BeppoSAX Research Articles

Comparing theρandχclass spectra of the microquasar GRS 1915+105 observed withBeppoSAX

Context. Beppo SAX observed GRS 1915+105 during two variability classes at the same 2–10 keV flux level. The ρ class is characterized by quasi-periodic flares recurring on a time-scale of 1 to 2 min, namely heartbeat , while the χ class is characterized by no strong temporal variability. Aims. The aim of this work is to coherently analyze the source spectrum in these two classes and to gain insight into the source conditions that inset the heartbeats . Methods. A single χ spectrum was accumulated, while ρ data were split in runs where five phase-resolved spectra were selected. In addition to the multicolor disc black body, the fitting model includes a hybrid Comptonization plus a Compton reflection component. Results. Our results show that the emission in the ρ class is dominated by the multi-temperature disk, while in the χ class the Comptonised component is dominant. The disk temperature varies between a maximum of 1.95 ± 0.11 keV reached at the peak and a minimum of 0.95 +0.08 -0.20 keV in the χ class. In both classes we detect a significant contribution from a non-thermal electron population to the total Comptonized emission. A broadened iron emission line is detected in the χ spectrum. We interpret the line shape as being due to reflection from an accretion disk extremely close to the black-hole ( R in = 4.1 +1.5 -0.9 gravitational radii), with an equivalent width of 200 ± 20 eV. Concomitantly, upper limits of ~150 eV can be derived from the ρ spectra. Conclusions. In the framework of coupled disc-corona models, these results point out that the source emission is strongly affected by the fractions of accretion energy distributed between the disk, the corona, and possibly the wind, with no indication on the conditions that inset the heartbeats .

THEINTEGRALSOURCE IGR J16328–4726: A HIGH-MASS X-RAY BINARY FROM THEBEPPOSAXERA

We report on temporal and spectral analysis of the INTEGRAL fast transient candidate IGR J16328-4726 observed with Beppo SAX in 1998 and more recently with INTEGRAL. The MECS X-ray data show a frequent micro activity typical of the intermediate state of Supergiant Fast X-ray Transients and a weak flare with duration of ~4.6 ks. The X-ray emission in the 1.5-10 keV energy range is well described through the different time intervals by an absorbed power law model. Comparing spectra from the lower emission level up to the peak of the flare, we note that while the power-law photon index was constant (~2), the absorption column density varied by a factor of up to ~6-7, reaching the value of ~2x10^{23}cm^{-2} at the peak of the flare. Analysis of the long-term INTEGRAL/IBIS light curve confirms and refines the proposed ~10.07 day period, and the derived ephemeris places the Beppo SAX observations away from periastron. Using the near and the mid-IR available observations, we constructed a spectral infrared distribution for the counterpart of IGR J16328-4726, allowing us to identify its counterpart as a High Mass OB type star, and to classify this source as a firm HMXB. Following the standard clumpy wind theory, we estimated the mass and the radius of the clump responsible of the flare. The obtained values of M~4x10^{22}g and R~4.4x10^6 km are in agreement with expected values from theoretical predictions.

History of gamma-ray telescopes and astronomy

Gamma-ray astronomy is devoted to study nuclear and elementary particle astrophysics and astronomical objects under extreme conditions of gravitational and electromagnetic forces, and temperature. Because signals from gamma rays below 1 TeV cannot be recorded on ground, observations from space are required. The photoelectric effect is dominant <100 keV, Compton scattering between 100 keV and 10 MeV, and electron–positron pair production at energies above 10 MeV. The sun and some gamma ray burst sources are the strongest gamma ray sources in the sky. For other sources, directionality is obtained by shielding / masks at low energies, by using the directional properties of the Compton effect, or of pair production at high energies. The power of angular resolution is low (fractions of a degree, depending on energy), but the gamma sky is not crowded and sometimes identification of sources is possible by time variation. The gamma ray astronomy time line lists Explorer XI in 1961, and the first discovery of gamma rays from the galactic plane with its successor OSO-3 in 1968. The first solar flare gamma ray lines were seen with OSO-7 in 1972. In the 1980’s, the Solar Maximum Mission observed a multitude of solar gamma ray phenomena for 9 years. Quite unexpectedly, gamma ray bursts were detected by the Vela-satellites in 1967. It was 30 years later, that the extragalactic nature of the gamma ray burst phenomenon was finally established by the Beppo–Sax satellite. Better telescopes were becoming available, by using spark chambers to record pair production at photon energies >30 MeV, and later by Compton telescopes for the 1–10 MeV range. In 1972, SAS-2 began to observe the Milky Way in high energy gamma rays, but, unfortunately, for a very brief observation time only due to a failure of tape recorders. COS-B from 1975 until 1982 with its wire spark chamber, and energy measurement by a total absorption counter, produced the first sky map, recording galactic continuum emission, mainly from interactions of cosmic rays with interstellar matter, and point sources (pulsars and unidentified objects). An integrated attempt at observing the gamma ray sky was launched with the Compton Observatory in 1991 which stayed in orbit for 9 years. This large shuttle-launched satellite carried a wire spark chamber “Energetic Gamma Ray Experiment Telescope” EGRET for energies >30 MeV which included a large Cesium Iodide crystal spectrometer, a “Compton Telescope” COMPTEL for the energy range 1–30 MeV, the gamma ray “Burst and Transient Source Experiment” BATSE, and the “Oriented Scintillation-Spectrometer Experiment” OSSE. The results from the “Compton Observatory” were further enlarged by the SIGMA mission, launched in 1989 with the aim to closely observe the galactic center in gamma rays, and INTEGRAL, launched in 2002. From these missions and their results, the major features of gamma ray astronomy are:

Autocorrelation analysis of GRBM–Beppo-SAX burst data

An autocorrelation function (ACF) analysis was performed on 17 gamma-ray bursts with known redshift, using data from the GRBM on board Beppo-SAX. When corrected from the cosmic time dilation effect, the ACFs show a bimodal distribution at about half-maximum, in agreement with a previous study based on BATSE and Konus burst data. Although the results show more dispersion, the separation between the two classes is highly significant.

The high energy view of blazars

Abstract Beppo SAX contributed substantially to our understanding of the physics of blazars. This has been made possible mainly by its wide energy range and especially by its high energy detector. Together with the information coming from still higher energies we know at last the entire spectral energy distribution (SED) of blazars. Different subclasses of blazars have different SEDs, which seem to form a sequence, whose main parameter is the bolometric luminosity. Physically, the blazar sequence can be the result of different cooling rates by those electrons emitting most of the radiation we see. Blazars are among the most active sources we know of, and the coordinated variability at high energies gives very important clues about the physics of their jets: initially, they must transport energy in a dissipationless way. They radiate most of the entire bolometric luminosity we see at a preferred distance of a few hundreds of Schwarzschild radii. For powerful blazars, the emitted radiation must be a small fraction of the power transported by the jet, since the bulk of it is required to energize the radio lobes. These are the jets which likely have bulk relativistic speeds up to hundreds of kiloparsecs, as suggested by the bright X–ray knots detected by Chandra in many extended jets.

Close and distant reprocessing media in Mkn 509 studied withBeppoSAX

We present the broad band analysis of two Beppo SAX observations of the Seyfert 1 Mkn 509. In 2000 the source was in a typical flux state, $F_{2-10~\rm keV}=2.7\times 10^{-11}~ \;\rm erg~cm^{-2}\,s^{-1}$, while in 1998 it was found in a high flux state, $F_{2-10~\rm keV}=5.7\times 10^{-11}~ \;\rm erg~cm^{-2}\,s^{-1}$. A comparison between the two states shows a energy–dependent flux variation of about a factor of three and a factor of two in the LECS (0.15-3 keV) and MECS (1.5-10 keV), respectively, while in the PDS (13-200 keV) the difference is marginal. A soft excess, a narrow iron line and a Compton reflection hump above 10 keV are clearly apparent in the residuals after fitting the spectra with a simple power law. We tested two alternative models. In the first the iron line and the high energy bump are well reproduced by reprocessing in a cold and Compton thick material. The intensity of the iron line (also consistent with a Chandra measurement) as well as the normalization of the reflection hump are consistent with a constant in the two epochs: this, combined with the fact that the line is narrow as observed by Chandra , suggests a common origin from distant and optically thick matter. This model further requires a component to model the soft excess: the empirical choice of two black bodies accounts well for the excess in both observations; their combined strength was a factor of about three higher in the high than in the low flux state defined above. However, the relative contribution of the soft excess is higher in the low flux state. In the second model we attempted to reproduce all spectral features, except for the narrow cold line, with reflection from an ionized disc. This model is successful only in the high flux state, but it fails in the low flux state, when the soft excess is only partially accounted for. In either model, the slope of the power law is greater in the high than in the low flux state, ($\Delta\Gamma \sim0.2$), in agreement with a behaviour known to be shared by several objects of the same type.

X-ray and optical observations of BL Lac objects: 3C 66A (B0219+428) and ON 325 (B1215+303)

We report the results of new observations of the two BL Lac objects 3C 66A and ON 325 performed with Beppo SAX and of nearly simultaneous photometric measurements in the bandpasses from I to B . 3C 66A was observed in January 1999 and in July 2001: its X-ray flux was $F(2{-}10~{\rm keV}) = 2.0^{+0.3}_{-0.5}\times 10^{-12}$ erg cm -2 s -1 in January 1999 and $F(2{-}10~{\rm keV}) = (2.6 \pm 0.6)\times 10^{-12}$ erg cm -2 s -1 in July 2001. In both observations the X-ray spectrum can be fitted by a single power law with an energy index $1.2\pm 0.1$, steeper than that measured in the optical, equal to $0.86\pm 0.05$. The Beppo SAX observation of ON 325 was performed in December 1998 when its X-ray flux was $F(2{-}10~{\rm keV}) = 8^{+3}_{-4}\times10^{-13}$ erg cm -2 s -1 . The X-ray spectrum cannot be fitted by a single power law, but it is found to be well represented by a broken power law with a break energy at around 4 keV and energy indices equal to $1.48\pm0.12$ and $-0.57\pm0.63$. Both sources are characterised by a synchrotron-dominated emission at energies lower than a few keV, with a sharp flattening toward higher energies due to the inverse Compton component. We obtained satisfactory modelling of the overall spectral energy distributions, from the optical to low energy X-rays, with log-parabolic laws.

Beppo SAX X-Ray Observations of PKS 1934–63 and S5 1946+708

AbstractX-ray observations of PKS 1934–63 and S5 1946+708 have been made with Beppo SAX lasting 100 and 40 ksec, respectively. Both sources were detected, and in both there is evidence (at 90% confidence) of a strong iron Kα line which would indicate that the nuclei are surrounded by Compton thick material (˜1 g cm−2).For 1946+708 VLBI H I absorption data are available from the literature. With the condition that free–free absorption should be modest for the source to be observable a minimum radius of around 20 pc is derived for the absorbing torus. The torus is predominantly molecular with a density of 108 cm−3. The corresponding pressure is rather high. Observations with XMM have been requested to verify the large equivalent width of Kα with a better confidence level.In 1934–63 no important H I absorption was previously detected and no nucleus has been seen in radio data at 8 GHz. If this is due to free–free absorption a molecular torus with a radius less than 5 pc could be responsible. The density would be 108 cm−3 or more.The relatively low S/N ratio of our observations does not allow a fully convincing conclusion to be reached. But our discussion shows that the combination of X-ray and high resolution 21-cm absorption data can provide important information on the physical parameters in the absorbing ‘torus’.Paper to appear in full in Astronomy and Astrophysics.

SC 1327–312 &amp; SC 1329–313: Two galaxy groups in-between a major merging event observed with Beppo–SAX

We present the results of two Beppo-SAX observations of the poor clusters SC1327-312 and SC1329-313. These objects are located in a huge structure, formed by three ACO clusters, which is probably the remnant of one of the largest and best studied major mergings known. Given the fact that these poor clusters are in between two interacting ACO clusters, the aim of this work is to study the physics of the intracluster medium and to look for the possible presence of shocks. We derived the gas distribution profiles, the global (i.e. within 0.3 Mpc) temperatures and abundances and the temperature profiles and maps for SC1327-312 and SC1329-313. Also the presence of soft excess in LECS and hard excess in PDS have been checked. We do not find evidence of regions where the gas is shocked or significantly heated. The image of SC1327-312 seems rather symmetric, while the gas profile of SC1329-313 shows disturbed, comet-like shaped isophotes, with the tail pointing toward A3562 and a compression toward SC1327-312. The presence of multiphase gas in SC1329-313, as claimed by Hanami et al. (1999) on ASCA data, has been found only at the 2 sigma confidence level. The lack of heating supports the hypothesis that the merging is at a late stage, after the first core--core encounter, when the main shock front had the time to travel to the external regions of the main clusters.

Spatially resolved spectroscopy of Cassiopeia A with MECS on board ${\vec Beppo}$SAX

We have performed the first detailed spatially resolved spectroscopy of Cas A in the 1.6-10 keV energy range, using data taken with the MECS spectrometer on board the Beppo SAX Observatory. The well calibrated point spread function in the central region of the MECS allowed us to perform a spatial deconvolution of the data at full energy resolution. We eventually generated a set of spectra, covering a region of ∼ 3′ radius around the centre of Cas A. The results obtained by fitting these spectra using a non-equilibrium ionisation plasma model and a power law, improve our knowledge about chemical and physical parameters of the Cas A SuperNova Remnant: (i) a single thermal component is sufficient to fit all the spectra; (ii) kT is rather uniformly distributed with a minimum in the east and a maximum in the west, and no evidence is found for high kT expected from the interaction of the main shock with the ISM; (iii) from the distribution of the values of the ionisation parameter $n_{\rm e}t$ we infer the presence of two distinct components: the first $(a)$ with n e in the range 1-10 cm -3 , the second ( b ) with values ten times higher; if we associate component a to the CSM and component b to the ejecta, the mass ratio $M(a)/M(b)\leq1/10$ indicates a progenitor star that lost only a small fraction of the envelope during its pre-SN life. In this hypothesis the distribution of component b across the remnant suggests that the explosion was not spherically symmetric; (iv) the distribution of abundances indicates that we are detecting a CSM component with almost solar composition, and an ejecta component enriched in heavier elements. Abundances found for α -elements are consistent with the current view that Cas A was produced by the explosion of a massive star. A low $\element[][]{Fe}$ overabundance can be an indication that at the moment of the explosion the mass-cut was rather high, locking most of the produced $\element[][]{Ni}$ 56 into the stellar remnant.

BeppoSAX discovery of the X-ray afterglow of GRB 971227

The Beppo SAX Narrow Field Instruments observed the region of sky containing GRB 971227 14 hours after the initial burst. A faint source (1SAXJ1257.3+5924) is detected in a position consistent with the Beppo SAX Wide Field Camera position, and disappears during the second half of the observation. In contrast to some other GRBs observed by Beppo SAX no optical and radio afterglows were unambiguously observed.

Ballerina – pirouettes in search of gamma bursts

The cosmological origin of gamma ray bursts has now been established with reasonable certainty. Many more bursts will need to be studied to establish the typical distance scale, and to map out the large diversity in properties which have been indicated by the first handful of events. We are proposing Ballerina, a small satellite to provide accurate positions and new data on the gamma-ray bursts. We anticipate a detection rate an order of magnitude larger than obtained from Beppo–SAX.

CSC][ITAL]BeppoSAX[/ITAL][/CSC] Observations of SGR 1900+14 in Quiescence and during an Active Period

We present results from two Beppo SAX Narrow Field Instrument (NFI) observations of SGR-1900+14 made during a quiescent and an active period of the source. We detect pulsations in the 1997 May 12-13 observation (quiescence) at 5.157190(7) sec and the 1998 September 15-16 observation (active period) at 5.16026](12) sec. Using results reported by Hurley et al. (1999a), we establish a long-term spin down rate during quiescence of 5.82(2)-approx. times 10(exp -11) s/s which implies a dipole magnetic field of sim 5.5 approx. times 10(exp 14) G. We confirm deviations from a constant spin down rate during the active period. We also find spectral similarities between SGR-1900+14 in quiescence and anomalous X-ray pulsars (AXPs).