Evidence for precession of the isolated neutron star

RXJ0720.4−3125

Jeng-Lwen, ChiuInstitute of Physics, NTHU

2006 / 04 / 27

This XMM-Newton X-ray image shows the portion of the sky around the pulsar named 'RX J0720.4-3125', which is the central bright object appearing in red. This spinning star has been observed by XMM-Newton over a few years, and has shown an unexpected variation in the observed thermal emission. According to astronomers, this effect is not due to a real variation in temperature, but instead to a changing viewing geometry. The pulsar is most probably a slowly tumbling star, which exposes different areas of the surface over time.

• RX J0720.4-3125• Radio-quiet INS

• P ~ 8.39 sec• v ~ 100 mas/yr • dP ~ (6.98±0.02)×10-14 s s-1 (K05)

• Tbb < 100 eV• Binf ~ 1013-1014 G

Table 1XMM-Newton EPIC-pn observations ofRXJ0720.4−3125

4.5 years

• Over the past four and a half years the temperature of one enigmatic object, named RX J0720.4-3125, kept rising. However, very recent observations have shown that this trend reversed and the temperature is now decreasing.

Fig. 1. Pulse-phase averaged EPIC-pn spectra of RXJ0720.4−3125 from the seven observations with the same instrumental setup (FF read-out mode, thin filter). The same detector efficiency for all the spectra allows a direct comparison and demonstrates the long-term spectral changes.The softest spectrum (the uppermost at low energies) was obtained in May 2000, while the hardest (the lowest at low energies) is that from May 2004. After May 2004 the spectra increased monotonically at low energies.

Softest

Hardest

Table 2Spectral analysis of pulse-phase averaged X-ray spectra

Common parameters obtained from the best fit:

• NH = 1.01±0.03 ×1020 cm−2

• Line energy of 280±6 eV

with a width of σ = 90±5 eV

• Errors are given for a 90% confidence level• Observed flux is given in the energy band 0.1−2.4 keV• Statistical uncertainties on the flux are less then 2×10−14 erg cm−2 s−1

May 2000

May 2004

(soft S: 0.12-0.4 keV)

(hard H: 0.4-1.0 keV)

2580 ± 180 days300 pc (assumed distance)

• It is very unlikely that the global temperature of the neutron star changes that quickly.

• People are rather seeing different areas of the stellar surface at different times.

• This is also observed during the rotation period of the neutron star when the hot spots are moving in and out of our line of sight, and so their contribution to the total emission changes.

• A similar effect on a much longer time scale can be observed when the neutron star precesses (similarly to a spinning top).

In that case the rotation axis itself moves around a cone leading to a slow change of the viewing geometry over the years.

• This preliminary analysis strongly supports the idea that RXJ0720.4−3125 is a precessing, nearly aligned rotator, seen almost equator-on. First indications seem to favour precession angles > 10◦, larger than those found for radio-pulsars.

• During the first XMM-Newton observation of RX J0720.4-3125 in May 2000, the observed temperature was at minimum and the cooler, larger spot was predominantly visible.

• On the other hand, four years later (May 2004) the precession brought into view mostly the second, hotter and sm

aller spot, that made the observed temperature increase.

Precession scenario

A model with a numerical code adapted from that discussed by Zane & Turolla (2006)

• The model is capable of reproducing the observed variations of the blackbody temperature and emitting area, and also their phase anti-correlation.

• An independent sinusoidal fit to published and new pulse timing residuals from a coherent analysis covering 12 years yields a consistent period of 7.7±0.6 years (2830±220 days) supporting the precession model.

Conclusion

Unclear…

• They do not account for the detector response and interstellar absorption nor do they look for parameter fine-tuning.

• It is also not clear if there are additional variations in the absorption line energy and width. (They assume to be constant in their current analysis)

• Free precession can be caused by a slight deformation of the star from a perfect sphere, which may have its origin in the very strong magnetic field.

• The results of a detailed timing analysis which directly includes precession in the spin frequency model will be presented in a forthcoming paper.

(Including other three ROSAT data points to extend to >12 yr (1.5 precession cycles))

Supplement

Reference• Haberl, F., Turolla, R., de Vries, C. P., Zane, S., Vink, J., Méndez,

M., & Verbunt, F. 2006, astro-ph/0603724• http://www.esa.int/esaCP/SEMSIWNFGLE_index_0.html• http://www.tam.gov.tw/news/2006/200604/06042201.htm

• de Vries, C. P., Vink, J., Méndez, M., & Verbunt, F. 2004, A&A, 415, L31 (D04)• Haberl, F., Zavlin, V. E., Trűmper, J., & Burwitz, V. 2004, A&A, 419, 1077 (H04)• Kaplan, D. L. & van Kerkwijk, M. H. 2005a, ApJ, 628, L45 (K05)• Akgűn, T., Link, B., &Wasserman, I. 2006,MNRAS, 365, 653 (PSR B1828-11) • Ketsaris, N. A., Kuster,M., Postnov, K. A., et al. 2000, in Proc. Intl. Workshop: Hot Points in Astroph

ysics, JINR, Dubna, Russia, 192 or “1999, A&A 348, 917” (Her X1) • Schwope, A. D., Hambaryan, V., Haberl, F., &Motch, C. 2005, A&A, 441, 597 (RBS 1223)

• http://sci.esa.int/science-e/www/area/index.cfm?fareaid=23

~ Thank You ~

H04

Phase (i.e. angle) dependent spectrum of single neutron stars (D04)

• The broad absorption features? 1. proton-cyclotron absorption feature

2. cyclotron-resonance scattering of the spectrum from the surface of the neutron star• For both interpretations the spectrum is likely to be angle and energy dependent, in a

ccordance with the variation of the X-ray spectrum (as measured by hardness ratio) with pulse phase.

• Gradual, long-term variation 1. the intrinsic spectrum of the neutron star changes

a) not explain the change in pulse shape

b) predict the total flux to increase with the temperature

2. our view of the neutron star changes

valid for Ruderman’s model (no specific prediction in the Ruderman model for changes in the magnetospheric plasma on a year-long time scale)

• Precession arises when the form of the neutron star deviates from a perfect sphere and its rotation is not around a principal axis (as reviewed by e.g. Link 2003). The changes in pulse form and phase of the radio pulsar B1828−11 are successfully described with free precession, and have a time scale of years (Stairs et al. 2000)