreement with observations: radio haloes are found in about 30 per cent of the more massive clusters in our synthetic population (M ≥ 1.8 x 10 15 M ○. ) and in about 4 per cent of the intermediate massive clusters (9 x 10 14 < M < 1.8 × 10 15 M ○. ), while the radio halo phenomenon is found to be extremely rare in the case of the smaller clusters.

Evidence for a Neutron Star in the non-pulsating massive X-ray binary 4U2206+54

We present an analysis of archival RXTE and BeppoSAX data of the X-ray source 4U2206+54. For the first time, high energy data (≥30 keV) are analyzed for this source. The data are well described by comptonization models (CompTT and BMC) in which seed photons with temperatures between 1.1 keV and 1.5 keV are comptonized by a hot plasma at 50 keV thereby producing a hard tail which extends up to, at least, 100 keV. We offer a new method of identification of neutron star systems using a temperature – luminosity relation. If a given X-ray source is characterized by a low bolometric luminosity and a relatively high color blackbody temperature (>1 keV) it has necessarily to be a neutron star rather than a black hole. From these arguments it is shown that the area of the soft photon source must be small ( km) and that the accretion disk, if present, must be truncated very far from the compact object. Here we report on the possible existence of a cyclotron line around 30 keV. The presence of a neutron star in the system is strongly favored by the available data.

The Nature of the Compact Object in the Hard X-Ray Source 4U2206+54

We present an analysis of archival RXTE and BeppoSAX data of the X-ray source 4U2206+54. For the first time, high energy data (≥ 30 keV) is analyzed. The data is well described by comptonization models in which seed photons with temperatures between 1.1 keV arid 1.5 keV are comptonized by a hot plasma at 50 keV thereby producing a hard tail which extends up to 100 keV. From luminosity arguments it is shown that the area of the soft photons source must be small (r ≈ 1 km) and that the presence of an accretion disk in this system is unlikely. Here we report on the possible existence of a cyclotron line around 30 keV . The presence of a neutron star in the system is strongly favored by the available data.

A BeppoSAX study of the Galactic Z-source GX 340+0

Abstract We present the results of a BeppoSAX broad band (0.1–200 keV) observation of the Z-source GX 340+0. The 1.8–30 keV continuum is well described by a blackbody ( kT BB ∼0.5 keV) plus a Comptonized component with seed photons temperature ∼ 1 keV and electron temperature ∼ 3 keV. A hard tail dominates the spectrum above 30 keV. It can be fitted using a bremsstrahlung component or, equivalently, a powerlaw (with a low-energy cutoff). We detect also a Gaussian line at ∼6.8 keV and an absorption edge at ∼9.2 keV. A low-energy (∼1 keV) unresolved feature needs further investigations.

X‐Ray Spectral Variability in Cygnus X‐1

Spectral variability in different energy bands of X-rays from Cyg X-1 in different states is studied with RXTE observations and time domain approaches. In the hard tail of energy spectrum above $\sim 10$ keV, average peak aligned shots are softer than the average steady emission and the hardness ratio decreases when the flux increases during a shot for all states. In regard to a soft band lower $\sim 10$ keV, the hardness in the soft state varies in an opposite way: it peaks when the flux of the shot peaks. For the hard and transition states, the hardness ratio in respect to a soft band during a shot is in general lower than that of the steady component and a sharp rise is observed at about the shot peak. For the soft state, the correlation coefficient between the intensity and hardness ratio in the hard tail is negative and decreases monotonically as the timescale increases from 0.01 s to 50 s, which is opposite to that in regard to a soft band. For the hard and transition states, the correlation coefficients are in general negative and have a trend of decrease with increasing timescale.

Two spectral states of the transient X-ray burster SAX J1747.0–2853

The neutron star binary SAX J1747.0-2853, located in the Galactic Center region at about 0.5 deg from Sgr A* and at a distance of 9 kpc, has been observed in outburst four times (1998, 1999, 2000 and 2001) by BeppoSAX and RossiXTE. At the time of its discovery in 1998 the source was observed in a low/hard state, showing a hard tail with a high energy cutoff of 70 keV. About two years later the source reappeared about one order of magnitude brighter in the X-rays (0.5-10 keV) and with a significantly steeper spectrum. As was the case for the low state, the data could be fitted by an input model based on two continuum primary components: a) a soft thermal excess, which is ~4 times more luminous than the one found in hard state; b) a non-thermal component which is compatible with either a power-law or a comptonization spectrum. The soft component is equally well described by pure blackbody or multi-color disk emission, with significantly higher temperature than observed in low state (~1.3 vs. the ~0.5 keV assuming pure blackbody). For this model, the flux of the non-thermal component below ~10 keV is a significant fraction of the total X-ray flux, i.e. greater than ~50% in the 2-10 keV band.

Is there a hard tail in the Coma Cluster X-ray spectrum?

We report results from a re-analysis of the BeppoSAX observation of Coma and from the analysis of a second, yet unpublished observation of the same object. From our re-analysis of the first observation we find that the statistical evidence for a hard tail is about 2 sigma. From the analysis of the second observation which, thanks to the lower background and the longer exposure time, is characterized by a larger signal to noise we find no evidence for a hard tail. From the upper limit on the flux of the hard tail, using the standard Inverse Compton formulae, we derive a lower limit for the magnetic of about 0.2-0.4 microGauss consistent with Faraday rotation measurements.

Disappearance of Hard X‐Ray Emission in the LastBeppoSAXObservation of the Z Source GX 349+2

We report on the results from two BeppoSAX observations of the Z source GX 349+2 performed in 2001 February and covering the broad energy range 0.12-200 keV. The light curve obtained from these observations shows a large flaring activity, the count rate varying from ~130 to ~260 counts s-1, indicating that the source was in the flaring branch during these observations. The average spectrum is well described by a soft blackbody (kTBB ~ 0.5 keV) and a Comptonized component having a seed photon temperature of kT0 ~ 1 keV, an electron temperature of kTe ~ 2.7 keV, and optical depth of τ ~ 11. To well fit the energy spectrum, three Gaussian lines are needed at 1.2, 2.6, and 6.7 keV with corresponding equivalent widths of 13, 10, and 39 eV, probably associated to L-shell emission of Fe XXIV, Lyα S XVI, and Fe XXV, respectively. These lines may be produced at different distances from the neutron star, which increase when the count rate of the source increases. An absorption edge is also needed at 9 keV with an optical depth of ~3 × 10-2. From the color-color diagram (CD), we selected five zones from which we extracted the corresponding energy spectra. The temperatures of the blackbody and of the Comptonized component tend to increase when the intensity of the source increases. We discuss our results comparing them to those obtained from a previous BeppoSAX observation, performed in 2000 March, during which the source was in a similar position on its Z-track. In particular, we find that, although the source showed similar spectral states in the 2000 and the 2001 observations, a hard tail that was significantly detected in 2000 March is not observed in these recent observations.