- V635 Cas -


X0115+634 (4U0115+63) is the most active of Be/X-ray transients, showing very bright X-ray outbursts at least once every three years. Its optical counterpart is the B0.2Ve (my best estimate of the spectral class, based on good spectroscopy taken during a disc-less phase) star V635 Cas - a most amazing Be star showing very large (at least 1 mag) variations in brightness and very rapid changes in the shape of emission lines.

I will try to provide a resource page containing information on this source, in order to facilitate multi-frequency collaborations. If I ever find the time, this page is likely to be a shameless copy of Paul Ray's LS I +61 303 page.

The field of V635 Cas

Central part of a 20-s white light exposure taken on 22nd August 1999 with the 1.52-m G. D. Cassini telescope at the Loiano Observatory, near Bologna, in Italy.

Some basic data

The source has a minimum brightness V = 15.53, with corresponding (B-V) = 1.39 . For the spectral type, this implies approximately E(B-V) = 1.7 , i.e., assuming standard interstellar reddening (which is favoured by model fittings), an extinction of 5.3 magnitudes. Therefore the source is relatively bright in the infrared.

Moreover, the source is very variable and can brighten up by at least one magnitude in V. Colour changes are, however, not so spectacular, which is taken as indication of an optically thick envelope.

If anyone is interested in monitoring the source, they can use the local photometric sequence for calibration. This sequence was worked out by Iain Steele from the Astrophysics Research Institute at Liverpool John Moores University using the observations detailed in Negueruela et al. 1997, MNRAS 284, 859.

B band sequence 
V band sequence 
R band sequence 
I band sequence 

System parameters

In our paper Negueruela & Okazaki (A&A, accepted), we develop a model for the system using observational data and sensible assumptions for the parameters not yet known. The orbital parameters were derived by Rappaport et al. (1978, ApJ 224, L1).

Observational Data
Pulse Period Ps = 3.6 s
Orbital Period Po = 24.3 days
Eccentricity e = 0.34
Proyected semimajor axis axsini = 140.1 lt-s
Spectral type B0.2Ve
Assumed Values (based on data)
Be star mass MBe = 18 Msun
Be star radius RBe = 8 Rsun
Neutron star mass Mx = 1.4 Msun
Effective temperature Teff = 26000 K
Derived Values (from data + asumptions)
Inclination angle i = 43°
Semimajor axis a = 6.6E10 m = 12 RBe
Periastron distance rper = 8 RBe


Other resources:

  • The X-ray lightcurve from the RXTE/ASM.
  • The hard X-ray lightcurve from BATSE.
  • News:  


    Written and maintained by Ignacio N.. Last update on 3rd April 2000.