Roberto Turolla

Isolated Neutron Stars: Accretors and Coolers

As many as 109 neutron stars populate the Galaxy, but only ≈103 are directly observed as pulsars or as accreting sources in X-ray binaries. In principle, also the accretion of the interstellar medium may make isolated neutron stars shine, and their weak luminosity could be detected in soft X-rays. Recent ROSAT observations have convincingly shown that neutron stars accreting from the interstellar medium are extremely rare, if observed at all, in contrast with earlier theoretical predictions. Until now two possible explanations for their elusiveness have been proposed: their velocity distribution may peak at ~200-400 km s–1, as inferred from pulsar statistics, and this would severely choke accretion; the magnetic field may decay on timescales ~108-109 yr, preventing a large fraction of neutron stars from entering the accretor stage. The search for accreting neutron stars has produced up to now a handful of promising candidates. While little doubt is left that these objects are indeed isolated neutron stars, the nature of their emission is still controversial. In particular, accreting objects can be confused with much younger, cooling neutron stars. However, a combination of observations and theoretical modeling may help in discriminating between the two classes.

X-RAY SPECTRA FROM NEUTRON STARS ACCRETING AT LOW RATES

The spectral properties of X–ray radiation produced in a static atmosphere around a neutron star accreting at very low rates are investigated. Previous results by Alme & Wilson (1973) are extended to the range 10 −7 � L=L Edd � 10 −3 to include the typical luminosities, L � 10 31 10 32 ergss −1 , expected from isolated neutron stars accreting the interstellar medium. The emergent spectra show an overall hardening with respect to the blackbody at the neutron star effective temperature in addition to a significant excess over the Wien tail. The relevance of present results in connection with the observability of low– luminosity X–ray sources is briefly discussed.

Proton cyclotron features in thermal spectra of ultramagnetized neutron stars

A great deal of interest has been recently raised in connection with the possibility that soft γ-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) contain magnetars, young neutron stars endowed with magnetic fields 1014 G. In this paper we calculate thermal spectra from ultramagnetized neutron stars for values of the luminosity and magnetic field believed to be relevant to SGRs and AXPs. Emergent spectra are found to be very close to a blackbody at the star effective temperature and exhibit a distinctive absorption feature at the proton cyclotron energy Ec,p 0.63(B/1014 G) keV. The proton cyclotron features (PCFs) are conspicuous (equivalent width of up to many hundreds eV) and relatively broad (ΔE/E ~ 0.05-0.2). The detection of the PCFs is well within the capabilities of present X-ray spectrometers, like the HETGS and METGS on board Chandra. Their observation might provide decisive evidence in favor of the existence of magnetars.

An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907

Spinning up an extragalactic neutron star Ultraluminous x-ray sources (ULXs) are strange objects in other galaxies that cannot be explained by conventional accretion onto stellar-mass objects. This has led to exotic interpretations, such as the long-sought intermediate-mass black holes. Israel et al. observed a ULX in the nearby galaxy NGC 5907 and found that it is instead a neutron star. The spinning neutron star is accreting material so fast that its spin period is quickly accelerating. The only way that it can consume enough material to explain these properties is if it has a strong multipolar magnetic field. Science, this issue p. 817 An ultraluminous x-ray source in NGC 5907 is a spinning neutron star with a complex magnetic field. Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any x-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC 5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43 seconds in 2003 to 1.13 seconds in 2014. It has an isotropic peak luminosity of ~1000 times the Eddington limit for a NS at 17.1 megaparsec. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity ≥ 1041 erg second−1) might harbor NSs.

AN XMM-NEWTON VIEW OF THE SOFT GAMMA REPEATER SGR 1806-20: LONG-TERM VARIABILITY IN THE PRE-GIANT FLARE EPOCH

The low-energy (<10 keV) X-ray emission of the soft gamma repeater SGR 1806-20 has been studied by means of four XMM-Newton observations carried out in the last two years, the latest performed in response to a strong sequence of hard X-ray bursts observed on 2004 October 5. The source was caught in different states of activity; over the 2003-2004 period, the 2-10 keV flux doubled with respect to the historical level observed previously. The long-term rise in luminosity was accompanied by a gradual hardening of the spectrum, with the power-law photon index decreasing from 2.2 to 1.5, and by a growth of the bursting activity. The pulse period measurements obtained in the four observations are consistent with an average spin-down rate of 5.5 × 10-10 s s-1, higher than the values observed in the previous years. The long-term behavior of SGR 1806-20 exhibits the correlation between spectral hardness and spin-down rate previously found only by comparing the properties of different sources (both SGRs and anomalous X-ray pulsars [AXPs]). The best-quality spectrum (obtained on 2004 September 6) cannot be fitted by a single power law but requires an additional blackbody component [kTBB = 0.79 keV, RBB = 1.9 (d/15 kpc)2 km], similar to the spectra observed in other SGRs and AXPs. No spectral lines were found in the persistent emission, with equivalent width upper limits in the range 30-110 eV. Marginal evidence for an absorption feature at 4.2 keV is present in the cumulative spectrum of 69 bursts detected in 2004 September-October.

A variable absorption feature in the X-ray spectrum of a magnetar

Soft-γ-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly rotating, isolated neutron stars that sporadically undergo episodes of long-term flux enhancement (outbursts) generally accompanied by the emission of short bursts of hard X-rays. This behaviour can be understood in the magnetar model, according to which these sources are mainly powered by their own magnetic energy. This is supported by the fact that the magnetic fields inferred from several observed properties of SGRs and AXPs are greater than—or at the high end of the range of—those of radio pulsars. In the peculiar case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing parameters, whereas a strong field has been proposed to reside in the stellar interior and in multipole components on the surface. Here we show that the X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which depend strongly on the star’s rotational phase. This line is interpreted as a proton cyclotron feature and its energy implies a magnetic field ranging from 2 × 1014 gauss to more than 1015 gauss.

Discovery of Cyclotron Resonance Features in the Soft Gamma Repeater SGR 1806-20

We report evidence of cyclotron resonance features from the Soft Gamma Repeater SGR 1806-20 in outburst, detected with the Rossi X-Ray Timing Explorer in the spectrum of a long, complex precursor that preceded a strong burst. The features consist of a narrow 5.0 keV absorption line with modulation near its second and third harmonics (at 11.2 and 17.5 keV, respectively). The line features are transient and are detected in the harder part of the precursor. The 5.0 keV feature is strong, with an equivalent width of similar to500 eV and a narrow width of less than 0.4 keV. Interpreting the features as electron-cyclotron lines in the context of accretion models leads to a large mass-to-radius ratio (M/R > 0.3 M. km(-1)) that is inconsistent with neutron stars or that requires a low (5-7) x 10(11) G magnetic field that is unlikely for SGRs. The line widths are also narrow compared with those of electron-cyclotron resonances observed so far in X-ray pulsars. In the magnetar picture, the features are plausibly explained as being ion-cyclotron resonances in an ultrastrong magnetic field that have recently been predicted from magnetar candidates. In this view, the 5.0 keV feature is consistent with a proton-cyclotron fundamental whose energy and width are close to model predictions. The line energy would correspond to a surface magnetic field of 1.0 x 10(15) G for SGR 1806-20, in good agreement with that inferred from the spin-down measure in the source.

Evidence for precession of the isolated neutron star RX J0720.4-3125

The XMM-Newton spectra of the isolated neutron star RXJ0720.4-3125 obtained over 4.5 years can be described by sinusoidal variations in the inferred blackbody temperature, the size of the emitting a e and the depth of the absorption line with a period of 7.1 +/- 0.5 years, which we suggest to be the precession period of the neutron star. Precession of a neutron star with two hot spots of different temperature and size, probably not located exactly in anti positions, may account for the variations in the X-ray spectra, changes in the pulsed fraction, shape of the light curve and the phase-lag between soft and hard energy bands observed from RXJ0720.4-3125. An independent sinusoidal fit to published and new pulse timing residuals from a coherent analysis covering similar to 12 years yields a consistent period of 7.7 +/- 0.6 years supporting the precession model.

A Strongly Magnetized Pulsar within the Grasp of the Milky Way's Supermassive Black Hole

We acknowledge support by grants AYA 2012-39303, SGR2009-811, iLINK 2011-0303, AYA 2010-21097-C03-02, Prometeo 2009/103, AYA2010-17631, P08-TIC-4075, INAF 2010 PRIN grant, Chandra Awards GO2-13076X, G03-14060X, GO3-14099X and G03-14121X, and an EU Marie Curie IEF (FP7-PEOPLE-2012-IEF-331095).

Post-glitch variability in the anomalous X-ray pulsar 1RXS J170849.0-400910

Here we report on the first XMM-Newton observation of the anomalous X-ray pulsar 1RXSJ170849.0-400910. The source was observed in 2003 August, and was found at a flux level factor of about 2 lower than previous observations. Moreover, a significant spectral evolution appears to be present, the source exhibiting a much softer spectrum than in the past. Comparison of the present properties of 1RXSJ170849.0-400910 with those from archival data shows a clear correlation between the X-ray flux and the spectral hardness. In particular, the flux and the spectral hardness reached a maximum level close to the two glitches the source experienced in 1999 and 2001, and successively decreased. Although the excellent XMM-Newton spectral resolution should in principle allow us to detect the absorption line reported in a phase-resolved spectrum with BeppoSAX, and interpreted as a cyclotron feature, we found no absorption features, neither in the phase-averaged spectrum nor in the phase-resolved spectra. We discuss in detail both the possibilities that the feature in the BeppoSAX data may have resulted from a spurious detection and that it is real and intrinsically variable. We then discuss a possible explanation for the glitches and for the softening of the source emission that followed the flux decrease, in the framework of the magnetar model.