N. Rea

Discovery of Pulsations and a Possible Spectral Feature in the X-ray Emission from Rotating Radio Transient J1819-1458

PSR J1819-1458 is a rotating radio transient (RRAT) source with an inferred surface dipole magnetic field strength of 5 × 1013 G and a 4.26 s spin period. We present XMM-Newton observations of the X-ray counterpart of this source, CXOU J181939.1-145804, in which we identify pulsations and a possible spectral feature. The X-ray pulsations are at the period predicted by the radio ephemeris, providing an unambiguous identification with the radio source and confirmation of its neutron star nature. The X-ray pulse has a 0.3-5 keV pulsed fraction of 34% and is aligned with the expected phase of the radio pulse. The X-ray spectrum is fit well by an absorbed blackbody with kT = 0.14 keV with the addition of an absorption feature at 1 keV, with total absorbed flux of 1.5 × 10-13 ergs cm-2 s-1 (0.3-5 keV). This absorption feature is well modeled by a Gaussian or resonant cyclotron scattering model, but its significance is dependent on the choice of continuum model. We find no evidence for any X-ray bursts or aperiodic variability on timescales of 6 ms to the duration of the observation and can place the most stringent limit to date of ≤3 × 10-9 ergs cm-2 s-1 on the absorbed 0.3-5 keV flux of any bursts.

Pronounced long term flux variability of the Anomalous X-ray Pulsar 1E 1048.1-5937

We present XMM-Newton and Chandra observations of 1E 1048.1-5937, being the first to show evidence for a significant variation in the X-ray luminosity of this anomalous X-ray pulsar (AXP). While during the first XMM-Newton (2000 December) and Chandra (2001 July) observations the source had a flux consistent with that measured on previous occasions (~5 × 10-12 ergs cm-2 s-1), two more recent observations found it at a considerably higher flux level of 2 × 10-11 ergs cm-2 s-1 (2002 August; Chandra) and 10-11 ergs cm-2 s-1 (2003 June; XMM-Newton). All the spectra are fit by the sum of a blackbody with kT ~ 0.6 keV and a power law with photon index ~3. No significant changes were seen in the spectral parameters, while the pulsed fraction in the 0.6-10 keV energy range decreased from ~90% in 2000 to ~53% in 2003. The spectral invariance does not support the presence of two physically distinct components in the AXP emission. The sparse coverage of the data does not permit us to unambiguously relate the observed variations to the two bursts seen from this source in the fall of 2001.

Evidence of a Cyclotron Feature in the Spectrum of the Anomalous X-Ray Pulsar 1RXS J170849–400910

We report the results of a long observation of the anomalous X-ray pulsar 1RXS J170849-400910 obtained with the BeppoSAX satellite in 2001 August. The best-fit phase-averaged spectrum was an absorbed power-law plus blackbody model, with a photon index of Γ ~ 2.4 and a blackbody temperature of kTbb ~ 0.4 keV. We confirm the presence of significant spectral variations with the rotational phase of the pulsar. In the spectrum corresponding to the rising part of the pulse, we found an absorption-like feature at ~8.1 keV (a significance of 4 σ), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9 × 1011 and 1.6 × 1015 G in the case of electrons and protons, respectively. If confirmed, this would be the first detection of a cyclotron feature in the spectrum of an anomalous X-ray pulsar.

Discovery and monitoring of the likely IR counterpart of SGR 1806–20 during the 2004 γ-ray burst-active state

The sky region including the Chandra position of SGR 1806-20 was monitored in the IR band during 2004, following its increased high energy bursting activity. Observations were performed using NAOS-CONICA, the adaptive optics IR camera mounted on Yepun VLT, which provided images of unprecedented quality (FWHM better than 0.1). After the 2004 December 27th giant flare, the source position has been nailed by VLA observations of its radio counterpart, reducing the positional uncertainty to 0.04. Using IR data from our monitoring campaign, we discovered the likely IR counterpart to SGR 1806-20 based on positional coincidence with the Chandra and VLA uncertainty regions and flux variability of a factor of about 2 correlated with that at higher energies. We compare our findings with other isolated neutron star classes thought to be related, at some level, with SGRs.

Correlated Infrared and X-ray variability of the transient Anomalous X-ray Pulsar XTE J1810-197

We report on observations aimed at searching for flux variations from the proposed IR counterpart of the Anomalous X-ray Pulsar (AXP) XTE J1810−197. These data, obtained in March 2004 with the adaptive optics camera NAOS-CONICA at the ESO VLT, show that the candidate proposed by Israel et al. (2004a, ApJ, 603, L97) was fainter by ∆H = 0.7 ± 0.2 and ∆Ks = 0.5 ± 0.1 with respect to October 2003, confirming it as the IR counterpart of XTE J1810−197. We also report on an XMM-Newton observation carried out the day before the VLT observations. The 0.5−10 keV absorbed flux of the source was 2.2 × 10 −11 erg cm −2 s −1 , which is less by a factor of about two compared to the previous XMM-Newton observation on September 2003. Therefore, we conclude that a similar flux decrease took place in the X-ray and IR bands. We briefly discuss these results in the framework of the proposed mechanism(s) responsible for the IR variable emission of AXPs.

Accurate X-Ray Position of the Anomalous X-Ray Pulsar XTE J1810–197 and Identification of Its Likely Infrared Counterpart*

We report the accurate subarcsecond X-ray position of the new anomalous X-ray pulsar (AXP) XTE J1810-197, derived with a Chandra High Resolution Camera Target of Opportunity observation carried out in 2003 November. We also report the discovery of a likely IR counterpart based on a Very Large Telescope (IR band) Target of Opportunity observation carried out in 2003 October. Our proposed counterpart is the only IR source (Ks = 20.8) in the X-ray error circle. Its IR colors as well as the X-ray/IR flux ratio are consistent with those of the counterparts of all other AXPs (at variance with field star colors). Deep Gunn i-band images obtained at the 3.6 m ESO telescope detected no sources down to a limiting magnitude of 24.3. Moreover, we find that the pulsed fraction and count rates of XTE J1810-197 remained nearly unchanged since the previous Chandra and XMM-Newton observations (2003 August 27 and September 8, respectively). We briefly discuss the implications of these results. In particular, we note that the transient (or at least highly variable) nature of this AXP might imply a relatively large number of hidden members of this class.

Swift and Chandra confirm the intensity-hardness correlation of the AXP 1RXS J170849.0-400910

Convincing evidence of long-term variations in the emission properties of the anomalous X-ray pulsar 1RXS J170849.0-400910 has been gathered in the past few years. In particular, and following the pulsar glitches of 1999 and 2001, XMM-Newton witnessed a decline in 2003 of the X-ray flux accompanied by a definite spectral softening. This suggested the existence of a correlation between the luminosity and the spectral hardness in this source, similar to what was seen in the soft gamma-repeater SGR 1806-20. Here we report on new Chandra and Swift observations of 1RXS J170849.0-400910 performed in 2004 and 2005, respectively. These observations confirm and strengthen the proposed correlation. The trend appears to have now reversed: the flux has increased and the spectrum is now harder. We briefly discuss the consequences of these observations for the twisted magnetosphere scenario for anomalous X-ray pulsars.

Constraints on Galactic intermediate mass black holes

Intermediate Mass Black Holes (IMBHs; 10 1.3−5 M⊙) are thought to form as relics of Population III stars or from the runaway collapse of stars in young clusters; their number and very existence are uncertain. We ran N-body simulations of Galactic IMBHs, modelling them as a halo population distributed according to a Navarro, Frenk & White (NFW) or a more concentrated Diemand, Madau & Moore (DMM) density profile. As IMBHs pass through Galactic molecular/atomic hydrogen regions, they accrete gas, thus becoming X-ray sources. We constrain the density of Galactic IMBHs, •, by comparing the distribution of simulated X-ray sources with the observed one. From the null detections of Milky Way Ultra-Luminous X-ray sources, and from a comparison of simulations with unidentified sources in the IBIS/ISGRI catalogue we find a strong upper limit • � 10 −2 b(� 10 −1 b) for a DMM (NFW) profile, if IMBHs accrete via ADAF disks. Slightly stronger constraints (• � 10 −3 b for a DMM profile; • � 10 −2 b for a NFW profile) can be derived if IMBHs accrete with higher efficiency, such as by forming thin accretion disks. Although not very tight, such constraints are the most stringent ones derived so far in the literature.

Very deep X-ray observations of the anomalous X-ray pulsar 4U 0142+614

We report on two new XMM‐Newton observations of the anomalous X-ray pulsar (AXP) 4U 0142+614 performed in 2004 March and July, collecting the most accurate spectrum for this source. Furthermore, we analyse two short archival observations performed in 2002 February and 2003 January in order to study the long-term behaviour of this AXP. 4U 0142+614 appears to be relatively steady in flux between 2002 and 2004, and the phase-averaged spectrum does not show any significant variability between the four epochs. We derive the deepest upper limits to date on the presence of lines in 4U 0142+614 spectrum as a function of energy: equivalent width in the 1‐3 keV energy range <4 and <8 eV for narrow and broad lines, respectively. A remarkable energy dependence in both the pulse profile and the pulsed fraction is detected, and consequently pulse‐phase spectroscopy shows spectral variability as a function of phase. By making use of XMM‐Newton and INTEGRAL data, we successfully model the 1‐250 keV spectrum of 4U 0142+614 with three models, namely the canonical absorbed blackbody plus two power laws, a resonant cyclotron scattering model plus one power-law and two logparabolic functions.

Linking the X-ray timing and spectral properties of the glitching AXP 1RXS J170849-400910

Aims. Previous studies of the X-ray flux and spectral properties of 1RXS J170849−400910 showed a possible correlation with the spin glitches that occurred in 1999 and 2001. However, due to the sparseness of spectral measurements and the paucity of detected glitches, no firm conclusion could be drawn. Methods. We retrieved and analysed archival Rossi-XTE pointings of 1RXS J170849−400910 covering the time interval between January 2003 and June 2006 and carried out a detailed timing analysis with phase fitting techniques. Results. We detected two large glitches ( ∆ν ν of 1.2 and 2.1 × 10 −6 ) that occurred in January and June 2005. The occurrence times of these glitches are in agreement with the predictions made in our previous studies. This finding strongly suggests a connection between the flux, spectral and timing properties of 1RXS J170849−400910.