Spectral monitoring of RX J1856.5-3754 withXMM-Newton

Abstract

Using a large set of XMM-Newton observations, we searched for the long-term spectral and flux variability of the isolated neutron star RX J1856.5-3754 in the time interval from April 2002 to October 2011. This is the brightest and most extensively observed source of a small group of nearby, thermally emitting isolated neutron stars, of which at least one member (RX J0720.4-3125) has shown long-term variability. A detailed analysis of the data obtained with the EPIC-pn camera in the 0.15−1.2 keV energy range reveals only small variations in the temperature derived with a single blackbody fit (of about 1% around an average value of kT ∞ ∼ 61 eV). These variations appear to be correlated with the position of the source on the detector and can be ascribed to an instrumental effect, most likely a spatial dependence of the channel-to-energy relation. For the sampled instrumental coordinates, we quantify this effect as variations of ∼4% and ∼15 eV in the gain slope and offset, respectively. Selecting only a homogeneous subset of observations, with the source imaged at the same detector position, we find no evidence of either spectral or flux variations of RX J1856. The high-quality-statistics source spectrum from the selected observations is best fitted with the sum of two blackbody models, with temperatures of kT ∞ h = 62.4 +0.6 −0.4 eV and kT ∞ s = 38.9 +4.9 −2.9 eV, which can also account for the flux seen in the optical band. No significant narrow or broad spectral features are detected, with upper limits of ∼6 eV on their equivalent width.

Polarized thermal emission from X-ray dim isolated neutron stars: the case of RX J1856.5−3754

“…The value of the radius is also in agreement with the estimates derived by Sartore et al (2012) and Ho et al (2007), assuming a source distance of 120 pc (Walter et al 2010). This choice translates into a gravitational red-shift factor at the star surface 1 + z = 1.26.…”

“…However, taken face value, the previous expression for T dip yields vanishingly small values near the magnetic equator. The analysis of Sartore et al (2012) has shown that the X-ray spectrum of RX J1856 is best modeled in terms of two blackbody components with kT hand side should be a matrix, and B.12, where cos 2 θ k − sin 2 θ k should be cos 2 θ k + sin 2 θ k = 1. Figure 3 for the condensed surface.…”

Superfluid Fermi atomic gas as a quantum simulator for the study of the neutron-star equation of state in the low-density region

“…This is because, once it is fixed, together with the Tolman-Oppenheimer-Volkoff (TOV) equation [43,44], we can obtain the so-called M−R relation [45], linking the neutron-star mass M and its radius R, which also gives the upper limit of the neutron-star mass. However, the determination of the EoS by astronomical observations is difficult, because even the known nearest neutron star (RX J1856.5-3754) is about 400 lightyears away from Earth [46]. Although neutron skins [47,48] and hallows [49,50] in neutronrich nuclei give information about neutron matter, it is still not enough to construct the neutron-star EoS, including the many-body effects associated with a strong neutron-neutron interaction [33].…”

Discovery of a keV-X-ray excess in RX J1856.5–3754

“…Observations of this source were repeated with Xray satellites after ROSAT (e.g. Burwitz et al 2003, Beuermann et al 2006, Sartore et al 2012 not only for science purpose but also for calibration purpose. The latter is due to the fact that the X-ray spectrum of J1856 is ideal for instrumental calibration in soft X-ray range (e.g.…”