T. Oosterbroek

A BEPPOSAX Study of the Pulsating Transient X0115+63: The First X-Ray Spectrum with Four Cyclotron Harmonic Features

The recurrent hard pulsating X-ray transient X0115+63 was observed with BeppoSAX on 1999 March 19, when the source was at a 2-10 keV flux level of ~310 mcrab. We report on the high-energy spectrum of the source, concentrating on cyclotron resonant scattering features. The spectrum is strongly pulse phase dependent, and absorption features are detected at virtually all phases. In particular, four absorption-like features at 12.74, 24.16, 35.74, and 49.5 keV are observed in the descending edge of the main peak of the pulse profile. The ratios between the centroid energies of the lines with respect to the first are 1 : (1.9) : (2.8) : (3.9). These values are close to the harmonic relation expected from cyclotron resonant scattering in a strong magnetic field when relativistic effects are taken into account. The equivalent widths of the second, third, and fourth harmonics are found to be larger than that of the first harmonic, confirming the key role of two-photon processes in the spectral formation. These results provide the first evidence for four harmonically spaced lines in the spectrum of an accreting X-ray pulsar, yielding the clearest confirmation to date of their magnetic origin.

The X-Ray, optical, and infrared counterpart to GRB 980703

We report on X-ray, optical, and infrared follow-up observations of GRB 980703. We detect a previously unknown X-ray source in the GRB error box; assuming a power-law decline, we find for its decay index α 1.3 × 1017 Hz. For this epoch we obtain an extinction of AV = 1.50 ± 0.11. From the X-ray data we estimate the optical extinction to be AV = 20.2+12.3-7.3, inconsistent with the former value. Our optical spectra confirm the redshift of z = 0.966. We compare the afterglow of GRB 980703 with that of GRB 970508 and find that the fraction of the energy in the magnetic field, B < 6 × 10-5, is much lower in the case of GRB 980703, as a consequence of the high frequency of the cooling break.

Post-glitch variability in the anomalous X-ray pulsar 1RXS J170849.0-400910

Here we report on the first XMM-Newton observation of the anomalous X-ray pulsar 1RXSJ170849.0-400910. The source was observed in 2003 August, and was found at a flux level factor of about 2 lower than previous observations. Moreover, a significant spectral evolution appears to be present, the source exhibiting a much softer spectrum than in the past. Comparison of the present properties of 1RXSJ170849.0-400910 with those from archival data shows a clear correlation between the X-ray flux and the spectral hardness. In particular, the flux and the spectral hardness reached a maximum level close to the two glitches the source experienced in 1999 and 2001, and successively decreased. Although the excellent XMM-Newton spectral resolution should in principle allow us to detect the absorption line reported in a phase-resolved spectrum with BeppoSAX, and interpreted as a cyclotron feature, we found no absorption features, neither in the phase-averaged spectrum nor in the phase-resolved spectra. We discuss in detail both the possibilities that the feature in the BeppoSAX data may have resulted from a spurious detection and that it is real and intrinsically variable. We then discuss a possible explanation for the glitches and for the softening of the source emission that followed the flux decrease, in the framework of the magnetar model.

An XMM-Newton study of the X-ray binary MXB 1659-298 and the discovery of narrow X-ray absorption lines

We report the discovery of narrow X-ray absorption lines from the low-mass X-ray binary MXB 1659- 298 during an XMM-Newton observation in 2001 February. The 7.1 hr orbital cycle is clearly evident with narrow X-ray eclipses preceded by intense dipping activity. A sinusoid-like OM B-band modulation with a peak-to-peak modulation of0.5 mag and a minimum coincident with the X-ray eclipse is visible. EPIC and RGS spectra reveal the presence of narrow resonant absorption features identied with O viii 1s-2p, 1s-3p and 1s-4p, Ne x 1s-2p, Fe xxv 1s-2p, and Fe xxvi 1s-2p transitions, together with a broad Fe emission feature at 6:47 +0:18 0:14 keV. The equivalent widths of the Fe absorption features show no obvious dependence on orbital phase, even during dipping intervals. The equivalent widths of the other features are consistent with having the same values during persistent and dipping intervals. Previously, the only X-ray binaries known to exhibit narrow X-ray absorption lines were two superluminal jet sources and it had been suggested that these features are related to the jet formation mechanism. This now appears unlikely, and instead their presence may be related to the viewing angle of the system. The MXB 1659-298 0.6{12 keV continuum is modeled using absorbed cuto power-law and blackbody components. During dips the blackbody is more strongly absorbed than the power-law. The spectral shape of the 3.6% of 0:5 10 keV emission that remains during eclipses is consistent with that during non-dipping intervals.

Nonthermal Hard X-Ray Emission in Galaxy Clusters Observed with the BeppoSAX PDS

We study the X-ray emission in a sample of galaxy clusters using the BeppoSAX PDS instrument in the 20-80 keV energy band. We estimate the nonthermal hard X-ray (HXR) cluster emission by modeling the thermal contribution from the cluster gas and the nonthermal contamination from the unobscured active galactic nuclei (AGNs) in the clusters. We also evaluate the systematic uncertainties due to the background fluctuations. Assuming negligible contamination from the obscured AGNs, the resulting nonthermal component is detected at a 2 σ level in ~50% of the nonsignificantly AGN-contaminated clusters: A2142, A2199, A2256, A3376, Coma, Ophiuchus, and Virgo. The data are consistent with a scenario whereby relaxed clusters have no hard X-ray component of nonthermal origin, whereas merger clusters do, with a 20-80 keV luminosity of ~1043-1044 h ergs s-1. The co-added spectrum of the above clusters indicates a power-law spectrum for the HXR emission with a photon index of 2.8 in the 12-115 keV band, and we find indication that it has extended distribution. These indications argue against significant contamination from obscured AGNs, which have harder spectra and a centrally concentrated distribution. These results are supportive of the assumption of the merger shock acceleration of electrons in clusters, which has been proposed as a possible origin of the nonthermal hard X-ray emission models. Assuming that the cosmic microwave background photons experience inverse Compton scattering from the merger-accelerated relativistic electrons and thus produce the observed HXR, the measured hard X-ray slope corresponds to a differential momentum spectra of the relativistic electrons with a slope of μ = 3.8-5.0. In presence of cluster magnetic fields this relativistic electron population produces synchrotron emission with a spectral index of 1.4-2.1, consistent with radio halo observations of merger clusters. Thus both hard X-ray and radio observations of merger clusters are consistent with the inverse Compton model. The observed slope of the HXR emission is also consistent with that predicted by the nonthermal bremsstrahlung, which thus cannot be ruled out by the fit to the current data, even though this model requires an extreme, untenable cluster energetics. Assuming a centrally concentrated distribution of HXR emission, the data require a harder slope for the HXR spectrum, which is consistent with secondary electron models, but this model yields a worse fit to the PDS data and thus seems to be disfavored over the primary electron inverse Compton model.We study the X-ray emission in a sample of galaxy clusters using the BeppoSAX PDS instrument in the 20 -- 80 keV energy band. The non-thermal hard X-ray cluster emission (HXR) is detected at a 2 sigma level in 50% of the non-significantly AGN-contaminated clusters: A2142, A2199, A2256, A3376, Coma, Ophiuchus and Virgo. The data are consistent with a scenario whereby relaxed clusters have no hard X-ray component of non-thermal origin, whereas merger clusters do, with a 20-80 keV luminosity of 10^(43-44) erg/s. The co-added spectrum of the above clusters indicates a power-law spectrum for the HXR with a photon index of 2.8+0.3-0.4 in the 12-115 keV band, and we find indication that it has extended distribution. These indications argue against significant contamination from obscured AGN, which have harder spectra and centrally concentrated distribution. These results are supportive of the assumption of the merger shock acceleration of electrons in clusters. Assuming that the Cosmic Microwave Background photons experience Inverse Compton scattering from the merger-accelerated relativistic electrons, and thus produce the observed HXR, the measured hard X-ray slope corresponds to a differential momentum spectra of the relativistic electrons with a slope of mu = 3.8-5.0. The observed slope of HXR is also consistent with that predicted by the non-thermal bremsstrahlung, which thus cannot be ruled by the fit to the current data, even though this model requires an extreme, untenable cluster energetics. Assuming centrally concentrated distribution of HXR, the data requires a harder slope for the HXR spectrum, which is consistent with secondary electron models, but this model yields a worse fit to the PDS data and thus seems to be disfavored over the primary electron Inverse Compton model.

BeppoSAX study of the broad-band properties of luminous globular cluster X{ray sources

We have performed a detailed study of the broadband spectra of the luminous (>10 36 erg s 1 ) globular cluster X{ray sources using BeppoSAX. With the exception of X 2127+119, located in NGC 7078, all the other spectra are well represented by a two component model consisting of a disk-blackbody and Comptonized emission. The measured low-energy absorptions are in good agreement with those predicted from optical measurements of the host globular clusters. This implies that there is little intrinsic X{ray absorption within the binaries themselves, and that the above spectral model provides a good representation of the low-energy continua. The sources can be divided into two groups. In the rst group, composed of 3 ultra-compact (orbital period <1 hr) sources, the disk-blackbody temperatures and inner-radii appear physically realistic and the Comptonization seed photons temperatures and radii of the emission areas are consistent with the disk temperatures and inner radii. For all the other sources, the disk-blackbody parameters appear not to be physically realistic and the Comptonization parameters are unrelated to those of the disk-blackbody emission. If this is a spectral signature of ultra-compact binaries, this implies that no other ultra-compact binaries are present among those studied here. It is unclear why this dierence between the two types of binaries should exist. One possibility may be related to the mass ratio, which is similar in the ultracompact systems and binaries containing black holes. In the latter systems the soft components are also well-t with disk-blackbody models, which appear to have physically realistic parameters.

Discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624-490 with XMM-Newton

We report the discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624 490 during an XMM-Newton observation in 2001 February. The features are identied with the K ab- sorption lines of Fe xxv and Fe xxvi and have energies of 6:72 0:03 keV and 7:00 0:02 keV and equivalent widths (EWs) of 7:5 +1:7 6:3 eV and 16:6 +1:9 5:9 eV, respectively. The EWs show no obvious dependence on orbital phase, except during a dip, and correspond to a column of>10 17:3 Fe atom cm 2 . In addition, faint absorption features tentatively identied with Ni xxvii K and Fe xxvi K may be present. A broad emission feature at 6:58 +0:07 0:04 keV with an EW of 78 +19 6 eV is also evident. This is probably the 6.4 keV feature reported by earlier missions since tting a single Gaussian to the entire Fe-K region gives an energy of 6:39 +0:03 0:04 keV. A deep ab- sorption feature is present during the dip with an energy consistent with Fe xxv K. This is the second dipping LMXRB source from which narrow Fe absorption features have been observed. Until recently the only X-ray bina- ries known to exhibit narrow X-ray absorption lines were two superluminal jet sources and it had been suggested that these features are related to the jet formation mechanism. It now appears likely that ionized absorption features may be common characteristics of accreting systems with accretion disks.

Pronounced long term flux variability of the Anomalous X-ray Pulsar 1E 1048.1-5937

We present XMM-Newton and Chandra observations of 1E 1048.1-5937, being the first to show evidence for a significant variation in the X-ray luminosity of this anomalous X-ray pulsar (AXP). While during the first XMM-Newton (2000 December) and Chandra (2001 July) observations the source had a flux consistent with that measured on previous occasions (~5 × 10-12 ergs cm-2 s-1), two more recent observations found it at a considerably higher flux level of 2 × 10-11 ergs cm-2 s-1 (2002 August; Chandra) and 10-11 ergs cm-2 s-1 (2003 June; XMM-Newton). All the spectra are fit by the sum of a blackbody with kT ~ 0.6 keV and a power law with photon index ~3. No significant changes were seen in the spectral parameters, while the pulsed fraction in the 0.6-10 keV energy range decreased from ~90% in 2000 to ~53% in 2003. The spectral invariance does not support the presence of two physically distinct components in the AXP emission. The sparse coverage of the data does not permit us to unambiguously relate the observed variations to the two bursts seen from this source in the fall of 2001.

Euclid: ESA's mission to map the geometry of the dark universe

Euclid is a space-borne survey mission developed and operated by ESA. It is designed to understand the origin of the Universes accelerating expansion. Euclid will use cosmological probes to investigate the nature of dark energy, dark matter and gravity by tracking their observational signatures on the geometry of the Universe and on the history of structure formation. The mission is optimised for the measurement of two independent cosmological probes: weak gravitational lensing and galaxy clustering. The payload consists of a 1.2 m Korsch telescope designed to provide a large field of view. The light is directed to two instruments provided by the Euclid Consortium: a visual imager (VIS) and a near-infrared spectrometer-photometer (NISP). Both instruments cover a large common field of view of 0.54 deg2, to be able to survey at least 15,000 deg2 for a nominal mission of 6 years. An overview of the mission will be presented: the scientific objectives, payload, satellite, and science operations. We report on the status of the Euclid mission with a foreseen launch in 2019.

Evidence of a Cyclotron Feature in the Spectrum of the Anomalous X-Ray Pulsar 1RXS J170849–400910

We report the results of a long observation of the anomalous X-ray pulsar 1RXS J170849-400910 obtained with the BeppoSAX satellite in 2001 August. The best-fit phase-averaged spectrum was an absorbed power-law plus blackbody model, with a photon index of Γ ~ 2.4 and a blackbody temperature of kTbb ~ 0.4 keV. We confirm the presence of significant spectral variations with the rotational phase of the pulsar. In the spectrum corresponding to the rising part of the pulse, we found an absorption-like feature at ~8.1 keV (a significance of 4 σ), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9 × 1011 and 1.6 × 1015 G in the case of electrons and protons, respectively. If confirmed, this would be the first detection of a cyclotron feature in the spectrum of an anomalous X-ray pulsar.