4U 1538-52

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Also known as: V* QV Nor (SIMBAD)

Monitoring data: CGRO/BATSE RXTE/ASM Swift/BAT MAXI Fermi/GBM pulsed flux

Coordinates

RA 15h 42m 23.36s, Dec -52° 23' 9.6"
RA 235°.597 , Dec -52°.386
l 327°.420 , b +2°.164

Binary system

Distance estimated 6.4±1.0 kpc by [1].

Neutron star mass: Mx=1.06+0.41-0.34 Msol [2].

QV Nor mass Mopt=16.4+5.2-4.0 Msol [2].

QV Nor mass Mopt=19.8±3.3 Msol [1].

Orbit

Parameter Value Unit Reference
Porb 3.728366±0.000032 days [3]
Tpi/2 50,450.206±0.014 MJD [3]
a sin i 56.6±0.7 lt-sec [3]
e 0.174±0.015 [3]
omega 64±9 deg [3]
P-dot(orb)/P(orb) (-2.9±2.1)x10-6 yr-1 [3]
a0 47,221.488 MJD [3]
a1 3.728366±0.000032 days [3]
a2 (-5.5±4.0)x10-8 days [3]

Tpi/2 = a0 + a1N + a2N2 where N is the orbit number after a0

Optical Companion

Names: QV Nor, 2MASS J15422335-5223095
B0Iab supergiant ([1]).

Parameter Value Unit Reference
B 16.3 mag Simbad
R 13.500 mag Simbad
J 10.385 mag Simbad
H 9.910 mag Simbad
K 9.677 mag Simbad

Available data

To be filled in later.

Background

The accreting X-ray pulsar, 4U 1538-522, has been observed by every X-ray satellite since UHURU, and quite possibly was detected in earlier rocket [4], balloon [5], and satellite [6] observations. [7] and [8] discovered pulsations at 528 s from a source designated A1540-53, whose position was later updated to be coincident with 4U 1538-522 [9]. [8] used the 9 day OSO-8 observation to derive a 3.73 day orbital period from the Doppler modulation of the observed pulse period. [10] localized the source to 40" with SAS-3, and then [11] further refined the position error box to #12 with HEAO-1, thereby reducing the number of candidate optical counterparts noted in [12] to a single one -- #12, a B0 supergiant designated as QV Nor. Independently, [13] obtained spectra of the stars in the SAS-3 error circle and identified star 12 to be the counterpart of 4U 1538-522. Later photometric observations by [14] detected the 3.73 d orbital variability in the absorption and emission line radial velocity curves. A mass estimate from the X-ray and optical data yields a neutron star mass Mx=1.06+0.41-0.34 Msol and companion star mass Mopt=16.4+5.2-4.0 Msol [2]. The 4U 1538-522 system is similar to other high mass X-ray binary systems, such as Vela X-1 [15] and 4U 1907+09 [16],[17], with long pulse periods (hundreds of seconds), short orbital periods (<10 days), and flaring. The spin and orbital periods place 4U 1538-522 among the X-ray binary systems with a supergiant companion that underfills its Roche lobe.

The Present Situation

Timing

Until the launch of the BATSE instrument, the sparse satellite-by-satellite pulse period measurements had shown that 4U 1538-522 had been spinning down. Somewhere in the mid to late 1980's this trend reversed and continuous BATSE measurements over 8 years revealed a continuous spin-up trend [18]. This trend has continued at least through 2003, e.g. RXTE results [19]. The recent spin frequency measurements by the GBM on the Fermi satellite [[1]] shows that the source underwent another torque reversal in 2008 (approximately) and has been spinning down since. The current pulse period is approximately 526.1 s.

Spectrum

Initial descriptions of the spectrum included a power law modified at low energies by photoelectric absorption plus an iron line, and at high energies by a cutoff to an exponential shape, e.g. [20]. A cyclotron resonant scattering feature (CRSF or cyclotron line) at 20 keV was discovered by [21] from Ginga observations, which also showed a modulation in the cyclotron line strength and centroid with pulse phase. Broad band spectra were obtained in 1998 using BeppoSAX [22], and in 1997/2001 using RXTE [23]. The cyclotron line was detected at ~21 keV with a marginal indication of a second feature at 51+4-3 keV from BeppoSAX [22]. Phase-resolved spectroscopy by RXTE in 8 broad phase bins [23] revealed that the cyclotron line centroid exhibited maxima coincident with the two pulse peaks, which may be an indication of emission from both magnetic poles. In addition, the power law index was seen to be flatter on the falling portions of the pulse peaks, as has been seen in other pulsars. The pulsed light curve from RXTE showed significant energy-dependent structure, including a single, broad, flat-topped main pulse which narrowed above 10 keV, a second smaller pulse above 2 keV, and variation in the shape of the secondary pulse with increasing energy [23].

References

  1. ↑ 1.0 1.1 1.2 Reynolds, A.P., Bell, S.A. & Hilditch, R.W. (NASA ADS)
  2. ↑ 2.0 2.1 2.2 van kerkwijk, M.H., van Paradijs, J., Zuiderwijk, E.J., 1995, A&A, 303, 497
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Clark, G.W., 2000, ApJ, 542, L131
  4. ↑ Friedman, H., Byrum, E.T., Chubb, T.A., 1967, Science, 156, 374
  5. ↑ Lewin, Clark, & Smith, 1968, ApJ 152, 48 (NASA ADS)
  6. ↑ Hudson, Peterson, & Schwartz, Sol. Phys. 6, 205 (NASA ADS)
  7. ↑ Davidson, P.J.N., 1977, MNRAS, 179, 35p
  8. ↑ 8.0 8.1 Becker, R.H. et al., 1977, ApJ, 216, L11
  9. ↑ Davidson, P.J.N., Watson, M.G., Pye, J.P., 1977, MNRAS, 181, 73p
  10. ↑ Apparo,K.M.V. et al., 1978 Nature, 271, 225
  11. ↑ Schwartz, D.A. et al., 1980, Nature, 275, 517
  12. ↑ Cowley, A.P. et al., 1977, ApJ, 218, L3
  13. ↑ PArkes, G.E., Murdin, P.G., Mason, K.O., 1978, MNRAS, 184, 73p
  14. ↑ Crampton, D., Hutchings, J.B., Cowley, A.P., 1978, ApJ, 225, L63
  15. ↑ Kreykenbohm I., Wilms J., Kretschmar P., et al., 2008, A&A 492, 511 (NASA ADS)
  16. ↑ in 't Zand, J. J. M., Baykal, A., Strohmayer, T. E., 1998, ApJ, 496, 386 (NASA ADS)
  17. ↑ Fritz, S., Kreykenbohm, I., Wilms, J., et al., 2006, aap, 458, 885 (NASA ADS)
  18. ↑ Rubin, B.C., Finger, M.H., Scott, D.M., Wilson, R.B., 1997, Apj, 488, 413
  19. ↑ Baykal, A., Inam, S.C., Beklen, E., 2006, A&A, 453, 1037
  20. ↑ White, N., Swank, J., & Holt, S. S., 1983, ApJ, 270, 711 (NASA ADS)
  21. ↑ Clark, G.W., Woo, J.W., Nagase, F., Makishima, K., Sakao, T., 1990, ApJ, 353, 274 (NASA ADS)
  22. ↑ 22.0 22.1 Robba, N.R. et al., 2001, ApJ, 562, 950
  23. ↑ 23.0 23.1 23.2 Coburn, W., Heindl, W.A., Rothschild, R.E., et al., 2002, ApJ, 580, 394 (NASA ADS)
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