243 research outputs found
The isolated neutron star X-ray pulsars RX J0420.0–5022 and RX J0806.4–4123 : new X-ray and optical observations
No full textWe 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
Modelling the spin pulse profile of the isolated neutron star RX J0720.4-3125 observed with XMM-Newton
No full textWe model the spin pulse intensity and hardness ratio profiles of the isolated neutron star RX J0720.4-3125 using XMM-Newton data. The observed variation is approximately sinusoidal with a peak-to-peak amplitude of 15%, and the hardness ratio is softest slightly before flux maximum. By using polar cap models we are able to derive maximum polar cap sizes and acceptable viewing geometries. The inferred sizes of the caps turn out to be more compatible with a scenario in which the neutron star is heated by accretion, and place limits on the magnetic field strength. The hardness ratio modulation can then be explained in terms of energy-dependent beaming effects, and this constrains the acceptable models of the emerging radiation to cases in which softer photons are more strongly beamed than harder photons. An alternative explanation in terms of spatially variable absorption co-rotating in the magnetosphere is also discussed
Periodicities in the high-mass X-ray binary system RXJ0146.9+6121/LSI+61 235
No full textThe high-mass X-ray binary RX J0146.9+6121, with optical counterpart LS I+61°235 (V831 Cas), is an intriguing system on the outskirts of the open cluster NGC 663. It contains the slowest Be type X-ray pulsar known with a pulse period of around 1400 s and, primarily from the study of variation in the emission line profile of Hα, it is known to have a Be decretion disc with a one-armed density wave period of approximately 1240 d. Here we present the results of an extensive photometric campaign, supplemented with optical spectroscopy, aimed at measuring short time-scale periodicities. We find three significant periodicities in the photometric data at, in order of statistical significance, 0.34, 0.67 and 0.10 d. We give arguments to support the interpretation that the 0.34 and 0.10 d periods could be due to stellar oscillations of the B-type primary star and that the 0.67 d period is the spin period of the Be star with a spin axis inclination of 23+10−8 degrees. We measured a systemic velocity of −37.0 ± 4.3 km s−1 confirming that LS I+61°235 has a high probability of membership in the young cluster NGC 663 from which the system's age can be estimated as 20–25 Myr. From archival RXTE All Sky Monitor (ASM) data we further find ‘super’ X-ray outbursts roughly every 450 d. If these super outbursts are caused by the alignment of the compact star with the one-armed decretion disc enhancement, then the orbital period is approximately 330 d
The ROSAT Galactic Plane Survey: analysis of a low latitude sample area in Cygnus
No full text. The analysis of the part of the ROSAT allsky survey covering the galactic plane is the scope of a dedicated project called the ROSAT Galactic Plane Survey. In order to statistically understand the nature of the ß 14,000 sources discovered by ROSAT at jbj 20 ffi , a number of sample areas have been chosen for follow-up optical identification. In this paper we present the X-ray and optical material gathered in a region located in the Cygnus constellation, centered at l = 90 ffi , b = 0 ffi and covering an area of 64.5 deg 2 . A total of 95 and 128 sources are detected with a maximum likelihood larger than 10 and 8 respectively. With a typical survey exposure time of the order of 700 to 900 s the flux completeness level is ß 0.02 cts s \Gamma1 corresponding to ß 2 10 \Gamma13 erg cm \Gamma2 s \Gamma1 . The position of the sample area allows to investigate the soft Xray content of a rather typical region of the galactic plane. In this paper we describe the details of t..
Mice Producing Reduced Levels of Insulin-Like Growth Factor Type 1 Display an Increase in Maximum, but not Mean, Life Span
No full textReduced signaling through the IGF type 1 (IGF-1) receptor increases life span in multiple invertebrate organisms. Studies on mammalian longevity suggest that reducing levels of IGF-1 may also increase life span. However, the data are conflicting and complicated by the physiology of the mammalian neuroendocrine system. We have performed life-span analysis on mice homozygous for an insertion in the Igf1 gene. These mice produce reduced levels of IGF-1 and display a phenotype consistent with a significant decrease in IGF-1. Life-span analysis was carried out at three independent locations. Although the life-span data varied between sites, the maximum life span of the IGF-1-deficient mice was significantly increased and age-specific mortality rates were reduced in the IGF-1-deficient mice; however, mean life span did not differ except at one site, where mean life span was increased in female IGF-1-deficient animals. Early life mortality was noted in one cohort of IGF-1-deficient mice. The results are consistent with a significant role for IGF-1 in the modulation of life span but contrast with the published life-span data for the hypopituitary Ames and Snell dwarf mice and growth hormone receptor null mice, indicating that a reduction in IGF-1 alone is insufficient to increase both mean and maximal life span in mice
NewXMM-Newtonobservation of the thermally emitting isolated neutron star 2XMM J104608.7-594306
No full textContext. The isolated neutron star (INS) 2XMM J104608.7-59430
VLT optical observations of the isolated neutron star RX J0420.0-5022
No full textContext: X-ray observations performed with the Röntgen Satellite (ROSAT) led to the discovery of seven radio-silent isolated neutron stars (INSs) which are detected only through the relatively dim and purely thermal X-ray emission from the cooling star surface. A few of these INSs (X-ray Dim INSs, or XDINSs) have been also detected at optical wavelengths where they seem to feature thermal spectra. Optical studies of XDINSs thus play a crucial role in mapping the temperature distribution on the neutron star surface and in investigating the existence of an atmosphere around the neutron star.
Aims: The aim of this work is to investigate the optical identification of the XDINS RX J0420.0-5022, tentatively proposed in the literature based on Very Large Telescope (VLT) observations.
Methods: We re-analysed the original VLT observations of the proposed counterpart to assess its detection significance and we performed deeper VLT observations aiming at a higher confidence detection.
Results: With a ̃ 2 σ detection significance and a re-computed flux of B=27.52 ± 0.61, we cannot rule out that the proposed counterpart was spurious and produced by the halo of a very bright nearby star. While we could not detect the proposed counterpart in our deeper VLT observations, we found evidence for a marginally significant (~ 3.9 σ) detection of a similarly faint object (B= 27.5± 0.3), ≈ 0.5 arcsec north of it and coincident with the updated Chandra position of RX J0420.0-5022. Interestingly, the angular separation is consistent with the upper limit on the RX J0420.0-5022 proper motion, which suggests that we might have actually detected the originally proposed counterpart. From the flux of the putative RX J0420.0-5022 counterpart we can rule out a > 7 optical excess with respect to the extrapolation of the XMM-Newton spectrum.
Conclusions: High spatial resolution observations with the refurbished Hubble Space Telescope (HST) are the only way to confirm the detection of the putative candidate counterpart and to validate its identification with RX J0420.0-5022
VLT/FORS2 observations of the optical counterpart of the isolated neutron star RBS 1774
No full textContext. The X-ray observations performed with the Röntgen Satellite (ROSAT) have led to the discovery of a group (seven to date) of X-ray dim and radio-silent, middle-aged isolated neutron stars (a.k.a. XDINSs), which are characterised by pure blackbody spectra (kT ≈ 40-100 eV) and long X-ray pulsations (P = 3-12 s), and appear to be endowed with relatively high magnetic fields, (B ≈ 1013-1014 G). Optical observations of XDINSs are important, together with the X-ray ones, for studying the cooling of the neutron star surface and for investigating the relation between XDINSs and other isolated neutron star classes. RBS 1774 is one of the few XDINSs with a candidate optical counterpart, which we discovered with the Very Large Telescope (VLT).
Aims: We aim at constraining the optical spectrum of RBS 1774, for which only two B-band flux measurements are available, and to determine whether its optical emission has either a thermal or a non-thermal origin.
Methods: We performed deep observations of RBS 1774 in the R band with the VLT to disentangle a non-thermal power-law spectrum from a Rayleigh-Jeans, whose contributions are expected to be very different in the red part of the spectrum.
Results: We did not detect the RBS 1774 candidate counterpart down to a 3σ limiting magnitude of R ~ 27. The constraint on its colour, (B - R) ≲ 0.6, rules out its being a background object, positionally coincident with the X-ray source. Our R-band upper limit is consistent with the extrapolation of the B-band flux (assuming a 3σ uncertainty) for a set of power laws Fν ∝ ν-α with spectral indices α ≤ 0.07. If the optical spectrum of RBS 1774 were non-thermal, its power-law slope would be very much unlike those of all isolated neutron stars with non-thermal optical emission, suggesting that it is most likely thermal. For instance, a Rayleigh-Jeans with temperature TO = 11 eV, for an optically emitting radius rO = 15 km and a source distance d = 150 pc, would be consistent with the optical measurements. The implied low distance is compatible with the 0.04 X-ray pulsed fraction if either the star spin axis is nearly aligned with the magnetic axis or with the line of sight or it is slightly misaligned with respect to both the magnetic axis and the line of sight by 5-10°.
Conclusions: New observations, both from the ground and from the Hubble Space Telescope (HST), are important to characterise the optical/near-ultraviolet (UV) spectrum of RBS 1774, to better constrain the values of rO, d, and TO and measure the source's proper motion from which indirect constraints on the source distance can be inferred
An optical counterpart candidate for the isolated neutron star RBS 1774
No full textMultiwavelength studies of the seven identified X-ray-dim isolated neutron stars (XDINSs) offer a unique opportunity to investigate their surface thermal and magnetic structure and the matter-radiation interaction in presence of strong gravitational and magnetic fields. As a part of an ongoing campaign aimed at a complete identification and spectral characterization of XDINSs in the optical band, we performed deep imaging with the ESO Very Large Telescope (VLT) of the field of the XDINS RBS 1774 (1RXS J214303.7 +065419). The recently upgraded FORS1 instrument mounted on the VLT provided the very first detection of a candidate optical counterpart in the B band. The identification is based on a very good positional coincidence with the X-ray source (chance probability ~2×10-3). The source has B=27.4+/-0.2 (1 σ confidence level), and the optical flux exceeds the extrapolation of the X-ray blackbody at optical wavelengths by a factor ~35 (+/-20 at 3 σ confidence level). This is barely compatible with thermal emission from the neutron star surface, unless the source distance is d~200-300 pc, and the star is an almost aligned rotator or its spin axis is nearly aligned with the line of sight. At the same time, such a large optical excess appears difficult to reconcile with rotation-powered magnetospheric emission, unless the source has an extremely large optical emission efficiency. The implications and possible similarities with the optical spectra of other isolated NSs are discussed
Identification of selected sources from the ROSAT Galactic Plane Survey. I.
No full textWe report on optical searches in the error circles of 93 ROSAT survey
sources located at low galactic latitudes (
< 20° ). These
sources were extracted from the ROSAT Galactic Plane Survey using various
selection criteria on hardness ratio, X-ray and optical brightness and
integrated galactic absorption in the direction of the source. We find
optical identifications in 76 cases, among which are 25 new AGN, 6 new
CVs and a new Be/X-ray binary. In order to illustrate the relevance of the
source selections applied here, we cross-correlated the ROSAT all-sky survey
bright source list with SIMBAD. Different classes of X-ray emitters populate
distinct regions of a multi dimensional parameter space involving flux
ratios, galactic latitude and NH. This relatively good segregation offers
the possibility to build source samples with enhanced probability of
identification with a given class. Complete optical identification of such
subsamples could eventually be used to compute meaningful probabilities of
identification for all sources using as basis a restricted set of multi-wavelength information
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