Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008acasn..49...18c&link_type=abstract
Acta Astronomica Sinica, vol. 49, no. 1, p. 18-28
Astronomy and Astrophysics
Astronomy
2
X-Rays: Stars, Pulsars: Individual, Binaries: Close
Scientific paper
In recent years, a third class of binaries has emerged, with wide orbits but low eccentricities. HMXB (High Mass X-ray Binary) 4U1901+03 was confirmed as a member of this growing class in which the neutron stars may have received much smaller kicks as a result of their natal supernova explosions. After a silence of roughly 32 years, the HMXB 4U 1901+03 came back in business by undergoing an outburst in Feb. 2003. With the ASM (All Sky Monitor) onboard, RXTE (Rossi X-ray Timing Explorer) made 4U 1901+03 as its ToO (Target of Opportunity), and scheduled a five-month observation between Feb.10th and Jul.16th. With the observational data, we investigated the timing and spectral character of the X-ray binaries in detail. Here we report analysis of 4U1901+03, especially caring for the timing evolution and the photon energy dependence of the pulse profiles. The results show that both pulse profiles and pulsed fractions evolve with X-ray intensity and photon energy, in a way correlating with the stage of the outburst. In detail, we find the following results: 1) As X-ray intensity evolves, the general evolution tendency of the pulse fraction is that it first decreases and then increases, roughly in anti-correlation with that of the X-ray intensity. And this evolution tendency is divided into stages by five minima. Meanwhile, the pulse profile during the whole outburst process also evolves in stages in accordance with that of the pulse fraction. That is, at the point when the pulse fraction drops to a minimum, the evolution of the pulse profile transits from one stage into the next. 2) The typical pulse profile of the outburst is a double-peaked shape consisting of a fan beam and a pencil beam. As X-ray intensity decreases, the contribution of the pencil beam becomes stronger, and that of the fan beam becomes weaker. In the latter part, there is only the pencil beam lasting with its single sinusoidal-like pulse profile. 3) As photon energy increases, the pencil beam decreases significantly or even vanishes. 4) As photon energy increases, the pulse fraction first increases and then decreases, reaching the peak at energies between 10 and 20 keV. The pulse profiles are rather complicated, and hard to be explained solely by either a single geometric or a physical model. The pulse profiles are of viewing angle dependent, and the observed variability might be caused by the evolution of the accretion process during the outburst. We discuss the observed phenomena of pulse profiles of 4U 1901+03 within the standard pulsar radiation scenario.
Chen Wei
Qu Jin Lu
Zhang Fangfu
Zhang Guo-Bin
Zhang Sheng
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