RX J0146.9+6121

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Also known as: LS I +61 235 Simbad

Monitoring data: RXTE/ASM


RA 01h 47m 00.21s Dec +61° 21' 23.7"
RA 26.750875 Dec +61.356583
l 129.54 b -0.800

Binary system

Porb= 330 d if the compact star lines up with the one-armed Be decretion disc enhancement Sarty et al.(2009)
Pspin=1412 s White et al. (1987) Mereghetti et al.(1993)

Astrophysical parameters of the optical companion

Reig et al.(1997)

Spectral type B1V
E(B-V) 0.93 ± 0.02
Teff 24000 ± 1500K
Radius 7 ± 1 RSun
Mass 11 ± 2 MSun
Distance 2.3 ± 0.5 kpc
log g 3.9 ± 0.2
vrot sin i 200 ± 30 km s-1

Main publications

  • Periodicities in the high-mass X-ray binary system RXJ0146.9+6121/LSI+61235 Sarty et al.(2009)
  • XMM-Newton observation of the Be/neutron star system RX J0146.9+6121: a soft X-ray excess in a low luminosity accreting pulsar La Palombara & Mereghetti(2006)
  • RossiXTE and BeppoSAX observations of the Be/neutron star system RX J0146.9+6121 Mereghetti et al.(2000)
  • Correlated V/R and infrared photometric variations in the Be/X-ray binary LS I +61{deg} 235/RX J0146.9+6121 Reig et6 al.(2000)
  • X-ray observations of the slowest known Be/X-ray pulsars RX J0146.9+6121 and X Persei Haberl et al.(1998)
  • RXTE observations of RXJ0146.9+6121: a new X-ray outburst of the Be/X-ray pulsar Haberl et al.(1998)
  • The Be/X-ray binary LS I +61 235/RX J0146.9+6121: physical parameters and V/R variability Reig et al.(1997)
  • A ROSAT observation of the X-ray pulsars X0142+614 and X0146+612 Hellier(1994)
  • On the nature of the 25-min periodicity from 4U 0142+614 : a nearby, slowly spinning neutron star/Be system? Mereghetti et al.(1993)

Available data

  • ROSAT: HRI 4 pointings (8.0, 13.7, 32.3, 38.6 ks)
  • RXTE: 19.9 ks (28 March 1996). 20 ks (21 March 1998). Caution: Anomalous X-ray pulsar 4U 0142+61 in the FOV (~24 arcmin)
  • XMM Newton: 41.9 ks (14 January 2004)


RX J0146.9+6121 is a BeXRB hosting a neutron star characterized by a rotational period of about 23 min, among the longest observed in X-ray pulsars.

This system is a member of the open cluster NGC 663.

The optical counterpart shows V/R variability with a quasi-period of 1240 ± 30 d Reig et al.(2000)


The 2-30 keV spectrum is well fit by an absorbed power law. The best power-law fit yields a photon index of 2.20±0.07 and photo-electric absorption of the spectrum by a column density of (8.8 ± 0.8) 1022 H cm-2. However, the addition of a high-energy cutoff with a cutoff energy of 3.8 keV and a folding energy of 11.3 keV improves the fit. In this case the power-law index is 1.4, more within the range typically observed from Be/X-ray pulsars.

Below 3 keV a significant excess at low energies over the main power-law spectral component is detected. The soft excess can be described by a black-body spectrum with a temperature of about 1 keV and an emitting region with a radius of ~140 m.


RXJ0146.9+6121 shows persistent low-level X-ray flux with little variations (< 10)

X-ray luminosity: ~(1-10) × 1034 erg s-1

Sporadically LX may increase a factor of ~ 5 over the maximum.

X-ray pulsations

A long-term spin-up trend is observed between August 1984 and January 2004. The spin period decrease in this period from 1455 s to 1396 s, implying a spin-up rate of dot(P) = 4.6 × 108 s s-1 (La Palombara & Mereghetti(2006))