A reversal of the X-ray spectral evolution of RX J0720.4-3125 : the blackbody temperature is decreasing

ATel #650; Jacco Vink (Univ. Utrecht), Frank Haberl (MPE), Cor de Vries (SRON), Silvia Zane (MSSL), Roberto Turolla (Univ. Padua), Mariano Mendez (SRON), and Frank Verbunt (Univ. Utrecht)
on 2 Nov 2005; 22:51 UT
Credential Certification: Jacco Vink (j.vink@astro.uu.nl)

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This telegram is prompted by the latest XMM-Newton observation of the isolated, radio quiet, neutron star RX J0720.4-3125, which shows that the blackbody temperature started decreasing (see Table below). This means that the spectral evolution observed over the last 4 years has reversed.

Much like other sources in its class, RX J0720.4-3125 exhibits a blackbody-like spectrum in the soft X-rays. However, since the first Chandra and XMM-Newton observations in 2000, the blackbody temperature underwent rather significant changes, rising from kT~86 eV to kT~95 eV in October 2003 (de Vries et al. 2004, Vink et al. 2004). At the same time, the spectral shape became more complicated exhibiting a significant broad absorption feature (only weakly present in the first observation). Moreover, the strength of the gaussian absorption component (Haberl et al. 2004) increased with time and appears to correlate with the blackbody temperature. The evolution was gradual from 2000 to May 2003, but was more rapid from May 2003 to October 2003. Subsequent observations by XMM-Newton and Chandra did not detect any further evolution from October 2003 to May 2005. The latest XMM-Newton observation (September 23, 2005), however, indicates that the source spectral evolution has reversed. A first hint for a trend reversal was already provided by the previous observation on April 28 2005, but with a lower significance.

In order to avoid inter-instrumental calibration issues we only report on the data taken with the XMM-Newton PN instrument in Full Frame Imaging mode with the "Thin" filter.

TABLE

Obs. Date	kTBB	BB Area	Abs. line Eq. Width
	(eV)	(km2 @ 300pc)	(eV)
13-05-2000	86.29±0.10	23.5	5.8
06/08-11-2002	88.39±0.09	21.8	16.6
22-05-2004	94.96±0.15	16.9	56.8
28-04-2005	94.56±0.11	18.5	56.0
23-09-2005	93.38±0.11	19.2	53.2

Errors on the temperature indicate 90% confidence ranges. The absorption column, and absorption line energy and width were coupled for all observations, with best-fit values N_H = 1.06x10²⁰ cm^-2, E = 305 eV and sigma=71 eV, respectively.

The cause of the spectral evolution is still unclear. It is unlikely that it is caused by a time-varying accretion rate, because the large proper motion makes accretion from the ISM highly inefficient (Motch et al. 2003). This suggests that the spectral evolution is either connected to the internal evolution of the neutron star, e.g. in the magnetic field strength/topology, or to changes in the viewing geometry of the X-ray emitting region, e.g. if the neutron star precesses. In the latter case a cyclic behavior of the spectral evolution may be expected.

In the light of the possible implications for the physical mechanism responsible for the observed changes, the reversal in the temperature evolution is of particular relevance. Note that the temperature evolution since October is modest, but as the rapid change in temperature between May and October 2003 indicates, the spectral evolution may suddenly accelerate on a time scale < 6 month.