Luciano Burderi

The 0.1-100 keV Spectrum of Centaurus X-3: Pulse Phase Spectroscopy of the Cyclotron Line and Magnetic Field Structure

We report spectral and temporal analysis of the X-ray pulsar Centaurus X-3 out of eclipse observed by BeppoSAX. The broadband spectrum (0.12-100 keV) is well described by an absorbed power law modified by a high-energy rollover at ~14 keV (e-folding energy ~8 keV) plus an iron emission line at ~6.7 keV. A soft excess below 1 keV is also present. Interpreted as a blackbody (kT 0.1 keV), it corresponds to 58% of the total unabsorbed flux. This component seems to originate from reprocessing of the primary radiation by an opaque shell located at the magnetosphere. An absorption feature at ~30 keV is also present. Interpreted as a cyclotron line, after correction for gravitational redshift, this corresponds to a surface magnetic field of ~3.5 × 1012 G. Phase-resolved spectroscopy reveals a variation by about 8 keV of the cyclotron resonance energy along the pulse profile. In particular, the line energy decreases from ~36 to ~28 keV along the peak, starting from the ascent. The asymmetric variations of the cyclotron line energy can be explained by assuming an offset of the dipolar magnetic field with respect to the neutron star center. The spectral results are discussed in terms of emission from magnetic caps, where Comptonization of soft photons occurs. The soft photons could come from either magnetically resonant double Compton scattering or from illumination of the polar cap by the primary radiation reprocessed at the magnetospheric surface, a feedback mechanism similar to that proposed for the formation of black hole spectra.

A Search for Pulsars in Quiescent Soft X-Ray Transients. I.

We have carried out a deep search at 1.4 GHz for radio pulsed emission from six soft X-ray transient sources observed during their X-ray quiescent phase. The commonly accepted model for the formation of millisecond radio pulsars predicts the presence of a rapidly rotating, weakly magnetized neutron star in the core of these systems. The sudden drop in accretion rate associated with the end of an X-ray outburst causes the Alfven surface to move outside the light cylinder, allowing the pulsar emission process to operate. No pulsed signal was detected from the sources in our sample. We discuss several mechanisms that could hamper the detection and suggest that free-free absorption from material ejected from the system by the pulsar radiation pressure could explain our null result.

On the Soft Excess in the X-Ray Spectrum of Circinus X-1: Revisitation of the Distance to Circinus X-1

We report on a 300 ks BeppoSAX (0.12-200 keV) observation of Circinus X-1 (Cir X-1) at phases between 0.62 and 0.84 and on a 90 ks BeppoSAX observation of Cir X-1 at phases 0.11-0.16. Using the canonical model adopted until now to fit the energy spectrum of this source, large residuals appear below 1 keV. These are well fitted using an equivalent hydrogen column of 0.66 × 1022 cm-2, adding absorption edges of O VII, O VIII, and Ne IX in the spectra extracted from the observation at phases 0.62-0.84 and adding absorption edges of O VII, O VIII, Mg XI, and Mg XII and absorption lines of O VIII and Mg XII in the spectra extracted from the observation at phases 0.11-0.16. During the observation at phases 0.62-0.84 the electron density associated with the ionized matter is ~1013 cm-3, remaining quite constant going away from the compact object. During the observation at phases 0.11-0.16, the electron density profile varies along the distance going from ~6 × 1013 cm-3 at ~1011 cm to ~9 × 1010 cm-3 at ~1013 cm. The equivalent hydrogen column toward Cir X-1 is 3 times lower than the value obtained from previous models. This low value would imply that Cir X-1 is at a distance of 4.1 kpc.

A Firm Upper Limit to the Radius of the Neutron Star in SAX J1808.4?3658

We show that observations of X-ray pulsing from SAX J1808.4-3658 place a firm upper limit of 13.8m1/3 km on the radius of the neutron star, where m is its mass in solar units. The limit is independent of distance, or assumptions about the magnetospheric geometry, and could be significantly tightened by observations of the pulsations in the near future. We discuss the implications for the equation of state and the possible neutron star mass.

The BeppoSAX 0.1–18 keV spectrum of the bright atoll source GX 9+1: an indication of the source distance

We report the results of a long, 350 ks, BeppoSAX observation of the bright atoll source GX 9+1 in the 0.12-18 keV energy range. During this observation GX 9+1 showed a large count rate variability in its lightcurve. From its color-color diagram we selected six zones and extracted the source energy spectrum from each zone. We find that the model, composed of a blackbody plus a Comptonized component absorbed by an equivalent hydrogen column of ~1.4 ? 1022 cm-2, fits the spectra in the energy range 1-18 keV well; however, below 1 keV a soft excess is present. We find that the spectrum of GX 9+1, in the 0.12-18 keV energy range, is well fitted by the model above, if we use an equivalent hydrogen column of ~0.8 ? 1022 cm-2, together with several absorption edges from ionized matter (O VII, O VIII, Ne IX, Ar XVII, and Ca XX) and an absorption line from Si XIV. From the study of these features we deduce that the electron density of the plasma and the equivalent hydrogen column density associated with the ionized matter is ~1012 cm-3 and ~1023 cm-2, respectively, at a distance from the central object of r ~ 1011 cm, while at larger distances the equivalent hydrogen column density associated to the ionized matter decreases down to ~1022 cm-2. The value of the equivalent hydrogen column of ~0.8 ? 1022 cm-2 associated to the interstellar matter implies that GX 9+1 is at a distance of 5 kpc, and not 8.5-10 kpc as usually assumed. This result implies that the atoll source GX 9+1 does not emit at the Eddington limit, as supposed until now, but has a luminosity of 6 ? 1037 erg/s in the 0.1-18 keV energy range, typical of LMXBs belonging to the bright atoll class.

Population Synthesis of Millisecond and Submillisecond Pulsars

Known millisecond pulsars have periods longer than 1.558 ms. Recycled in binary systems, neutron stars can attain very short spin periods. In this Letter, we investigate the expected properties of the millisecond pulsar distribution by simulating synthetic populations under different assumptions for the neutron star equation of state and the decay of the magnetic field. We find evidence that a tail in the distribution of millisecond pulsars may exist at periods shorter than those observed.

An Upper Limit on the Power-Law Index of the Radio Spectrum of Geminga

Among the ~20 isolated neutron stars that emit in more than one energy band, only Geminga was radio quiet. Very recently, three groups at the Pushchino Radio Astronomy Observatory reported independently the detection of a weak pulsed radio emission from this source at ~100 MHz, bringing Geminga back into the standard group of radio-loud, isolated neutron stars. Here we report a series of observations of Geminga performed with the Arecibo radio telescope at various frequencies between 318 and 1400 MHz, performed from 1982 to 1984. No pulsed emission was detected. The high sensitivity of the Arecibo telescope allowed us to put a stringent upper limit to the spectral index in the radio band: α≤-3.6. If the 100 MHz pulsed emission is confirmed, Geminga will retain its peculiar nature of a radio pulsar with an extremely steep radio spectrum. This would severely constrain the models for the radio emission from isolated rotating neutron stars.

A newly discovered accreting pulsar in Terzan 5

The 11 Hz accreting pulsar, IGR J17480-2446, was recently di scovered in the globular cluster Terzan 5. The analisys of the Doppler shifts induced by the or bital motion reveals how the neutron star belongs to a∼ 21.3 hr binary system with a companion star of 0.4-1.5 M ⊙. The X-ray pulsar spins up while accreting at an average rate of 1.48(2) ×10−12 Hz s−1, in agreement with the accretion of disc matter angular momentum given the observe d luminosity. From the presence of pulsations at different accretion rates we constrain the ma gnetic field to lie within∼2×108 and ∼2×1010 G. From this estimate, the value of the spin period and of the o bs rved spin-up rate, we associate this source with the still poorly sampled populat ion of slow, mildly recycled, accreting pulsars. The pulse profile is generally modelled by using two harmonic components, and a timing analysis performed on the pulse phases shows how the second harmonic i s much less affected by timing noise than the first one. The temporal evolution of the pulse p hases computed over the second harmonic is also much more in agreement with the expected dep endence of the spin-up rate on the accretion luminosity. The assumption that this harmoni c component is a good tracer of the neutron star spin frequency opens the possibility of a dynam ic l measure of the accretion torque exerted on this neutron star during accretion.

Temporal Analysis of the Millisecond X‐ray Pulsar SAX J1808.4‐3658 During the 2000 Outburst

We report a temporal analysis of the millisecond X‐ray Pulsar SAX J1808.4‐3658 during the 2000 outburst, observed with RXTE.The observed maximum luminosity was approximately a factor of ten lower than in the other outbursts exhibited by the source, and this low flux level forced us to use a technique based on the χ2 obtained with an epoch folding search to discriminate between different possible orbital solutions, in order to correct the data for the orbital motion.In the subsequent searches for periodicities we clearly detected the 401Hz pulsation in at least two observations, but in the faintest the pulsed fraction varied from 20 % ca. to the absence of signs of coherent pulsation at all, while the measured flux remained at an almost constat level. This erratic behaviour is discussed in the context of the centrifugal inhibition of accretion.

BeppoSAX observation of the X-ray pulsar 4U 1538-52

We report preliminary results of the temporal and spectral analysis performed on the X-ray pulsar 4U1538-52 out of eclipse observed by BeppoSAX. We obtain a new estimate of the pulse period of the neutron star P=528.24±0.01 s (corrected for the orbital motion of the X-ray source): the source is still in the spin-up state, as since 1988. The broad band (0.12–100 keV) spectral analysys shows the presence of an absorption feature at ∼21 keV is present, interpreted as due to cyclotron resonant scattering. Another absorption feature at ∼51 keV seems also to be present (at 99% confidence level). If confirmed this might be interpreted as a cyclotron line, but its energy is not compatible with being double than the energy of the first line.