The BeppoSAX view of the galactic high-mass X-ray binary 4U 0114+65

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, INAF, via Gobetti 101, 40129 Bologna, Italy (formerly IASF/CNR, Bologna) e-mail: masetti@bo.iasf.cnr.it
² Istituto Tecnologie e Studio sulla Radiazione Extraterrestre, CNR, via Gobetti 101, 40129 Bologna, Italy
³ Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, INAF, via La Malfa 153, 90146 Palermo, Italy (formerly IASF/CNR, Palermo)
⁴ Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
⁵ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany

Received: 17 June 2005
Accepted: 6 September 2005

Abstract

A pointed observation of the galactic high-mass X-ray binary 4U 0114+65 was carried out with BeppoSAX to compare the X-ray spectral and timing characteristics observed by this satellite over the broadest range of energies thus far (1.5–100 keV) with the information previously obtained with other spacecraft. The light curve of 4U 0114+65 shows a large flare at the beginning of the BeppoSAX pointing and no significant hardness evolution either during the flare or in the low state occurring after the flare itself. The modulation at ~2.7 h, attributed to the accreting neutron star (NS) spin periodicity, is not significantly detected in our data, although fluctuations with timescales of ~3 h can be seen in the 2–10 keV light curve. Shorter modulations down to timescales of minutes are also found and interpreted as due to accretion of matter onto the NS. The flaring and the low state spectra of 4U 0114+65 can be equally well fitted either with a power law modulated by a high-energy exponential cutoff or with a Comptonization model. During the low state the presence, although tentative, of a thermal component (with  keV) at low energies, possibly produced by an ionized plasma cloud around the NS, cannot be excluded. Contrary to previous claims, a cyclotron resonant feature in absorption at ~22 keV was not detected in the BeppoSAX spectroscopic data, whereas evidence for a Fe emission line around 6.4 keV is found only during the low state emission. Using all of the above information, a scenario for the system in which the NS is embedded in, and accreting from, a low angular momentum gas cloud is envisaged.