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Strange Star? Neutron Star?. XMM-Newton finds the leader of the Magnificent Seven in a spin ESA-Space Science, 9 March 2007. XMM-NEWTON DISCOVERY OF 7 s PULSATIONS IN THE ISOLATED NEUTRON STAR RX J1856.5 -3754 ApJ., 657: L101-L104, 2007 March 10. VLA image of RX J1856 sky region.
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Strange Star? Neutron Star? XMM-Newton finds the leader of the Magnificent Seven in a spin ESA-Space Science, 9 March 2007 XMM-NEWTON DISCOVERY OF 7 s PULSATIONS IN THE ISOLATED NEUTRON STAR RX J1856.5 -3754 ApJ., 657: L101-L104, 2007 March 10 VLA image of RX J1856 sky region
Magnificent Seven? 豪勇七洨蛟龍?! Properties: 1. Nearby radio-quiet isolated NSs discovered in ROSAT data and characterized by thermal X-ray spectra. 2. Despite intensive searches their number remained constant since 2001. 3. Five of the stars exhibit pulsations in their X-ray flux with periods in the range [3-12]s and pulsed fractions between 4% and 18% . 4. XMM-Newton observations indicate more complex X-ray spectra with multiple absorption lines among these sources.
Neutron stars By J. L. Chiu; 2004 9/30
RX J1856.5-3754 Features: The soft X-ray source RX J1856.5-3754 belongs to a small group of seven nearby, INSs discovered with ROSAT and often dubbed XDINS. RX J1856.5-3754 received particular interest since it is the brightest XDIN, its X-ray flux doesn’t show any apparent contamination from non thermal magnetospheric emission, and a parallactic distance (117 12 pc) has been reported. It is an ideal target to derive information on the neutron star radius and thus constrain the equation of state of matter at super nuclear density through detailed modeling of its surface thermal emission. Even the X-ray spectra with the highest resolution and the best statistics do not show any line feature and can be fit by a simple Planckian with tempearture kT = 63 eV. The tight upper limit on the pulsed fraction (<1.3% at 2) obtained for periods between 0.02 and 1000s and another upper limit on the pulsed fraction of 2.1% (at 1) was reported for periods in the range 1-20 ms.
Quark Star? Neutron Star? The accurate parallactic distance (500 Lyrs) of RX J1856 which was supplied by HST has allowed astronomers to use the brightness to estimate its radius. The estimated radius came out to be smaller than 10 km and this was taken as possible evidence that RX J1856 was an exotic object, known as a quark star. Using data collected by XMM-Newton, A. Tiengo and S. Mereghetti found a repeating 7s pulsation over a 19 hr observations of the source performed in Oct. 2006. ˝The pulsations are a typical characteristic of a neutron star!˝ says Tiengo. The pulsations in RX J1856 have very low amplitude; this explains why they were not seen before. XMM-Newton view of RX J1856.5-3754 and its pulation
Periodicity Search A 70ks long observation of RX J1856 was carried out with XMM staring on 2006 Oct. 24 at 00:30 UT. After data reduction, the event files result in net exposure times of 47 and 68 ks for the pn and MOS. Due to the very soft spectrum of RX J1856, they started timing analysis using the data of pn camera and selecting energy range 0.15-1.2 keV. They used a circular extraction region centered at the source position and with radius 40˝ and then resulted in ~356600 counts, of which ~2700 can be ascribed to the background. They rebinned the counts at ~0.278s and omputed the Fourier power spectrum. A significant peak was found at the frequency 0.14174Hz. Taking into account the number of searched periods, the peak value 38.46 corresponds to a prob. 6x10-4. To better estimate the period, they used the epoch folding technique and fitted the peak in the 2 versus trial period distribution obtaining P = 7.05514± 0.00007s. The pulsed fraction, derived by fitting sinusoid to the background subtracted pn light curve is 1.6%±0.2%. In order to confirm the periodicity seen in the pn data of 2006 Oct., they analyzed the other XMM observations listed in the table with Z2-test by assuming s s-1.
Random prob. 3x10-5 7x10-5 5x10-4 2x10-3 2x10-2
Periodicity Search The values of chance probabilities indicate that the periodicity is detected in all the observations, although sometimes with small Z2-values, as expected for the case of a small pulsed fraction.
The verified all the folded light curves by means of Kolmogorov-Smirnov test, taking into account the unknown relative phase alignment, all the figures are fully compatible with the same profile.
Periodicity Search All the period measurements, spanning more than 4 yr, are consistent with average value of P =7.05515±0.00004s. With a linear fit to the period values, they can limit the period derivative to -1.2x10-12 s s-1 < dP/dt < 1.9 x 10-12 s s-1.(90% c.l.) The upper limit derived for period derivative can be used to infer a characteristic age >6x104 yr, a magnetic field B < 1.2x1014G, and a spin-down luminosity < 2x1032 ergs s-1. So their results indicate that, expect for the striking absence of deviations from a pure blackbody in X-ray spectrum, RX J1856.5-3754 shares most of the properties of the other members of the XDINS group. "We don’t rule out the quark star interpretation but the pulsations show that the object is also consistent with neutron star models", says Tiengo. More importantly, the discovery of pulsations opens the way for the determination of timing-dependent parameters that are crucial for more detailed modeling of the star’s surface emission properties.
