arXiv subjects

E. Massaro

Publications and source records attributed to E. Massaro.

At least 73 records · Page 4Linked to original sources

arXiv2003

BeppoSAX Observations of Centaurus A: the Hard Continuum and the Iron Line Feature

The radio galaxy Centaurus A was observed by the BeppoSAX satellite five times from 1997 to 2000. From July 6 1999 to August 17 1999, the source was also simultaneously pointed by COMPTEL on-board of the satellite CGRO. Centaurus A has a complex spectrum with multiple extended components and a strongly absorbed (N_H\sim 10^{23}$ cm${-2}) nucleus well fitted by a power law (Γ\sim 1.8) which bends at high energies. When the BeppoSAX and COMPTEL observations are combined together, an exponential cutoff with e-folding energy \sim 1000 keV gives an adequate description of the spectral steepening. A complex feature in emission at 6-7 keV is resolved into two Fe components, one narrow cold line and an ionized line centred at 6.8 keV. Significant variations have been observed in the iron feature, with the less prominent ionized line seemingly being the only one responsible for them: its variations do not appear to correlate with the strength of the continuum. The high energy cutoff and the Fe feature suggest the presence of an accretion flow in the Centaurus A nucleus. However the absence of a significant reflection, the narrowness of the cold line as well as the lack of correlation between the continuum and 6.8 keV line variations disfavour a standard cold/ionized thin disk (at least in the inner regions). A more plausible configuration might be a hot thick optically thin accretion flow surrounded by material with different opacities. Finally, we note that high energy break observed by BeppoSAX and COMPTEL could be also reasonably explained by Inverse Compton radiation from a jet. If this is the case, a structured jet with outer slow layers surrounding a beamed inner region is necessary to explain the strong Fe feature observed by BeppoSAX.

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arXiv2003

Optical and radio behaviour of the BL Lacertae object 0716+714

Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while radio light curves extend back to 1978. Many of these data are presented here for the first time. The long-term trend shown by the optical light curves seems to vary with a characteristic time scale of about 3.3 years, while a longer period of 5.5-6 years seems to characterize the radio long-term variations. In general, optical colour indices are only weakly correlated with brightness. The radio flux behaviour at different frequencies is similar, but the flux variation amplitude decreases with increasing wavelength. The radio spectral index varies with brightness (harder when brighter), but the radio fluxes seem to be the sum of two different-spectrum contributions: a steady base level and a harder-spectrum variable component. Once the base level is removed, the radio variations appear as essentially achromatic, similarly to the optical behaviour. Flux variations at the higher radio frequencies lead the lower-frequency ones with week-month time scales. The behaviour of the optical and radio light curves is quite different, the broad radio outbursts not corresponding in time to the faster optical ones and the cross-correlation analysis indicating only weak correlation with long time lags. However, minor radio flux enhancements simultaneous with the major optical flares can be recognized, which may imply that the mechanism producing the strong flux increases in the optical band also marginally affects the radio one.

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arXiv2003

Timing, glitches and braking index of PSR B0540-69

We report a pulse-time history of PSR B0540$-$69 based on the analysis of an extended Data set including ASCA, BeppoSAX and RXTE observations spanning a time interval of about 8 years. This interval includes also the epoch of the glitch episode reported by Zhang et al. (2001). Our analysis shows the presence of a relevant timing noise and does not give a clear evidence of the glitch occurrence. We performed an accurate evaluation of the main timing parameters, $ν$, $\dotν$ and $\ddotν$ and derived a mean braking index of $n=2.125\pm0.001$ quite different from the lower value found by Zhang et al. (2001), but in rather good agreement with other several values reported in the literature.

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arXiv2003

The BL Lac objects OQ 530 and S5 0716+714. Simultaneous observations in the X-rays, radio, optical and TeV bands

We present the results of the BeppoSAX observations of two BL Lacs, OQ 530 and S5 0716+714, as part of a ToO program for the simultaneous observation at radio, optical, X-ray and TeV energies. Both sources are detected in the LECS and MECS, with S5 0716+714 visible also in the PDS band, up to about 60 keV. The X-ray spectra of both sources are better fitted by a double power-law model, with a steep soft X-ray component flattening at harder energies, with breaks at 0.3 and 1.5 keV, respectively. The concave shape of the spectra in both objects is consistent with soft X-rays being produced by the synchrotron and harder X-rays by the inverse Compton processes. Also the X-ray variability properties confirm this scenario, in particular for S5 0716+714 our observation shows variations by about a factor 3 over one hour below 3 keV and no variability above. Their simultaneous broad band energy spectral distributions can be successfully interpreted within the frame of a homogeneous synchrotron and inverse Compton model, including a possible contribution from an external source of seed photons with the different spectral states of S5 0716+714 being reproduced by changing the injected power. The resulting parameters are fully consistent with the two sources being intermediate objects within the "sequence" scenario proposed for blazars.

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arXiv2002

BL Lacertae: the multiwavelength campaign of 2000

We present two BeppoSAX observations of BL Lacertae as part of a multiwavelength radio-to-TeV campaign. During the first observation we observe a faint Compton spectrum, while during the second, we detect a synchrotron spectrum with the highest [2-10] keV flux ever measured; above 10 keV an inverse Compton component begin to dominate. The synchrotron flux is very variable with time scales of 1 hr. We describe four different SED shifting the synchrotron peak both in frequency and flux intensity and we sketch a scenario in which a blob moves along a jet and can be located in or outside the BLR. This implies different radiative mechanism at work, SSC or external Compton, producing different high energy spectra.

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arXiv2002

BeppoSAX observations of the three Gamma-ray pulsars PSR B0656+14, PSR B1055-52 and PSR B1706-44

We report the results of the observations of the three gamma-ray pulsars PSR B0656+14, PSR B1055-52 and PSR B1706-44 performed with BeppoSAX. We detected a pulsed emission only for PSR B1055-52: in the range 0.1-6.5 keV the pulse profile is sinusoidal and the statistical significance is 4.5 sigma. The pulsed fraction was estimated 0.64+/-0.17. This pulsation was detected also at energies greater than 2.5 keV suggesting either a non-thermal origin or a quite high temperature region on the neutron star surface. Spectral analysis showed that only the X-ray spectrum of PSR B1706-44 can be fitted by a single power-law component, while that of PSR B1055-52 requires also a blackbody component (kT = 0.075 keV) and that of PSR B0656+14 two blackbody components (kT_1 = 0.059, kT_2 = 0.12 keV).

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arXiv2002

Optical and Radio monitoring of S5 1803+74

The optical (BVRI) and radio (8.4 GHz) light curves of S5 1803+784 on a time span of nearly 6 years are presented and discussed. The optical light curve showed an overall variation greater than 3 mag, and the largest changes occured in three strong flares. No periodicity was found in the light curve on time scales up to a year. The variability in the radio band is very different, and shows moderate oscillations around an average constant flux density rather than relevant flares, with a maximum amplitude of $\sim$30%, without a simultaneous correspondence between optical and radio luminosity. The optical spectral energy distribution was always well fitted by a power law. The spectral index shows small variations and there is indication of a positive correlation with the source luminosity. Possible explanations of the source behaviour are discussed in the framework of current models.

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arXiv2002

BL Lacertae: complex spectral variability and rapid synchrotron flare detected with BeppoSAX

We report on two BeppoSAX observations of BL Lac (2200+420) performed respectively in June and December 1999, as part of a ToO program to monitor blazars in high states of activity. During both runs the source has been detected up to 100 keV, but it showed quite different spectra: in June it was concave with a very hard component above 5-6 keV (alpha_1 ~ 1.6; alpha_2 ~ 0.15); in December it was well fitted by a single power law (alpha ~ 0.6). During the first BeppoSAX observation BL Lac showed an astonishing variability episode: the 0.3 - 2 keV flux doubled in ~ 20 minutes, while the flux above 4 keV was almost constant. This frequency-dependent event is one of the shortest ever recordered for BL Lac objects and places lower limits on the dimension and the magnetic field of the emitting region and on the energy of the synchrotron radiating electrons. A similar but less extreme behaviour is detected also in optical light curves, that display non-simultaneous, smaller fluctuations of ~ 20% in 20 min. We fit the spectral energy distributions with a homogeneous, one-zone model to constrain the emission region in a very simple but effective SSC + external Compton scenario, highlightning the importance of the location of the emitting region with respect to the Broad Line Region and the relative spectral shape dependence. We compare our data with historical radio to gamma-ray Spectral Energy Distributions.

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arXiv2001

The hard X-ray emission from the complex SNR MSH 15-52 observed by BeppoSAX

We present the results of a BeppoSAX observation of the Supernova Remnant MSH 15-52, associated with the pulsar PSR B1509-58, and discuss its main morphological and spectroscopic properties in the 1.6--200 keV energy range (MECS and PDS instruments). The two main structures of the remnant, the Southern Nebula, the plerion centered on the pulsar, and the Northern Nebula, are clearly visible in the MECS, with the former showing a much a harder spectrum. Furthermore, a diffuse extended emission surrounds the whole remnant up to ~ 17' from the center. Non-thermal flux is detected in the PDS up to 200 keV as well, and it appears that also in this energy range the emission is not concentrated in the central region around the pulsar. These data imply that the plerion extends up to a few tens of parsecs from the pulsar.

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arXiv2001

Optical and radio variability of the BL Lac object AO 0235+16: a possible 5-6 year periodicity

New optical and radio data on the BL Lacertae object AO 0235+16 have been collected in the last four years by a wide international collaboration, which confirm the intense activity of this source. The optical data also include the results of the Whole Earth Blazar Telescope (WEBT) first-light campaign organized in November 1997. The optical spectrum is observed to basically steepen when the source gets fainter. We have investigated the existence of typical variability time scales and of possible correlations between the optical and radio emissions by means of visual inspection, Discrete Correlation Function analysis, and Discrete Fourier Transform technique. The major radio outbursts are found to repeat quasi-regularly with a periodicity of about 5.7 years; this period is also in agreement with the occurrence of some of the major optical outbursts, but not all of them.

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arXiv2001

Wide-band X-ray variability of GRS 1915+105 observed with BeppoSAX

The Galactic Microquasar GRS 1915+105 was observed by the Narrow Field Instruments onboard BeppoSAX in two pointings during year 2000. We present the preliminary results of wide-band study of the observed variability, carried out also with the wavelet analysis.

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arXiv2001

Two-sided radio emission in ON231 (W Comae)

Recent radio images of the BL Lac object ON231 (W Com, 1219+285) show remarkable new features in the source structure compared to those previously published. The images were obtained from observations made with the European VLBI Network plus MERLIN at 1.6 GHz and 5 GHz after the exceptional optical outburst occurred in Spring 1998. The up-to-date B band historic light curve of ON231 is also presented together with the R band luminosity evolution in the period 1994--1999. We identify the source core in the radio images with the brightest component having the flattest spectrum. A consequence of this assumption is the existence of a two--sided emission in ON231 not detected in previous VLBI images. A further new feature is a large bend in the jet at about 10 mas from the core. The emission extends for about 20 mas after the bend, which might be due to strong interaction with the environment surrounding the nucleus. We suggest some possible interpretations to relate the changes in the source structure with the optical and radio flux density variation in the frame of the unification model.

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arXiv2000

Variability Modes of Blazars from Intensive Optical Monitoring

We report the main results of our six year long intensive optical monitoring on blazars ON 231 (W Com), BL Lac, and 3C 273. Intensive optical monitoring is an indispensable tool to obtain well sampled light curves and thus to understand the correlation with the variability in other bands and to discriminate among the proposed emission models. The curves based on our work show different variability modes: intermittent mode, quasi-regular mode and mixed mode. It is not clear if different variability modes can be present, at various times, in the life of an object suggesting that their occurrence can be related to the evolution of blazars. An optimized and nearly continuous optical monitoring is the only way to know the activity status of the sources and to retrieve useful information on their physical dynamics. Also, the rapid availability of information about the luminosity of a source is very important to trigger space based observations and to activate large multifrequency collaborations. We stress that a significant sample of blazars can be observed with small size telescopes (0.35-0.80 cm) equipped with CCD cameras. A world-wide network of several instruments in different countries, is very useful to increase the time coverage and to reduce the number of nights lost for bad meteorological conditions.

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arXiv2000

Two BeppoSAX observations of BL Lac

We present the results of two ToO BeppoSAX observations of BL Lac. During the first observation we detected both the synchrotron and the Compton components. Fast time variability was present, but only for the synchrotron component. During the second observation the spectrum was flatter and only the Compton component was present. Four different SED are presented and can be described by a blob moving along a jet and responsible for the SSC emission. The seed photons for the Compton scattering are the synchrotron photons themself plus possibly external photons coming from the broad line region.

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arXiv2000

Fine phase resolved spectroscopy of the X-ray emission of the Crab pulsar (PSR B0531+21) observed with BeppoSAX

The Crab Nebula and the Pulsar have been pointed several times by the BeppoSAX satellite. We analysed all these observations, summed together, to study the main spectral properties with a good phase resolution. A new accurate estimate of the hydrogen column density in the Crab direction N_H=3.23 x 10^(21) cm^(-2) is given as derived from the analysis of the off-pulse emission at low energies. We studied the changes of the spectral index with the pulse phase and showed that in the interpeak region it is systematically harder than in the main peaks of about 0.4. We observed also a significant spectral steepening of the pulsed emission and showed that this distribution can be represented by a unique law with a linear variable log slope and apply it to the spectrum of the first peak. A two component model is proposed to explain the phase evolution of the spectrum. One component has the same profile observed in the optical while the other presents a maximum at the phase 0.4 with essentially the spectrum of the interpeak region. Two possible interpretations for the origin of the latter component are discussed.

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arXiv2000

Pulse shape and spectrum of PSR J0218+4232 observed with BeppoSAX

We present the results of a BeppoSAX observation of PSR J0218+4232 which, for the first time, provides detailed information on the pulsar's temporal and spectral emission properties in the broad band 1-10 keV. We detected X-ray pulses with a pulsed fraction of 73 +/- 12 %. The pulse profile is characterized by two peaks phase separated by 0.47 +/-0.05. The pulsed spectrum is best described by a power-law of photon index 0.61 +/- 0.32 with an unabsorbed (2-10 keV) X-ray flux of 4.1E-13 erg/cm^2/s implying a luminosity of Lx = 1.3E32 Theta (d/5.7 kpc})^2 erg/s and an X-ray efficiency of 4.8E-4 Theta (d/5.7 kpc})^2 where Theta is the solid angle spanned by the emission beam.

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arXiv1999

The concave X-ray spectrum of the blazar ON 231: the signature of intermediate BL Lac objects

ON 231 was observed with BeppoSAX in May and June 1998, following an exceptional optical outburst which occured in April--May. We measured the X-ray spectrum from 0.1 up to 100 keV. In both occasions the spectrum had a concave shape, with a break detected at about 4 and 2.5 keV, respectively. We interpret the steeper component at energies below the break as due to synchrotron emission and the extremely flat component at energies above the break as due to inverse Compton emission. This is so far the best example in which both the synchrotron and the Inverse Compton component are detected simultaneously and with the same instruments in the X-ray spectrum of a blazar. We detect a decrease of the X-ray flux of about 30% below the break between the first and the second observation, and smaller variability amplitude between 4 and 10 keV. During the May observation we also detected a fast variability event with the flux below 4 keV increasing by about a factor of three in 5 hours. Above 4 keV no variability was detected. We discuss these results in the framework of synchrotron self-Compton models.

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arXiv1999

Synchrotron and Inverse Compton Variability in the BL Lacertae object S5 0716+714

We report intensity variations of different spectral components in the BL Lac object S5 0716+714 detected during coordinated BeppoSAX and optical observations in 1996 and 1998. The transition between synchrotron and inverse Compton emission has been clearly detected as sharp X-ray spectral breaks at around 2-3 keV on both occasions. Correlated optical and soft X-ray variability was found during the second BeppoSAX pointing when intensive optical monitoring could be arranged. The hard (Compton) component changed by a factor of 2 between the two observations, but remained stable within each exposure. During events of rapid variability S5 0716+714 showed spectral steepening with intensity, a behaviour rarely observed in BL Lacs. We interpret these findings as the probable consequence of a shift of the synchrotron peak emission from the IR/optical band to higher energies, causing the synchrotron tail to push into the soft X-ray band more and more as the source brightens.

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