Dim Isolated Neutron Stars Research Articles

ERRATUM: “NEW LIMITS ON RADIO EMISSION FROM X-RAY DIM ISOLATED NEUTRON STARS” (2009, ApJ, 702, 692)

We have carried out a search for radio emission at 820 MHz from six X-ray dim isolated neutron stars with the Robert C. Byrd Green Bank Radio Telescope. No transient or pulsed emission was found using fast folding, fast Fourier transform, and single-pulse searches. The corresponding flux limits are about 0.01 mJy for pulsed emission, depending on the integration time for the particular source and assuming a duty cycle of 2%, and 20 mJy for single dispersed pulses. These are the most sensitive limits to date on radio emission from X-ray dim isolated neutron stars. There is no evidence for isolated radio pulses, as seen in a class of neutron stars known as rotating radio transients. Our results imply that either the radio luminosities of these objects are lower than those of any known radio pulsars, or they could simply be long-period nearby radio pulsars with high magnetic fields beaming away from the Earth. To test the latter possibility, we would need around 40 similar sources to provide a 1 sigma probability of at least one of them beaming toward us. We also give a detailed description of our implementation of the Fast Folding Algorithm.

NEW LIMITS ON RADIO EMISSION FROM X-RAY DIM ISOLATED NEUTRON STARS

We have carried out a search for radio emission at 820 MHz from six X-ray dim isolated neutron stars (XDINSs) with the Robert C. Byrd Green Bank Radio Telescope. No transient or pulsed emission was found using fast folding, fast Fourier transform, and single-pulse searches. The corresponding flux limits are about 0.01 mJy for pulsed emission, depending on the integration time for the particular source and assuming a duty cycle of 2%, and 20 mJy for single dispersed pulses. These are the most sensitive limits to date on radio emission from XDINSs. There is no evidence for isolated radio pulses, as seen in a class of neutron stars known as rotating radio transients. Our results imply that either the radio luminosities of these objects are lower than those of any known radio pulsars, or they could simply be long-period nearby radio pulsars with high magnetic fields beaming away from the Earth. To test the latter possibility, we would need around 40 similar sources to provide a 1σ probability of at least one of them beaming toward us. We also give a detailed description of our implementation of the Fast Folding Algorithm.

Conversion of dark matter axions to photons in magnetospheres of neutron stars

We propose a new method to detect observational appearance of Dark Matter axions. The method utilizes observations of neutron stars (NSs) in radio. It is based on the conversion of axions to photons in strong magnetic fields of NSs (Primakoff effect). Whether the conversion takes place, the radio spectrum of the object would have a very distinctive feature -- a narrow spike at a frequency corresponding to the rest mass of the axion. For example, if the coupling constant of the photon-axion interaction is $M=10^{10}$ GeV, the density of Dark Matter axions is $\rho=10^{-24} {\rm g cm^{-3}}$, and the axion mass is $5 {\rm \mu eV}$, then a flux from a strongly magnetized ($10^{14}$ G) NS at the distance 300 pc from the Sun is expected to be about few tenths of mJy at the frequency $\approx 1200$ MHz in the bandwidth $\approx 3$ MHz. Close-by X-ray dim isolated neutron stars are proposed as good candidates to look for such radio emission.

Flares from a candidate Galactic magnetar suggest a missing link to dim isolated neutron stars

Magnetars are young neutron stars with very strong magnetic fields of the order of 10(14)-10(15) G. They are detected in our Galaxy either as soft gamma-ray repeaters or anomalous X-ray pulsars. Soft gamma-ray repeaters are a rare type of gamma-ray transient sources that are occasionally detected as bursters in the high-energy sky. No optical counterpart to the gamma-ray flares or the quiescent source has yet been identified. Here we report multi-wavelength observations of a puzzling source, SWIFT J195509+261406. We detected more than 40 flaring episodes in the optical band over a time span of three days, and a faint infrared flare 11 days later, after which the source returned to quiescence. Our radio observations confirm a Galactic nature and establish a lower distance limit of approximately 3.7 kpc. We suggest that SWIFT J195509+261406 could be an isolated magnetar whose bursting activity has been detected at optical wavelengths, and for which the long-term X-ray emission is short-lived. In this case, a new manifestation of magnetar activity has been recorded and we can consider SWIFT J195509+261406 to be a link between the 'persistent' soft gamma-ray repeaters/anomalous X-ray pulsars and dim isolated neutron stars.

Near infrared VLT/MAD observations of the isolated neutron stars RX J0420.0-5022 and RX J1856.5-3754

Context. High-energy observations have unveiled peculiar classes of isolated neutron stars which, at variance with radio pulsars, are mostly radio silent and not powered by the star rotation. Among these objects are the magnetars, hyper-magnetized neutron stars characterized by transient X-ray/gamma-ray emission, and neutron stars with purely thermal, and in most cases stationary, X-ray emission (a.k.a., X-ray dim isolated neutron stars or XDINSs). While apparently dissimilar in their high-energy behavior and age, both magnetars and XDINSs have similar periods and unusually high magnetic fields. This suggests a tantalizing scenario where the former evolve into the latter.Aims. Discovering so far uninvestigated similarities between the multi-wavelength properties of these two classes would be a further step forward to establish an evolutionary scenario. A most promising channels is the near infrared (NIR) one, where magnetars are characterized by a distinctive spectral flattening with respect to the extrapolation of the soft X-ray spectrum.Methods. We observed the two XDINSs RX J0420.0-5022 and RX J1856.5-3754 with the Multi-Conjugate Adaptive Optics Demonstrator (MAD) at the Very Large Telescope (VLT), as part of the instrument guaranteed time observations program, to search for their NIR counterparts.Results. Both RX J1856.5-3754 and RX J0420.0-5022 were not detected down to K-s similar to 20 and K-s similar to 21.5, respectively.Conclusions. In order to constrain the relation between XDINSs and magnetars it would be of importance to perform deeper NIR observations. A good candidate is 1RXS J214303.7+065419 which is the XDINS with the highest inferred magnetic field.

SGRs and AXPs proposed as ancestors of the Magnificent Seven

The recently suggested correlation between the surface temperature and the magnetic field in isolated neutron stars does not seem to work well for SGRs, AXPs and X-ray dim isolated neutron stars (XDINs; specifically the Magnificent Seven or M 7). Instead, by appealing to a color-flavor locked quark star (CFLQS) we find a more natural explanation. In this picture, the heating is provided by magnetic flux expulsion from a crust-less superconducting quark star. Combined with our previous studies concerning the possibility of SGRs, AXPs, and XDINs as CFLQSs, this provides another piece of evidence that these objects are all related. Specifically, we propose that XDINs are the descendants of SGRs and AXPs.

The birthrate of magnetars

ABSTRACT Magnetars, neutron stars with ultra strong magnetic elds (B ˘10 14 10 15 G), mani-fest their exotic nature in the form of soft gamma-ray repeaters and anomalous X-raypulsars. This study estimates the birthrate of magnetars to be ˘0.22 per century witha galactic population comprising of ˘17 objects. A population synthesis was carriedout based on the ve anomalous X-ray pulsars detected in the ROSAT All-Sky Surveyby comparing their number to that of massive OB stars in a well de ned volume. Ad-ditionally, the group of seven X-ray dim isolated neutron stars detected in the samesurvey were found to have a birthrate of ˘2 per century with a galactic populationof ˘22,000 objects.Key words: pulsar: general | stars: neutron | stars: statistics | stars: magnetic elds | supernovae: general 1 INTRODUCTIONThe end products of massive stars with initial masses in therange 8M known SGRs (SGR 1806-20, SGR 1900+14, SGR 1627-41) <M initial <25M , neutron stars (NSs) consti-tute ˘0:0001% of the total stellar population of the MilkyWay in the form of active pulsars (Lyne & Graham-Smith2006). It is now understood that not all NSs turn out to ex-hibit radio pulsations. This missing link manifests itself in ahighly energetic class of NSs called magnetars (Zhang 2003).Duncan & Thompson (1992) proposed the existence of mag-netars with ultra strong magnetic elds 100 times strongerthan that observed for normal radio pulsars (B˘10

Quark-nova remnants

We explore the formation and evolution of debris ejected around quark stars in the Quark Nova scenario, and the application to Soft Gamma-ray Repeaters (SGRs) and Anomolous X-ray Pulsars (AXPs). If an isolated neutron star explodes as a Quark Nova, an Iron-rich shell of degenerate matter forms out of the fall-back (crust) material. Our model can account for many of the observed features of SGRs and AXPs such as: (i) the two types of bursts (giant and regular); (ii) the spin-up and spin-down episodes during and following the bursts with associated persistant increases in $\dot{P}$; (iii) the energetics of the boxing day burst, SGR1806$+$20; (iv) the presence of an Iron line as observed in SGR1900$+$14; (v) the correlation between the far-Infrared and the X-ray fluxes during the bursting episode and the quiescent phase; (vi) the hard X-ray component observed in SGRs during the giant bursts, and (vii) the discrepancy between the ages of SGRs/AXPs and their supernova remnants. We also find a natural evolutionary relationship between SGRs and AXPs in our model which predicts that only the youngest SGRs/AXPs are most likely to exhibit strong bursting. Many features of X-ray Dim Isolated Neutron stars (XDINs) are also accounted for in our model such as, (i) the two-component blackbody spectra; (ii) the absorption lines around 300 eV; and (iii) the excess optical emission.

Deep VLT infrared observations of X-ray dim isolated neutron stars

Context. X-ray observations have unveiled the existence of a family of radio-quiet Isolated Neutron Stars whose X-ray emission is purely thermal, and hence are dubbed X-ray dim isolated neutron stars (XDINSs). While optical observations have allowed thermal emission to be related to the neutron star cooling and the neutron star surface thermal map to be built, IR observations are critical to pinpoint a spectral turnover produced by an as yet unseen magnetospheric component, or the presence of a fallback disk. The detection of such a turnover can provide further evidence of a link between this class of isolated neutron stars and the magnetars, which show a distinctive spectral flattening in the IR. Aims. We present the deepest IR observations reported to date of five XDINSs, which we use to constrain a spectral turnover in the IR and the presence of a fallback disk. Methods. We used archived VLT observations of these neutron stars performed with the ISAAC instrument in the H-band (1.65 µm) and the available fallback disk models. Results. It was not possible to identify the IR counterpart down to limiting magnitudes H ∼ 21.5–22.9 for any of our targets. Although these limits are the deepest ever obtained for neutron stars of this class, they are not deep enough to rule out the existence and the nature of a possible spectral flattening in the IR. We also derive, using disk models, the upper limits on the mass inflow rate in a fallback disk. We find the existence of a putative fallback disk consistent (although not confirmed) with our observations.

Persistent and transient blank field sources

Blank field sources (BFS) are good candidates for hosting dim isolated neutron stars (DINS). The results of a search of BFS in the ROSAT HRI images are revised. We then focus on transient BFS, arguing that they belong to a rather large population. The perspectives of future research on DINS are then discussed.

Dim isolated neutron stars, cooling and energy dissipation

The cooling and reheating histories of dim isolated neutron stars(DINs) are discussed. Energy dissipation due to dipole spindown with ordinary and magnetar fields, and due to torques from a fallback disk are considered as alternative sources of reheating which would set the temperature of the neutron star after the initial cooling era. Cooling or thermal ages are related to the numbers and formation rates of the DINs and therefore to their relations with other isolated neutron star populations. Interaction with a fallback disk, higher multipole fields and activity of the neutron star are briefly discussed.

The proper motion of the isolated neutron star RX J1605.3+3249

Aims. We present a new optical observation of the X-ray dim isolated neutron star RX J1605.3+3249, performed in 2005 with the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS) on board Hubble Space Telescope, aimed at measuring the neutron star proper motion.Methods. We have compared the target position with the one measured in a previous HST image of the field obtained in 2001.Results. We infer a proper motion of mu = 155.0 +/- 3.1 mas/yr, corresponding to a star velocity V-T similar to 73 km s(-1) at 100 pc distance. Our measurement refines the result recently reported by Motch et al. ( 2005, A&A, 429, 257), and confirms the optical source as the counterpart of the neutron star.

A tale of two populations: rotating radio transients and X-ray dim isolated neutron stars

Abstract We highlight similarities between recently discovered rotating radio transients and X-ray dim isolated neutron stars. In particular, it is shown that X-ray dim isolated neutron stars have a birthrate comparable to that of rotating radio transients. On the contrary, magnetars have a formation rate that is too low to account for the bulk of the radio transient population. The consequences of the recent detection of a thermal X-ray source associated with one of the rotating radio transients on the proposed scenarios for these sources are also discussed.

XMM‐NewtonDetection of Pulsations and a Spectral Feature in the X‐Ray Emission of the Isolated Neutron Star 1RXS J214303.7+065419/RBS 1774

We report on the results of a deep XMM-Newton observation of RBS 1774, the most recent dim isolated neutron star candidate found in the ROSAT archive data. Spectral and timing analysis of the high-quality PN and MOS data confirm the association of this source with an isolated neutron star. The spectrum is thermal and blackbody-like, and there is evidence at a significance level > 4sigma that the source is an X-ray pulsar, with spin period of 9.437 s. Spectral fitting reveils the presence of an absorption feature at ~0.7 keV, but at this level data do not have enough resolution to allow us to discriminate between an absorption line or an edge. We compare the newly measured properties of RBS 1774 with those of other known dim isolated neutron stars, and discuss possible interpretations for the absorption feature.

The isolated neutron star X-ray pulsars RX J0420.0–5022 and RX J0806.4–4123: New X-ray and optical observations

We report on the analysis of new X-ray data obtained with XMM-Newton and Chandra from two ROSAT-discovered X-ray dim isolated neutron stars (XDINs). RX J0806.4−4123 was observed with XMM-Newton in April 2003, 2.5 years after the first observation. The EPIC-pn data confirm that this object is an X-ray pulsar with 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature kT = 96 eV and N H = 4 × 10 19 cm −2 without significant changes between the two observations. Four XMM-Newton observations of RX J0420.0−5022 between December 2002 and July 2003 did not confirm the 22.7 s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453 s period. A fit to the X-ray spectrum using an absorbed black-body model yields kT = 45 eV, the lowest value found from the small group of XDINs and N H = 1.0 × 10 20 cm −2. Including a broad absorption line improves the quality of the spectral fits considerably for both objects and may indicate the presence of absorption features similar to those reported from RBS1223, RX J1605.3+3249 and RX J0720.4−3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RX J0420.0−5022 with B = 26.6 ± 0.3 mag from VLT imaging.

XMM-Newton observations of the isolated neutron star RX J0806.4-4123

The isolated neutron star was observed with XMM-Newton in November 2000. The data from the three EPIC instruments allowed us (i) to derive an improved X-ray position to an accuracy of 2–3´´, (ii) to accumulate the first medium-resolution soft X-ray spectra of high statistical quality and (iii) to find a candidate for the neutron star rotation period. Although this period of 11.3714 s is formally detected at a 3.5σ level in the EPIC-pn data, the similar pulse profiles deduced from all three EPIC instruments increase the confidence that the period is real. The pulsed fraction of ~6% would then be the weakest X-ray flux modulation detected from dim isolated neutron stars. We fitted the X-ray spectra with blackbody and neutron star atmosphere models and discuss the results with respect to the brightness limit placed by optical images. The reduced size of the error circle on the X-ray position should allow deeper searches for an optical counterpart.