Hard X-Ray flux upper limits of central compact objects in supernova remnants

We searched for hard X-ray (20–300 keV) emission from nine central compact objects (CCOs) 1E 1207.4−5209, 1WGA J1713−3949, J082157.5−430017, J085201.4−461753, J1601−5133, J1613483−5055, J181852.0−150213, J185238.6+004020, and J232327.9+584843 with the INTEGRAL observatory. We applied spectral imaging analysis and did not detect any of the sources with luminosity upper limits in the range of 1033-1034 ergs/s in the 20-75 keV band. For nearby CCOs (< 4 kpc) the upper limit luminosities are an order of magnitude lower than the measured persistent hard X-ray luminosities of AXPs. This may indicate that the central compact objects are low magnetic field systems with fallback disks around them

Hard X-ray timing and spectral properties of PSR B0540-69

We report the hard X-ray properties of the young Crab-like LMC pulsar PSR B0540-69, using archival RXTE PCA (2 - 60 keV) and RXTE HEXTE (15 - 250 keV) data. Making use of the very long effective exposure of 684 ks, we derived a very detailed master pulse profile for energies 2 - 20 keV. We confirm the broad single-pulse shape with a dip in the middle and with a significant fine structure to the left of the dip. For the first time pulse profiles in the 10 - 50 keV energy interval are shown. Remarkably, the coarse pulse shape is stable from the optical up to X-ray energies analogous to the case of the Crab pulsar (PSR B0531+21). The profiles can be described with two Gaussians with a phase separation of ~0.2; the distribution of the ratios between the two components from the optical to the X-ray range is consistent with being flat. Therefore we cannot conclude that the profile consists of two distinct components. We also derived a new total pulsed spectrum in the ~0.01 - 50 keV range in a consistent analysis including also archival ROSAT PSPC (0.01 - 2.5 keV) data. This spectrum cannot be described by a single power-law, but requires an additional energy dependent term. The bending of the spectrum around 10 keV resembles that of the Crab pulsar spectrum. Although model calculations using Outer Gap scenarios could probably explain the high-energy characteristics of PSR B0540-69 as they successfully do for the Crab, our measurements do not entirely agree with the latest calculations by Zhang & Cheng (2000). The small discrepancies are likely to be caused by uncertainties in the pulsar's geometry.Comment: 7 pages and 4 figures. Accepted for publication in Astronomy & Astrophysic

Supernova remnants with magnetars: clues to magnetar formation

I discuss the lack of observational evidence that magnetars are formed as rapidly rotating neutron stars. Supernova remnants containing magnetars do not show the excess of kinetic energy expected for such a formation scenario, nor is there any evidence for a relic pulsar wind nebula. However, it could be that magnetars are formed with somewhat slower rotation periods, or that not all excess rotational energy was used to boost the explosion energy, for example as a result of gravitational radiation. Another observational tests for the rapid initial period hypothesis is to look for statistical evidence that about 1% of the observed supernovae have an additional 1E40-1E44 erg/s excess energy during the first year, caused by the spin down luminosity of a magnetar. An alternative scenario for the high magnetic fields of magnetars is the fossil field hypothesis, in which the magnetic field is inherited from the progenitor star. Direct observational tests for this hypothesis are harder to formulate, unless the neutron star formed in the SN1987A explosion emerges as a slowly rotating magnetar. Finally, I point out the possible connection between the jets in Cas A and its X-ray point source: the jets in Cas A may indicate that the explosion was accompanied by an X-ray flash, probably powered by a rapidly rotating compact object. However, the point source in Cas A does not seem to be a rapidly rotating neutron star, suggesting that the neutron star has slowed down considerably in 330 yr, requiring a magnetar-like field.Comment: Accepted by Advances in Space Research (Cospar 2006, Beijing, Session E1.4

Subaru optical observations of the old pulsar PSR B0950+08

We report the B band optical observations of an old (17.5 Myr) radiopulsar PSR B0950+08 obtained with the Suprime-Cam at the Subaru telescope. We detected a faint object, B=27.07(16). Within our astrometrical accuracy it coincides with the radio position of the pulsar and with the object detected earlier by Pavlov et al. (1996) in UV with the HST/FOC/F130LP. The positional coincidence and spectral properties of the object suggest that it is the optical counterpart of PSR B0950+08. Its flux in the B band is two times higher than one would expect from the suggested earlier Rayleigh-Jeans interpretation of the only available HST observations in the adjacent F130LP band. Based on the B and F130LP photometry of the suggested counterpart and on the available X-ray data we argue in favour of nonthermal origin of the broad-band optical spectrum of PSR B0950+08, as it is observed for the optical emission of the younger, middle-aged pulsars PSR B0656+14 and Geminga. At the same time, the optical efficiency of PSR B0950+08, estimated from its spin-down power and the detected optical flux, is by several orders of magnitude higher than for these pulsars, and comparable with that for the much younger and more energetic Crab pulsar. We cannot exclude the presence of a compact, about 1'', faint pulsar nebula around PSR B0950+08, elongated perpendicular to the vector of its proper motion, unless it is not a projection of a faint extended object on the pulsar position.Comment: 8 pages, LaTeX, aa.cls style, 5 PS figures, submitted to A&A. Image is available in FITS format at http://www.ioffe.rssi.ru/astro/NSG/obs/0950-subar

X-ray observations of the Large Magellanic Cloud pulsar PSR B0540-69 and its PWN

PSR B0540-69 is a young pulsar in the Large Magellanic Cloud that has similar properties with respect to the Crab Pulsar, and is embedded in a Pulsar Wind Nebula. We have analyzed the complete archival RXTE dataset of observations of this source, together with new Swift-XRT and INTEGRAL-IBIS data. Accurate lightcurves are produced in various energy bands between 2 and 60 keV, showing no significant energy variations of the pulse shape. The spectral analysis shows that the pulsed spectrum is curved, and is best fitted up to 100 keV by a log-parabolic model: this strengthens the similarities with the Crab pulsar, and is discussed in the light of a phenomenologic multicomponent model. The total emission from this source is studied, the relative contributions of the pulsar and the PWN emission are derived, and discussed in the context of other INTEGRAL detected pulsar/PWN systems.Comment: 11 pages, 11 figures. Accepted for publication in MNRA

Spin-down Measurement of PSR J1852+0040 in Kesteven 79: Central Compact Objects as Anti-Magnetars

Using XMM-Newton and Chandra, we achieved phase-connected timing of the 105 ms X-ray pulsar PSR J1852+0040 that provides the first measurement of the spin-down rate of a member of the class of Central Compact Objects (CCOs) in supernova remnants. We measure P-dot = 8.68(9)E-18, and find no evidence for timing noise or variations in X-ray flux over 4.8 yr. In the dipole spin-down formalism, this implies a surface magnetic field strength B_s = 3.1E10 G, the smallest ever measured for a young neutron star, and consistent with being a fossil field. In combination with upper limits on B_s from other CCO pulsars, this is strong evidence in favor of the "anti-magnetar" explanation for their low luminosity and lack of magnetospheric activity or synchrotron nebulae. While this dipole field is small, it can prevent accretion of sufficient fall-back material so that the observed X-ray luminosity of L_x = 5.3E33(d/7.1 kpc)^2 erg/s must instead be residual cooling. The spin-down luminosity of PSR J1852+0040, E-dot = 3.0E32 erg/s, is an order-of-magnitude smaller than L_x. Fitting of the X-ray spectrum to two blackbodies finds small emitting radii, R_1 = 1.9 km and R_2 = 0.45 km, for components of kT_1 = 0.30 keV and kT_2 = 0.52 keV, respectively. Such small, hot regions are ubiquitous among CCOs, and are not yet understood in the context of the anti-magnetar picture because anisotropic surface temperature is usually attributed to the effects of strong magnetic fields.Comment: 11 pages, 7 figures, Added text and figures, acccepted by The Astrophysical Journa

A Two-dimensional Map of Color Excess in NGC 3603

Using archival HST/WFC3 images centered on the young HD 97950 star cluster in the giant HII region NGC 3603, we computed the pixel-to-pixel distribution of the color excess, E(B-V)g, of the gas associated with this cluster from its H_alpha/Pa_beta flux ratio. At the assumed distance of 6.9 kpc, the resulting median color excess within 1 pc from the cluster center is E(B-V)g =1.51 \pm 0.04 mag. Outside the cluster (at r > 1 pc), the color excess is seen to increase with cluster-centric distance towards both North and South, reaching a value of about 2.2 mag at r = 2 pc from the cluster center. The radial dependence of E(B-V)g westward of the cluster appears rather flat at about 1.55 mag over the distance range 1.2 pc < r < 3 pc. In the eastern direction, E(B-V)g steadily increases from 1.5 mag at r = 1 pc to 1.7 mag at r = 2 pc, and stays nearly constant at 1.7 mag for 2 pc < r < 3 pc. The different radial profiles and the pixel-to-pixel variations of E(B-V)g clearly indicate the presence of significant differential reddening across the 4.9 pc \times 4.3 pc area centered on the HD 97950 star cluster. We interpret the variations of E(B-V)g as the result of stellar radiation and stellar winds interacting with an inhomogeneous dusty local interstellar medium (ISM) whose density varies spatially. From the E(B-V)g values measured along the rims of the prominent pillars MM1 and MM2 in the southwest and southeast of the HD 97950 cluster we estimate an H2 column density of log10(N(H2))=21.7 and extrapolate it to log10(N(H2))=23 in the pillars' interior. We find the pillars to be closer to us than the central ionizing cluster and suggest that star formation may be occurring in the pillar heads.Comment: 14 pages, 6 figures, 2 tables, accepted for publication in A

Multifrequency Strategies for the Identification of Gamma-Ray Sources

More than half the sources in the Third EGRET (3EG) catalog have no firmly established counterparts at other wavelengths and are unidentified. Some of these unidentified sources have remained a mystery since the first surveys of the gamma-ray sky with the COS-B satellite. The unidentified sources generally have large error circles, and finding counterparts has often been a challenging job. A multiwavelength approach, using X-ray, optical, and radio data, is often needed to understand the nature of these sources. This chapter reviews the technique of identification of EGRET sources using multiwavelength studies of the gamma-ray fields.Comment: 35 pages, 22 figures. Chapter prepared for the book "Cosmic Gamma-ray Sources", edited by K.S. Cheng and G.E. Romero, to be published by Kluwer Academic Press, 2004. For complete article and higher resolution figures, go to: http://www.astro.columbia.edu/~muk/mukherjee_multiwave.pd

X-ray emission from isolated neutron stars

X-ray emission is a common feature of all varieties of isolated neutron stars (INS) and, thanks to the advent of sensitive instruments with good spectroscopic, timing, and imaging capabilities, X-ray observations have become an essential tool in the study of these objects. Non-thermal X-rays from young, energetic radio pulsars have been detected since the beginning of X-ray astronomy, and the long-sought thermal emission from cooling neutron star's surfaces can now be studied in detail in many pulsars spanning different ages, magnetic fields, and, possibly, surface compositions. In addition, other different manifestations of INS have been discovered with X-ray observations. These new classes of high-energy sources, comprising the nearby X-ray Dim Isolated Neutron Stars, the Central Compact Objects in supernova remnants, the Anomalous X-ray Pulsars, and the Soft Gamma-ray Repeaters, now add up to several tens of confirmed members, plus many candidates, and allow us to study a variety of phenomena unobservable in "standard'' radio pulsars.Comment: Chapter to be published in the book of proceedings of the 1st Sant Cugat Forum on Astrophysics, "ICREA Workshop on the high-energy emission from pulsars and their systems", held in April, 201

Constraining compactness and magnetic field geometry of X-ray pulsars from the statistics of their pulse profiles

The light curves observed from X-ray pulsars and magnetars reflect the radiation emission pattern, the geometry of the magnetic field, and the neutron star compactness. We study the statistics of X-ray pulse profiles in order to constrain the neutron star compactness and the magnetic field geometry. We collect the data for 124 X-ray pulsars, which are mainly in high-mass X-ray binary systems, and classify their pulse profiles according to the number of observed peaks seen during one spin period, dividing them into two classes, single- and double-peaked. We find that the pulsars are distributed about equally between both groups. We also compute the probabilities predicted by the theoretical models of two antipodal point-like spots that emit radiation according to the pencil-like emission patterns. These are then compared to the observed fraction of pulsars in the two classes. Assuming a blackbody emission pattern, it is possible to constrain the neutron star compactness if the magnetic dipole has arbitrary inclinations to the pulsar rotational axis. More realistic pencil-beam patterns predict that 79% of the pulsars are double-peaked independently of their compactness. The theoretical predictions can be made consistent with the data if the magnetic dipole inclination to the rotational axis has an upper limit of 40+/-4 deg. We also discuss the effect of limited sensitivity of the X-ray instruments to detect weak pulses, which lowers the number of detected double-peaked profiles and makes the theoretical predictions to be consistent with the data even if the magnetic dipole does have random inclinations. This shows that the statistics of pulse profiles does not allow us to constrain the neutron star compactness. In contrast to the previous claims by Bulik et al. (2003), the data also do not require the magnetic inclination to be confined in a narrow interval.Comment: 14 pages, 10 figures, Astronomy and Astrophysics, in pres