Radio-quiet Neutron Stars Research Articles

A Radio Frequency Study of the Accreting Millisecond X-ray Pulsar, IGR J16597–3704, in the Globular Cluster NGC 6256

Abstract We present Karl G. Jansky Very Large Array radio frequency observations of the new accreting millisecond X-ray pulsar (AMXP), IGR J16597−3704, located in the globular cluster NGC 6256. With these data, we detect a radio counterpart to IGR J16597−3704, and determine an improved source position. Pairing our radio observations with quasi-simultaneous Swift/XRT X-ray observations, we place IGR J16597−3704 on the radio–X-ray luminosity plane, where we find that IGR J16597−3704 is one of the more radio-quiet neutron star low-mass X-ray binaries known to date. We discuss the mechanisms that may govern radio luminosity (and in turn jet production and evolution) in AMXPs. Furthermore, we use our derived radio position to search for a counterpart in archival Hubble Space Telescope and Chandra X-ray Observatory data, and estimate an upper limit on the X-ray luminosity of IGR J16597−3704 during quiescence.

The origin of RX J1856.5−3754 and RX J0720.4−3125 - updated using new parallax measurements

RXJ1856 and RXJ0720 are the only young isolated radio-quiet neutron stars (NSs) for which trigonometric parallaxes were measured. Due to detection of their thermal emission in X-rays they are important to study NS cooling and to probe theoretical cooling models. Hence, a precise determination of their age is essential. Recently, new parallax measurements of RXJ1856 and RXJ0720 were obtained. Considering that NSs may originate from binary systems that got disrupted due to an asymmetric supernova, we attempt to identify runaway stars which may have been former companions to the NS progenitors. Such an identification would strongly support a particular birth scenario with time and place. We trace back each NS, runaway star and the centres of possible birth associations to find close encounters. The kinematic age is then given by the time since the encounter. We use Monte Carlo simulations to account for observational uncertainties. Using the most recent parallax measurement of 8.16+/-0.80 mas for RXJ1856, we find that it originated in the U Sco association 0.46+/-0.05 Myr ago. This is slightly larger than the value we reported earlier (0.3 Myr). Our result is strongly supported by its current radial velocity that we predict to be 6+19-20 km/s. This implies an inclination angle of 88+/-6 deg consistent with the bow shock. No suitable runaway star was found to be a potential former companion of RXJ1856. Making use of a recent parallax measurement for RXJ0720 of 3.6+/-1.6 mas, we find that this NS was possibly born in Tr 10 0.85+/-0.15 Myr ago. This is somewhat larger than the one obtained using the old parallax value (0.5 Myr). We suggest the B0 runaway supergiant HIP 43158 as a candidate for a former companion. Then, the current distance of RXJ0720 to the Sun should be 286+27-23 pc, in agreement with recent measurements. We then expect the radial velocity of RXJ0720 to be -76+34-17 km/s.

Identifying birth places of young isolated neutron stars

Young isolated radio-quiet neutron stars are still hot enough to be detectable at X-ray and optical wavelengths due to their thermal emission and can hence probe cooling curves. An identification of their birth sites can constrain their age. For that reason we try to identify the parent associations for four of the so-called Magnificent Seven neutron stars for which proper motion and distance estimates are available. We are tracing back in time each neutron star and possible birth association centre to find close encounters. The associated time of the encounter expresses the kinematic age of the neutron star which can be compared to its characteristic spin-down age. Owing to observational uncertainties in the input data, we use Monte-Carlo simulations and evaluate the outcome of our calculations statistically. RX J1856.5-3754 most probably originated from the Upper Scorpius association about 0.3 Myr ago. RX 0720.4-3125 was either born in the young local association TWA about 0.4 Myr ago or in Tr 10 0.5 Myr in the past. Also RX J1605.3+3249 and RBS 1223 seem to come from a nearby young association such as the Sco-Cen complex or the extended Corona-Australis association. For RBS 1223 also a birth in Sct OB2 is possible. We also give constraints on the observables as well as on the radial velocity of the neutron star. Given the birth association, its age and the flight time of the neutron star, we estimate the mass of the progenitor star. Some of the potential supernovae were located very nearby (<100pc) and thus should have contributed to the 10Be and 60Fe material found in the Earth's crust. In addition we reinvestigate the previously suggested neutron star/ runaway pair PSR B1929+10/ zeta Ophiuchi and conclude that it is very likely that both objects were ejected during the same supernova event.

A deep XMM-Newton serendipitous survey of a middle-latitude area

<i>Context. <i/>The radio-quiet neutron star 1E1207.4-5209 has been the target of several <i>XMM-Newton<i/> observations, with a total exposure of ~350 ks. The source is located at intermediate galactic latitude (), i.e. in a sky region with an extremely interesting mix of both galactic and extra-galactic X-ray sources.<i>Aims. <i/>The aim of our work is to investigate the properties of both the intermediate-latitude galactic and extra-galactic X-ray source populations in the 1E1207.4-5209 field.<i>Methods. <i/>We performed a coherent analysis of the whole <i>XMM-Newton<i/> observation data set to build a catalogue of serendipitous X-ray sources detected with high confidence and to derive information on the source flux, spectra, and time variability. In addition, we performed a complete multi-band (<i>UBVRI<i/>) optical coverage of the field with the <i>Wide Field Imager<i/> (<i>WFI<i/>) of the ESO/MPG 2.2 m telescope (La Silla) to search for candidate optical counterparts to the X-ray sources, down to a <i>V<i/>-band limiting magnitude of ~24.5.<i>Results. <i/>From the combined observation data set we detected a total of 144 serendipitous X-ray sources. We find evidence that the source distribution may be different from those computed either in the Galactic plane or at high galactic latitudes. Thanks to the refined X-ray positions and to the <i>WFI<i/> observations, we found candidate optical counterparts for most of the X-ray sources in our compilation. For most of the brightest ones we proposed a likely classification based on both the X-ray spectra and the optical colours.<i>Conclusions. <i/>Our results indicate that at intermediate galactic latitude the X-ray source population is dominated by the extra-galactic component, but with a significant contribution from the galactic component in the soft energy band, below 2 keV.

Spectral and temporal variations of the isolated neutron star RX J0720.4-3125: new XMM-Newton observations

In the past, the isolated, radio-quiet neutron star RX J0720.4-3125 showed variations in the spectral parameters (apparent radius, temperature of the emitting area and equivalent width of the absorption feature) seen in the X-ray spectra, not only during the spin period of 8.39s, but also over time scales of years. New X-ray observations of RX J0720.4-3125 with XMM Newton extend the coverage to about 7.5 years with the latest pointing performed in November 2007. Out of a total of fourteen available EPIC-pn datasets, eleven have been obtained with an identical instrumental setup (full frame read-out mode with thin filter), and are best suited for a comparative investigations of the spectral and timing properties of this enigmatic X-ray pulsar. We analysed the new XMM Newton observations together with archival data in order to follow the spectral and temporal evolution of RX J0720.4-3125 All XMM-Newton data were reduced with the standard XMM-SAS software package. A systematic and consistent data reduction of all these observations was emphasised in order to reduce systematic errors as far as possible. We investigate the phase residuals derived from data from different energy bands using different timing solutions for the spin period evolution and confirm the phase lag between hard and soft photons. The phase shift in the X-ray pulses between hard and soft photons varies with time and changes sign around MJD=52800 days, regardless of the chosen timing solution. The phase residuals[abridge]

Searching for substellar companions of young isolated neutron stars

<i>Context. <i/>Only two planetary systems orbiting old ms-pulsars have been discovered. Young radio pulsars and radio-quiet neutron stars cannot be analysed by the usually-applied radio-pulse-timing technique. However, finding substellar companions orbiting these neutron stars would be of significant importance: the companion may have had an exotic formation, its observation may also enable us to study neutron-star physics.<i>Aims. <i/>We investigate the closest young neutron stars to Earth to search for orbiting substellar companions.<i>Methods. <i/>Young, thus warm substellar companions are visible in the Near infrared, in which the neutron star itself is much fainter. Four young neutron stars are at sufficient speed to enable a common proper-motion search for substellar companions within few years. <i>Results. <i/>For Geminga, RX J0720.4-3125, RX J1856.6-3754, and PSR J1932+1059 we found no comoving companion of masses as low as 12, 15, 11, and 42 Jupiter masses, respectively, for assumed ages of 1, 1, 1, and 3.1 Myr, and distances of 250, 361, 167, and 361 pc, respectively. Near infrared limits are presented for these four and five additional neutron stars for which we have observations for only one epoch.<i>Conclusions. <i/>We conclude that young, isolated neutron stars rarely have brown-dwarf companions.

Using radioactivities to improve the search for nearby radio-quiet neutron stars

Neutron stars (NS) and black holes (BH) are sources of gravitational waves (GW) and the investigation of young isolated radio-quiet NS can in principle lead to constraints of the equation of state (EoS). The GW signal of merging NSs critically depends on the EoS. However, unlike radio pulsars young isolated radio-quiet neutron stars are hard to detect and only seven of them are known so far. Furthermore, for GW projects it is necessary to confine regions in the sky where and of which quantity sources of GW can be expected. We suggest strategies for the search for young isolated radio-quiet NSs. One of the strategies is to look for radioactivities which are formed during a supernova (SN) event and are detectable due to their decay. Radioactivities with half lives of ∼1Myr can indicate such an event while other remnants like nebulae only remain observable for a few kyrs. Here we give a brief overview of our strategies and discuss advantages and disadvantages.

Ionization and dissociation equilibrium in strongly magnetized helium atmosphere

Recent observations and theoretical investigations of neutron stars indicate that their atmospheres consist not of hydrogen or iron but possibly other elements such as helium. We calculate the ionization and dissociation equilibrium of helium in the conditions found in the atmospheres of magnetized neutron stars. For the first time, this investigation includes the internal degrees of freedom of the helium molecule. We found that at the temperatures and densities of neutron star atmospheres, the rotovibrational excitations of helium molecules are populated. Including these excitations increases the expected abundance of molecules by up to two orders of magnitude relative to calculations that ignore the internal states of the molecule; therefore, if the atmospheres of neutron stars indeed consist of helium, helium molecules and possibly polymers will make the bulk of the atmosphere and leave signatures on the observed spectra from neutron stars. We applied our calculation to nearby radio-quiet neutron stars (RQNS) with B dipole ∼ 10 13 -10 14 G. If helium comprises their atmospheres, our study indicates that isolated neutron stars with T BB ∼ 10 6 K such as RX J0720.4-3125 and RX J1605.3+3249 will have He + ions predominantly, while isolated neutron stars with lower temperature (T BB ∼ 5 x 10 5 K) such as RX J1856.5-3754 and RX J0420.0-5022 will have some fraction of helium molecules. We found that ionization, dissociation and electric excitation energies of helium molecules are larger than 100 eV at B 2≥ 10 13 G. On the other hand, rotovibrational excitation energies are in the range of 10-100 eV at B = 10 12 -10 14 G. If helium molecules are abundant, their spectroscopic signatures may be detected in the optical, ultraviolet and X-ray band.

POSSIBLE EVOLUTION OF DIM RADIO-QUIET NEUTRON STAR 1E 1207.4-5209 BASED ON A B-DECAY MODEL

Dim radio-quiet neutron star (DRQNS) 1E 1207.4-5209 is one of the most heavily examined isolated neutron stars. Wide absorption lines were observed in its spectrum obtained by both XMM-Newton and Chandra X-ray satellites. These absorption lines can be interpreted as a principal frequency centered at 0.7 keV and its harmonics at 1.4, 2.1 and possibly 2.8 keV. The principal line can be formed by resonant proton cyclotron scattering leading to a magnetic field which is two orders of magnitude larger than the perpendicular component of the surface dipole magnetic field (B) found from the rotation period (P) and the time rate of change in the rotation period [Formula: see text] of 1E 1207.4-5209. Besides, age of the supernova remnant (SNR) G296.5 + 10.0 which is physically connected to 1E 1207.4-5209 is two orders of magnitude smaller than the characteristic age [Formula: see text] of the neutron star. These huge differences between the magnetic field values and the ages can be explained based on a B-decay model. If the decay is assumed to be exponential, the characteristic decay time turns out to be several thousand years which is three orders of magnitude smaller than the characteristic decay time of radio pulsars represented in an earlier work. The lack of detection of radio emission from DRQNSs and the lack of point sources and pulsar wind nebulae in most of the observed SNRs can also be partly explained by such a very rapid exponential decay. The large difference between the characteristic decay times of DRQNSs and radio pulsars must be related to the differences in the magnetic fields, equation of states and masses of these isolated neutron stars.

The puzzling X-ray source in RCW103

1E 161348-5055 (1E) is a compact object lying at the center of the 2000 year old Supernova Remnant (SNR) RCW103. Its original identification as an isolated, radio-quiet neutron star has been questioned in recent years by the observation of a significant long-term variability, as well as by reports of a possible periodicity at ∼6 hours. Here we report conclusive evidence for a strong (nearly 50%) periodic modulation of 1E at 6.67±0.03 hours, discovered during a long (90 ks) XMM-Newton observation performed in August 2005, when the source was in a “low state”. The source spectrum varies along the 6.67 hr cycle. No fast pulsations are seen. 1E could be a very young binary system, possibly composed of a compact object and a low-mass star in an eccentric orbit. This would be the first example of a low-mass X-ray binary (LMXB) associated with a SNR, and thus the first LMXB for which we know the precise birth date, just 2000 years ago. Alternatively, if it is an isolated neutron star, the unprecedented combination of age, period and variability may only fit in a very unusual scenario, featuring a peculiar magnetar, dramatically slowed-down over 2000 years, possibly by a supernova debris disc.

Joint Discussion 2 On the present and future of pulsar astronomy

Neutron stars are formed in supernova explosions. They manifest themselves in many different ways, for example, as pulsars, anomalous X-ray pulsars (AXPs) and soft γ-ray repeaters (SGRs) and the so-called ‘radio-quiet neutron stars’. These objects are made visible by high-energy processes occurring on their surface or in the surrounding region. In most of these objects, ultra-strong magnetic fields are a crucial element in the radio, optical, X-ray and gamma-ray emission processes which dominate the observed spectrum.

A deep XMM-Newton serendipitous survey of a middle-latitude area

The radio-quiet neutron star 1E1207.4-5209 was the target of a 260 ks XMM-Newton observation, that yielded, as a by product, a harvest of about 200 serendipitous X-ray sources above a limiting flux of 2 10-15 erg cm-2 s-1 in the 0.3–8 keV energy range. In view of the intermediate latitude of our field (), it comes as no surprise that the log N–log S distribution of our serendipitous sources is different from those measured either in the Galactic Plane or at high galactic latitudes. Here we concentrate on analyzing of the brightest sources in our sample, which unveiled a previously unknown Seyfert-2 galaxy.

Spatially Resolved Spectroscopy of theChandraField Containing the Supernova Remnant G292.2−0.5

We present observations with the Chandra Advanced CCD Imaging Spectrometer (ACIS-S) of the western interior of the supernova remnant (SNR) G292.2-0.5 associated with the high magnetic field pulsar (PSR) J1119-6127. Chandra's excellent resolution revealed 18 point sources on the S3 chip and allowed the study of the diffuse emission from G292.2-0.5 independently from these point sources. The diffuse emission from G292.2-0.5 can be adequately fitted with nonequilibrium ionization models; however, these fits result in an unrealistically high temperature (kT > 3 keV), indicating the presence of a hard nonthermal component. By performing a spatially resolved spectroscopic study of the remnant, we identify the nonthermal emission as originating from the region closer to the pulsar. Its spectrum is well described with a power law with a photon index of Γ ~ 1-2. As a result of the flat spectrum and location close to the pulsar, we propose that this nonthermal emission arises from the injection of high-energy particles originating from PSR J1119-6127 and its pulsar wind nebula (PWN) into the SNR shell. As for the 18 point sources revealed by Chandra, only 3 sources have previously known counterparts at other wavelengths. Two of these sources exhibit enough counts to extract a spectrum, one being a new source CXOU J111911.8-613026 and the other being the X-ray counterpart of PSR J1119-6127 studied by us in a previous work. The spectrum of the new source CXOU J111911.8-613026 is soft (power-law photon index Γ ~ 3.6-4.6 or blackbody temperature kT ~ 0.2-0.36 keV) with a luminosity comparable to that of faint radio-quiet neutron stars. The nature of CXOU J111911.8-613026, however, cannot be determined with the present data and has to await further observations.

The neutral gas in the environs of the Geminga gamma-ray pulsar

We present a high-resolution (24″) study of the HI interstellar gas distribution around the radio-quiet neutron star Geminga. Based on very large array and MPIfR Effelsberg telescope data, we analyzed a 40′ × 40′ field around Geminga. These observations have revealed the presence of a neutral gas shell, 0.4 pc in radius, with an associated HI mass of 0.8 M ⊙, which surrounds Geminga at a radial velocity compatible with the kinematical distance of the neutron star. In addition, morphological agreement is observed between the internal face of the HI shell and the brightest structure of Geminga’s tail observed in X-rays. We explore the possibility that this morphological agreement is the result of a physical association.

Environments of Supernova Remnants Which Contain Different Types of Neutron Star

In the last few years new types of neutron stars having various degrees of activity have been identified. Among these are there dim radio quiet neutron stars and anomalous X-ray pulsars. In order to get some information about masses of progenitors of these neutron stars as well as about masses of progenitors of radio pulsars and single strong X-ray pulsars, we have examined environments of the supernova remnants connected to these neutron stars and pulsars. We have approached the problem through 4 independent ways: we have examined surface brightness – radius, surface brightness – age, and radius – age graphs of the supernova remnants as well as the observational data about the densities in the shock fronts and in the ambient media of the supernova remnants. We have also examined the environments of C- and S-type supernova remnants in which no point source has been observed. Densities of the ambient media of the supernova remnants connected to these different types of neutron stars are not very different from each other that there seems to be no reliable relation between mass of the progenitor star and the degree of activity or passivity of the neutron star. The S-type supernova remnants without point sources are expanding in less dense media that their progenitors might be less massive stars compared to the progenitors of the neutron stars and pulsars mentioned above.

Observations of the neutral hydrogen surrounding the radio-quiet neutron star RX J0822-4300 in Puppis A

We have observed the HI distribution in an area 40' x 40' around the neutron star candidate RX J0822-4300, which is located in the supernova remnant Puppis A. The observations of the 21 cm line were obtained with the Australia Telescope Compact Array (ATCA) and were combined with single dish data from the Southern Galactic Plane Survey. The spatial resolution is 90, and the velocity resolution, 1 km/s . A sensitivity of ~ 0.7 K was achieved. The results revealed a double lobed feature of reduced emission at +16 km/s, centered on the central compact object (CCO), and aligned with an HI hole blueshifted by 13 km/s. The HI depressions have probably been created by the sweeping up of ~ 2 solar masses. The alignement between the lobes and the optical expansion centre of Puppis A suggests that the CCO could be ejecting two opposite jets. The velocity at which the two lobes are best defined allowed us to confirm that the distance to Puppis A is 2.2 kpc, based on a systemic velocity of +16 km/s. The hydrogen column density computed using this systemic velocity is consistent with estimates from models for X-ray spectra, thus reinforcing our conclusion that the kinematic distance is 2.2 k pc.

Matter and Radiation in Superstrong Magnetic Fields and Thermal Emission from Neutron Stars

Thermal surface emissions have now been detected from more than a dozen isolated neutron stars, including radio pulsars, radio-quiet neutron stars and magnetars. These detections can potentially provide important information on the interior physics, magnetic fields, and surface composition neutron stars. Understanding the properties of matter and radiative transfer in strong magnetic fields is essential for the proper interpretation of the observations. We review current theoretical works on modeling magnetized neutron star atmospheres/surface layers, discussing some of the novel properties of matter and radiative transfer in strong magnetic fields. Of particular interest is the effect of the strong-field vacuum polarization, which dramatically changes the radiative transfer and the emergent X-ray spectra from magnetars.

ON PERIOD AND BURST HISTORIES OF AXPs AND SGRs AND THE POSSIBLE EVOLUTION OF THESE OBJECTS ON THE $P -\dot P$ DIAGRAM

In this paper, timing data for anomalous X-ray pulsars and soft gamma repeaters are compiled. Timing properties of these objects are investigated. The effect of bursts of soft gamma repeaters on their period history is investigated. The [Formula: see text] diagram for pulsars, X-ray binaries, anomalous X-ray pulsars, soft gamma repeaters and dim radio quiet neutron stars is constructed. The possible evolutionary tracks for anomalous X-ray pulsars, soft gamma repeaters and dim radio quiet neutron stars are examined.

On Young Neutron Stars as Propellers and Accretors with Conventional Magnetic Fields

The similarity of rotation periods of the anomalous X-ray pulsars (AXPs), the soft gamma-ray repeaters (SGRs), and the dim isolated thermal neutron stars (DTNs) suggests a common mechanism with an asymptotic spin-down phase, extending through the propeller and early accretion stages. The DTNs are interpreted as sources in the propeller stage. Their luminosities arise from frictional heating in the neutron star. If the 8.4 s rotation period of the DTN RX J0720.4-3125 is close to its rotational equilibrium period, the estimated propeller torque indicates a magnetic field in the 1012 G range. The mass inflow rate onto the propeller is on the order of the accretion rates of the AXPs. The limited range of rotation periods, taken to be close to equilibrium periods, and conventional magnetic fields in the range 5 × 1011 to 5 × 1012 G correspond to a range of mass inflow rates 3.2 × 1014 g s-1< < 4.2 × 1017 g s-1. Observed spin-down rates of the AXPs and SGRs also fit in with estimates for these magnetic fields and equilibrium periods. The source of the mass inflow is a remnant accretion disk formed as part of the fallback during the supernova explosion. These classes of sources thus represent the alternative pathways for those neutron stars that do not become radio pulsars. For the highest mass inflow rates the propeller action may support enough circumstellar material so that the optical thickness to electron scattering destroys the X-ray beaming, and the rotation period is not observable. These are the radio-quiet neutron stars at the centers of supernova remnants Cas A, Puppis A, RCW 103, and 296.5+10. The statistics and ages of DTNs suggest that sources in the propeller phase are quite common, maybe accounting for the majority of neutron stars formed in supernovae. AXPs are the rare cases whose history has allowed them to evolve rapidly to the post-propeller accretion phase. The different classes represent alternative pathways rather than consecutive phases of evolution. Thus, for example, the AXPs are not descendants of the DTNs. This model obviates the need to postulate magnetars for AXPs and DTNs. Frequently sampled timing observations of AXPs, SGRs, and DTNs can distinguish between this explanation and the magnetar model.

Nondetection of a Pulsar-powered Nebula in Puppis A and the Implications for the Nature of the Radio-quiet Neutron Star RX J0822−4300

We report on a deep radio search for a pulsar wind nebula associated with the radio-quiet neutron star RX J0822-4300 in the supernova remnant Puppis A. The well-determined properties of Puppis A allow us to constrain the size of any nebula to less than 30''; however, we find no evidence for such a source on any spatial scale up to 30'. These nondetections result in an upper limit on the radio luminosity of any pulsar-powered nebula that is 3 orders of magnitude below what would be expected if RX J0822-4300 was an energetic young radio pulsar beaming away from us, and they cast doubt on a recent claim of X-ray pulsations from this source. The lack of a radio nebula leads us to conclude that RX J0822-4300 has properties very different from most young radio pulsars and that it represents a distinct population that may be as numerous, or even more so, than radio pulsars.