On the spectral evolution of Cygnus X–2 along its color-color diagram

¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² Physics Department, University of Ferrara, Via Paradiso 12, 44100 Ferrara, Italy
³ Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone (Roma), Italy
⁴ Istituto TESRE, CNR, Via P. Gobetti 101, 40129 Bologna, Italy
⁵ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁶ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁷ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy

Corresponding author: T. Di Salvo, disalvo@astro.uva.nl

Received: 21 November 2001
Accepted: 12 February 2002

Abstract

We report on the results of a broad band (0.1–200 keV) spectral study of Cyg X–2 using two BeppoSAX observations taken in 1996 and 1997, respectively, for a total effective on-source time of ~100 ks. The color-color (CD) and hardness-intensity (HID) diagrams show that the source was in the horizontal branch (HB) and normal branch (NB) during the 1996 and 1997 observation, respectively. Five spectra were selected around different positions of the source in the CD/HID, two in the HB and three in the NB. These spectra are fit to a model consisting of a disk blackbody, a Comptonization component, and two Gaussian emission lines at ~1 keV and ~6.6 keV, respectively. The addition of a hard power-law tail with photon index ~2, contributing ~1.5% of the source luminosity, improves the fit of the spectra in the HB. We interpret the soft component as the emission from the inner accretion disk, with inner temperature, , varying between ~0.8 and ~1.7 keV and inner radius, R_in, varying between ~26 and ~11 km (assuming an inclination angle of the system of ). The Comptonization component is probably emitted by hot plasma (electron temperature varying between ~3 and ~20 keV, optical depth , seed-photon temperature  keV) surrounding the NS. The changes in the parameters of the blackbody component indicate that the inner rim of the disk approaches the NS surface when the source moves from the HB to the NB, i.e. as the (inferred) mass accretion rate increases. The parameters of the Comptonized component also change significantly when the source moves from the HB to the NB. We discuss possible scenarios which can explain these changes.