Additional Soft Component Research Articles

Deviations from the Flux–Recurrence Time Relationship in GS 1826−238: Potential Transient Spectral Changes

The low-mass X-ray binary GS 1826� 238 is presently unique for its consistently regular bursting behavior. In previousRossiX-RayTimingExplorer(RXTE)measurementsbetween1997Novemberand2002July,thissource exhibited (nearly) limit-cycle bursts with recurrence times that decreased proportionately as the persistent flux increased.HerewereportadditionalmeasurementsoftheburstrecurrencetimebyRXTE,Chandra,andXMM-Newton, as well as observations of optical bursts. On a few occasions we measured burst recurrence times which deviated significantly from the earlier fluxYrecurrence time relationship, and most of these bursts occurred earlier than would bepredictedbasedontheX-rayfluxlevel.Theepochswithearlyburstswerealsoaccompaniedbyunusualbroadband timing signatures, with the entire power spectrum shifting to higher frequencies. Concurrent XMM-Newton observations during one of these occasions, in 2003 April, indicate that an additional soft component may be present in the spectrum containing enough flux (� 30% of the total) to account for the burst recurrence time discrepancy. A selfconsistent interpretation for the increase in soft flux and accompanying timing changes during 2003 April is that the accretion disk extends down to smaller radial distances from the source than during the other observing epochs. The RXTE observations since 2003 April show that the spectral and timing properties have nearly returned to the previously established level. Subject headingg X-rays: binaries — X-rays: bursts — X-rays: individual (Ginga 1826� 238)

New flaring of an ultraluminous X-ray source in NGC 1365

We have studied a highly variable ultraluminous X-ray source (ULX) in the Fornax galaxy NGC 1365, with a series of 12 Chandra and XMM-Newton observations between 2002 and 2006. In 2006 April, the source peaked at a luminosity ≈3 x 10 40 erg s -1 in the 0.3-10 keV band (similar to the maximum luminosity found by ASCA in 1995), and declined on an e-folding time-scale ≈3 d. The X-ray spectrum is always dominated by a broad power-law-like component. When the source is seen at X-ray luminosities ≈10 40 erg s -1 , an additional soft thermal component (which we interpret as emission from the accretion disc) contributes ≈1/4 of the X-ray flux; when the luminosity is higher, ≈3 x 10 40 erg s -1 , the thermal component is not detected and must contribute <10 per cent of the flux. At the beginning of the decline, ionized absorption is detected around ∼0.5-2 keV; it is a possible signature of a massive outflow. The power law is always hard, with a photon index r ≈1.7 (and even flatter at times), as is generally the case with bright ULXs. We speculate that this source and perhaps most other bright ULXs are in a high/hard state: as the accretion rate increases well above the Eddington limit, more and more power is extracted from the inner region of the inflow through non-radiative channels, and is used to power a Comptonizing corona, jet or wind. The observed thermal component comes from the standard outer disc; the transition radius between outer standard disc and Comptonizing inner region moves further out and to lower disc temperatures as the accretion rate increases. This produces the observed appearance of a large, cool disc. Based on X-ray luminosity and spectral arguments, we suggest that this accreting black hole has a likely mass ∼50-150M ⊙ (even without accounting for possible beaming).

The XMM-SSC survey of hard-spectrum XMM–Newton sources – I. Optically bright sources

We present optical and X-ray data for a sample of serendipitous XMM-Newton sources that are selected to have 0.5-2 versus 2-4.5 keV X-ray hardness ratios which are harder than the X-ray background. The sources have 2-4.5 keV X-ray flux , and in this paper we examine a subsample of 42 optically bright (r sources; this subsample is 100 per cent spectroscopically identified. All but one of the optical counterparts are extragalactic, and we argue that the single exception, a Galactic M star, is probably a coincidental association rather than the correct identification of the X-ray source. The X-ray spectra of all the sources are consistent with heavily absorbed power laws (21.8 10(44) erg s(-1), and two of these sources have optical spectra which are dominated by narrow emission lines, that is, are type 2 QSOs. Only a small fraction of the sources (7/42) show broad optical emission lines, and all of these have N(H) 100 times the Galactic gas-to-dust ratio are rare in AGN absorbers (at most a few per cent of the population), and may be restricted to broad absorption line QSOs. Seven objects appear to have an additional soft X-ray component in addition to the heavily absorbed power law; all seven are narrow emission-line objects with z 4 per cent of broad-line AGN (BLAGN) have ionized absorbers that attenuate their soft X-ray flux by > 50 per cent. In at least one of the X-ray-absorbed BLAGN in our sample the X-ray spectrum requires an ionized absorber, consistent with this picture.

Constraints on the late X-ray radiation from the low-energy gamma-ray burst of December 3, 2003: INTEGRAL data

Comparison of the INTEGRAL upper limits on the hard X-ray flux before and after the low-energy GRB 031203 with the XMM measurements of the dust-scattered radiation at lower energies suggests that a significant fraction of the total burst energy could be released in the form of soft X-rays at an early afterglow stage with a characteristic duration of ~100-1000s. The overall time evolution of the GRB 031203 afterglow may have not differed qualitatively from the behavior of standard (i.e., more intense) bursts studied by the SWIFT observatory. The available data also admit the possibility that the dust-scattered radiation was associated with an additional soft component in the spectrum of the gamma-ray burst itself.

Broad-band spectra of Cygnus X-1 and correlations between spectral characteristics

We present the results of the spectral analysis of 42 simultaneous broad-band Ginga-OSSE and RXTE-OSSE observations of Cygnus X-1 carried out in 1991 and 1996–1999. The broad-band spectra from 3 to ∼1000 keV can be well described by the thermal Comptonization model with reflection from the cold disc, with an additional soft component visible below 10 keV. The relative contribution of this component to the total energy flux appears to be higher in the spectra with larger reflection amplitude and steeper photon index of the thermal Comptonized component. We consider a number of physically realistic models to describe the shape of the E≲ 10 keV excess. The additional soft component can result from thermal Comptonization by electrons with a low Compton parameter, or can be a part of a non-thermal, power-law-like emission extending above 1 MeV. We study correlations between parameters obtained from the spectral fits with different models. We confirm a general correlation between the photon index Γ and the amplitude of reflection R. We find that simple phenomenological models (such as power law plus Compton reflection) applied to the narrow band (3–20 keV) data overestimated the values of R and Γ, although the simple models did rank correctly the spectra according to R and Γ, as demonstrated in the original publications on this subject. The dynamic corona model provides a satisfactory description of the observed correlation, while the hot inner disc models have problems in reproducing it quantitatively. On the other hand, in the context of the dynamic corona model it is difficult to understand correlations with the timing characteristics, which seems natural in the hot disc scenario. We do not find significant correlation between the electron temperature and other spectral parameters, while the optical depth of the hot medium seems to decrease when the spectrum becomes softer. It is also shown that spectral parameters are well correlated with the timing characteristics of the source.

Three Additional Quiescent Low‐Mass X‐Ray Binary Candidates in 47 Tucanae

We identify through their X-ray spectra one certain (W37) and two probable (W17 and X4) quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars in a long Chandra X-ray exposure of the globular cluster 47 Tucanae, in addition to the two previously known qLMXBs. W37's spectrum is dominated by a blackbody-like component consistent with radiation from the hydrogen atmosphere of a 10 km neutron star. W37's light curve shows strong X-ray variability, which we attribute to variations in its absorbing column depth, and eclipses with a probable 3.087 hr period. For most of our exposures, W37's blackbody-like emission (assumed to be from the neutron star surface) is almost completely obscured, yet some soft X-rays (of uncertain origin) remain. Two additional candidates, W17 and X4, present X-ray spectra dominated by a harder component, fitted by a power law of photon index ~1.6-3. An additional soft component is required for both W17 and X4, which can be fitted with a 10 km hydrogen atmosphere neutron star model. X4 shows significant variability, which may arise from either its power-law or hydrogen atmosphere spectral component. Both W17 and X4 show rather low X-ray luminosities, LX(0.5-10 keV) ~ 5 × 1031 ergs s-1. All three candidate qLMXBs would be difficult to identify in other globular clusters, suggesting an additional reservoir of fainter qLMXBs in globular clusters that may be of similar numbers as the group of previously identified objects. The number of millisecond pulsars inferred to exist in 47 Tuc is less than 10 times larger than the number of qLMXBs in 47 Tuc, indicating that for typical inferred lifetimes of 10 and 1 Gyr, respectively, their birthrates are comparable.

ChandraandASCAObservations of the X‐Ray–brightest T Tauri Stars in the ρ Ophiuchi Cloud

We present the Chandra Advanced CCD Imaging Spectrometer and ASCA Gas Imaging Spectrometer results for a series of four long-term observations on DoAr 21, ROXs 21, and ROXs 31, the X-ray-brightest T Tauri stars in the ? Ophiuchi cloud. In the four observations with a net exposure of ~600 ks, we found six, three, and two flares from DoAr 21, ROXs 21, and ROXs 31, respectively; hence, the flare rate is fairly high. The spectra of DoAr 21 are well fitted with a single-temperature plasma model, while those of ROXs 21 and ROXs 31 need an additional soft plasma component. Since DoAr 21 is younger (~105 yr) than ROXs 21 and ROXs 31 (~106 yr), these results may indicate that the soft component gradually increases as T Tauri stars age. The abundances are generally subsolar and vary from element to element. Both high first ionization potential (FIP) and low-FIP elements show enhancement over the mean abundances. An unusual giant flare is detected from ROXs 31. The peak luminosity and temperature are ~1033 ergs s-1 and ~10 keV, respectively. The temperature reaches its peak value before the flux maximum and is nearly constant (4-5 keV) during the decay phase, indicating successive energy release during the flare. The abundances and absorption show dramatic variability from the quiescent to flare phase.

The BeppoSAX broad-band spectrum and variability of the Seyfert 1 NGC 3783

BeppoSAX observed the Seyfert 1 galaxy NGC 3783 for ~ 5 days from 1998 June 6 to 1998 June 10. The average flux during the observation was F(2-10 keV) ~ 6 X 10^{-11} erg cm^-2 s^-1. The long exposure provided a very high quality spectrum that exhibits all the typical features of Seyfert 1s. The high energy cut-off of the intrinsic continuum, at E_c=340+560/-107 keV, was detected for the first time in this object. During the observation the source showed modest flux variations (~ 20% in 2-10 keV) and spectral variability that can be completely explained by a change of the intrinsic slope \Delta\Gamma ~ 0.1. NGC 3783 is one of the few Seyfert 1s that exhibits a soft emission component in the BeppoSAX spectrum. This soft excess is well reproduced by a black body component with temperature kT ~ 0.2 keV. A warm absorber is also required to fit the soft X-ray spectrum. Both reprocessing features, the Compton reflection hump and the Fe emission line, were detected in our observation. In addition to a narrow iron line (as observed by Chandra) a broad component with sigma=0.72 +1.28/-0.27 keV and EW=115+/-76 eV, is required to fit the line. The observed Compton hump and the Fe line are well reproduced with the ionized (lg(xi)=2.73+0.04/-0.05) disc reflection model of Ross & Fabian. In this case the soft excess observed below 2 keV can be accounted for by the disc emissivity and no additional soft X-rays component is required to fit the spectrum.

Using the Bulge of M31 as a Template for the Integrated X‐Ray Emission from Low‐Mass X‐Ray Binaries

We have performed joint ASCA + ROSAT PSPC spectral fitting on the inner 5' of the bulge of M31. We find that single-component spectral models provide an inadequate fit to the spectrum, in contrast to previous studies by Einstein and Ginga. Although the 2-10 keV spectrum can be fitted adequately with a bremsstrahlung model with kT = 7.4 ± 0.3 keV, an additional soft component with kT = 0.38 ± 0.03 keV is required to fit the spectrum below 2 keV. This soft component comprises 38% ± 6% of the total emission in the 0.1-2.0 keV band, and possibly more depending on the absorption value used in the fit. Since previous spatial studies of the bulge of M31 indicate that less than 25% of the X-ray emission from the bulge in this band is from a diffuse gaseous component, this implies that stellar sources, namely, low-mass X-ray binaries (LMXBs), are responsible for some if not all of the soft component. The spectral properties of M31 are very similar to those of the X-ray-faint early-type galaxy NGC 4382. This supports the claim that the unexplained soft X-ray emission seen previously in these galaxies also emanates from LMXBs.

Multiwavelength Observations of GX 339−4 in 1996. I. Daily Light Curves and X‐Ray and Gamma‐Ray Spectroscopy

As part of our multiwavelength campaign of GX 339-4 observations in 1996 we present our radio, X-ray, and gamma-ray observations made in July, when the source was in a hard state (= soft X-ray low state). The radio observations were made at the time when there was a possible radio jet. We show that the radio spectrum was flat and significantly variable, and that the radio spectral shape and amplitude at this time were not anomalous for this source. Daily light curves from our pointed observation July 9-23 using OSSE, from BATSE, and from the ASM on RXTE also show that there was no significant change in the X- and gamma-ray flux or hardness during the time the possible radio jet-like feature was seen. The higher energy portion of our pointed RXTE observation made July 26 can be equally well fit using simple power law times exponential (PLE) and Sunyaev-Titarchuk (ST) functions. An additional soft component is required, as well as a broad emission feature centered on 6.4 keV. This may be an iron line that is broadened by orbital Doppler motions and/or scattering off a hot medium. Its equivalent width is 600 eV. Our simplistic continuum fitting does not require an extra reflection component. Both a PLE and a ST model also fit our OSSE spectrum on its own. Although the observations are not quite simultaneous, combining the RXTE and CGRO spectra we find that the PLE model easily fits the joint spectrum. However, the ST model drops off too rapidly with increasing energies to give an acceptable joint fit.

ROSAT Spectra of Quasars

X–ray observations of AGN with Einstein, EXOSAT and Ginga have shown, that the spectra of quasars in the energy range 2 to 10 keV can be approximately described by a single power law model with a photon index of 1.7 to 2.0. They also suggested that a soft X-ray excess component (below ≈ 1 keV) is a common feature in many quasars. In order to investigate whether a soft excess is characteristic for a certain class of objects we analysed the data of the pointed ROSAT PSPC observations of the six radio-loud quasars PG0007+106, PKS0135-247, QSO0537-286, QSO0923+392, PG1225+317, 3C273 and the radio-quiet quasar PG0804+761. In a first step the observed spectra were fitted with an absorbed single power law model. The hydrogen column density was fixed to its galactic value and the normalisation at 1 keV and the spectral index α were the free fit parameters. In order to decide whether a soft component is present in a source, the resulting power law index was compared with the hard X-ray power law index (2–10 keV) determined in the past with other instruments. A steep ROSAT PSPC spectrum indicates the presence of an additional soft X–ray component. In four cases (PKS0135-247, PG0804+761, QSO0923+392, 3C273) we find that the spectra in the PSPC band are considerably steeper than the spectra above 2 keV and therefore suggest the presence of a soft excess. In order to quantify the contribution of the soft excess these spectra were successively fitted with a model containing a hard power law component and an additional soft component described either by a power law, thermal bremsstrahlung or black body model. For the other three members of our sample (0007+106, 0537-286, 1225+317) the fitted power law index is not enhanced. This means that no soft component has been detected, but not necessarily that it does not exist. There are two effects which render more difficult the detection of a soft component in ROSAT spectra, the absorption of photons by interstellar material and the shift of the spectra towards lower energies due to the redshift. Both processes have first an effect on the soft part of the observed spectrum and it is therefore evident, that this leads to a decrease of the sensitivity for soft X–rays of the emitted spectrum. For the three quasars in our sample, where no soft excess has been detected, either the column density (0007+106) or the redshift (0537-286, 1225+317) is especially large and therefore an eventually present soft component could have remained undetected. In these cases we calculated upper limits for the strength of such a soft component (P. Bühler et al., to be published in A&amp;A.)

Structural and magnetic properties of laser-annealed magneto-optic thin films

In order to enhance the performance of rare-earth-transition-metal thin films, an attempt has been made to allow the as-sputtered film structure to relax by use of pulsed laser beam irradiation. This technique has several advantages over other forms of annealing, in particular the ability to tune the laser wavelength to a region which is absorbing in only one layer of the disk, in order to confine the effects of the anneal to that layer. By tuning a laser to an absorbing wavelength in Tb-Fe-Co media it was possible to produce local transient temperatures up to 3000 degrees C, modifying the magnetic behavior of the samples without significantly affecting their macroscopic layer structure. Anneals to a temperature of about 690 degrees C were found to introduce an additional soft magnetic component to the hysteresis loop, doubling the values of both the saturation and the remanent magnetization. On the other hand, anneals near 3000 degrees C destroyed the perpendicular anisotropy. The structure has been probed by neutron and X-ray reflectivity measurements, and the magnetic properties have been studied by vibrating sample magnetometry. >