Deep optical observations of the central X-ray source in the Puppis A supernova remnant

X-ray observations reveiled a group of radio-silent isolated neutron stars (INSs) at the centre of young supernova remnants (SNRs), dubbed central compact objects or CCOs, with properties different from those of classical rotation-powered pulsars. In at least three cases, evidence points towards CCOs being low-magnetized INSs, born with slow rotation periods, and possibly accreting from a debris disc of material formed out of the supernova event. Understanding the origin of the diversity of the CCOs can shed light on supernova explosion and neutron star formation models. Optical/infrared (IR) observations are crucial to test different CCO interpretations. The aim of our work is to perform a deep optical investigation of the CCO RX J0822.0-4300 in the Puppis A SNR, one of the most poorly understood in the CCO family. By using as a reference the Chandra X-ray coordinates of RX J0822.0-4300, we performed deep optical observations in the B, V and I bands with the Very Large Telescope (VLT). We found no candidate optical counterpart within 3 sigma of the computed Chandra X-ray position down to 5 sigma limits of B~27.2, V~26.9, and I~25.6, the deepest obtained in the optical band for this source. These limits confirm the non-detection of a companion brighter than an M5 dwarf. At the same time, they do not constrain optical emission from the neutron star surface, while emission from the magnetosphere would require a spectral break in the optical/IR.Comment: 4 pages, 2 figures, Astronomy and Astrophysics, accepte

The Spectral Evolution of Transient Anomalous X-ray Pulsar XTE J1810--197

(Abridged) We present a multi-epoch spectral study of the Transient Anomalous X-ray Pulsar XTE J1810-197 obtained with the XMM X-ray telescope. Four observations taken over the course of a year reveal strong spectral evolution as the source fades from outburst. The origin of this is traced to the individual decay rates of the pulsar's spectral components. A 2-T fit at each epoch requires nearly constant temperatures of kT=0.25 & 0.67 keV while the component luminosities decrease exponentially with tau=900 & 300d, respectively. One possible interpretation is that the slowly decaying cooler component is the radiation from a deep heating event that affected a large fraction of the crust, while the hotter component is powered by external surface heating at the foot-points of twisted magnetic field lines, by magnetospheric currents that are decaying more rapidly. The energy-dependent pulse profile of XTE J1810-197 is well modeled at all epochs by the sum of a sine and triangle function. These profiles peak at the same phase, suggesting a concentric surface emission geometry. The spectral and pulse evolution together argue against the presence of a significant ``power-law'' contribution to the X-ray spectrum below 8 keV. The extrapolated flux is projected to return to the historic quiescent level, characterized by an even cooler blackbody spectrum, by the year 2007.Comment: 12 pages, 6 Figures, Latex, emulateapj. To appear in the Astrophysical Journa

Imaging X-ray, Optical, and Infrared Observations of the Transient Anomalous X-ray Pulsar XTE J1810-197

We report X-ray imaging, timing, and spectral studies of XTE J1810-197, a 5.54s pulsar discovered by Ibrahim et al. (2003) in recent RXTE observations. In a set of short exposures with the Chandra HRC camera we detect a strongly modulated signal (55+/-4% pulsed fraction) with the expected period located at (J2000) 18:09:51.08, -19:43:51.7, with a uncertainty radius of 0.6 arcsec (90% C.L.). Spectra obtained with XMM-Newton are well fitted by a two-component model that typically describes anomalous X-ray pulsars (AXPs), an absorbed blackbody plus power law with parameters kT = 0.67+/-0.01 keV, Gamma=3.7+/-0.2, N_H=(1.05+/-0.05)E22 cm^-2, and Fx(0.5-10 keV) = 3.98E-11 ergs/cm2/s. Alternatively, a 2T blackbody fit is just as acceptable. The location of CXOU J180951.1-194351 is consistent with a point source seen in archival Einstein, Rosat, & ASCA images, when its flux was nearly two orders-of-magnitude fainter, and from which no pulsations are found. The spectrum changed dramatically between the "quiescent" and "active" states, the former can be modeled as a softer blackbody. Using XMM timing data, we place an upper limit of 0.03 lt-s on any orbital motion in the period range 10m-8hr. Optical and infrared images obtained on the SMARTS 1.3m telescope at CTIO show no object in the Chandra error circle to limits V=22.5, I=21.3, J=18.9, & K=17.5. Together, these results argue that CXOU J180951.1-194351 is an isolated neutron star, one most similar to the transient AXP AX J1844.8-0256. Continuing study of XTE J1810-197 in various states of luminosity is important for understanding and possibly unifying a growing class of isolated, young neutron stars that are not powered by rotation.Comment: 12 pages, 7 figures, AAS LaTex, uses emulateapj5.sty. Updated to include additional archival data and a new HRC observation. To appear in The Astrophysical Journa

Investigating CXOU J163802.6-471358: a new pulsar wind nebula in the Norma region?

We present the first analysis of the extended source CXOU J163802.6--471358, which was discovered serendipitously during the {\em Chandra} X-ray survey of the Norma region of the Galactic spiral arms. The X-ray source exhibits a cometary appearance with a point source and an extended tail region. The complete source spectrum is fitted well with an absorbed power law model and jointly fitting the {\em Chandra} spectrum of the full source with one obtained from an archived {\em XMM-Newton} observation results in best fit parameters $N_{\rm H}$ $=1.5^{+0.7}_{-0.5}\times10^{23} \text{cm}{^{-2}}$ and $\Gamma=1.1^{+0.7}_{-0.6}$ (90$$ confidence uncertainties). The unabsorbed luminosity of the full source is then $L_X\sim 4.8\times10^{33}d_{10}^2$ergs s$^{-1}$ with $d_{10}=d/10$kpc, where a distance of 10 kpc is a lower bound inferred from the large column density. The radio counterpart found for the source using data from the Molonglo Galactic Plane Survey epoch-2 (MGPS-2) shows an elongated tail offset from the X-ray emission. No infrared counterpart was found. The results are consistent with the source being a previously unknown pulsar driving a bow shock through the ambient medium

Discovery of a highly energetic pulsar associated with IGR J14003-6326 in a young uncataloged Galactic supernova remnant G310.6-1.6

We report the discovery of 31.18 ms pulsations from the INTEGRAL source IGR J14003-6326 using the Rossi X-ray Timing Explorer (RXTE). This pulsar is most likely associated with the bright Chandra X-ray point source lying at the center of G310.6-1.6, a previously unrecognised Galactic composite supernova remnant with a bright central non-thermal radio and X-ray nebula, taken to be the pulsar wind nebula (PWN). PSR J1400-6325 is amongst the most energetic rotation-powered pulsars in the Galaxy, with a spin-down luminosity of Edot = 5.1E+37 erg.s-1. In the rotating dipole model, the surface dipole magnetic field strength is B_s = 1.1E+12 G and the characteristic age tau_c = P/2Pdot = 12.7 kyr. The high spin-down power is consistent with the hard spectral indices of the pulsar and the nebula of 1.22 +/- 0.15 and 1.83 +/- 0.08, respectively, and a 2-10 keV flux ratio F_PWN/F_PSR ~ 8. Follow-up Parkes observations resulted in the detection of radio emission at 10 and 20 cm from PSR J1400-6325 at a dispersion measure of ~ 560 cm-3 pc, which implies a relatively large distance of 10 +/- 3 kpc. However, the resulting location off the Galactic Plane of ~ 280 pc would be much larger than the typical thickness of the molecular disk, and we argue that G310.6-1.6 lies at a distance of ~ 7 kpc. There is no gamma-ray counterpart to the nebula or pulsar in the Fermi data published so far. A multi-wavelength study of this new composite supernova remnant, from radio to very-high energy gamma-rays, suggests a young (< 1000 yr) system, formed by a sub-energetic (~ 1E+50 ergs), low ejecta mass (M_ej ~ 3 Msun) SN explosion that occurred in a low-density environment (n_0 ~ 0.01 cm-3).Comment: 9 pages, 6 figures, 2 tables. Accepted for publication in ApJ (after responding to referee's comments, expanded version after the radio detection of the pulsar

The Next Geminga: Deep Multiwavelength Observations of a Neutron Star Identified with 3EG J1835+5918

We describe Chandra, HST, and radio observations that reveal a radio-quiet but magnetospherically active neutron star in the error circle of the high-energy gamma-ray source 3EG J1835+5918, the brightest of the unidentified EGRET sources at high Galactic latitude. A Chandra ACIS-S spectrum of the ultrasoft X-ray source RX J1836.2+5925, suggested by Mirabal & Halpern as the neutron star counterpart of 3EG J1835+5918, requires two components: a blackbody of T~3x10^5 K and a hard tail that can be parameterized as a power law of photon index Gamma~2. An upper limit of d < 800 pc can be derived from the blackbody fit under an assumption of R = 10 km. Deep optical imaging with the HST STIS CCD failed to detect this source to a limit of V > 28.5, thus f_X/f_V > 6000 and d > 250 pc assuming the X-ray fitted temperature for the full surface. Repeated observations with the 76 m Lovell telescope at Jodrell Bank place an upper limit of < 0.1 mJy on the flux density at 1400 MHz for a pulsar with P > 0.1 s, and < 0.25 mJy for a ~10 ms pulsar at the location of RX J1836.2+5925. All of this evidence points to an older, possibly more distant version of the highly efficient gamma-ray pulsar Geminga, as the origin of the gamma-rays from 3EG J1835+5918.Comment: 4 pages, 4 figures, accepted for publication in ApJ Letter

An X-ray Image of the Composite SNR G16.7+0.1

We have observed the Galactic supernova remnant G16.7+0.1 for 13 ks using the EPIC cameras aboard the XMM-Newton X-ray Observatory, producing the first X-ray image of the remnant. This composite radio remnant has a core radio flux density of only 100 mJy, making it one of the faintest radio synchrotron nebulae yet detected, although the core-to-shell flux ratio at 6 cm is typical of the growing class of composite remnants. Our image is seriously contaminated by bright arcs produced by singly reflected X-rays from the X-ray binary GX17+2 which lies just outside the field of view, providing an interesting data analysis challenge. Nonetheless, the remnant's synchrotron core is clearly detected. We report on the spectrum and intensity of the core emission as well as on our search for emission from the thermal shell, and describe the constraints these observations provide on the remnant's distance, age, and central pulsar properties.Comment: 12 pages, 4 figures. Scheduled to appear in the September 1 Astrophysical Journa

X-ray Spectra of Young Pulsars and Their Wind Nebulae: Dependence on Spin-down Energy

An observational model is presented for the spectra of young rotation-powered pulsars and their nebulae based on a study of nine bright Crab-like pulsar systems observed with the Chandra X-ray observatory. A significant correlation is discovered between the X-ray spectra of these pulsars (PSRs) and that of their associated pulsar wind nebulae (PWNe), both of which are observed to be a function of the spin down energy, Edot . The 2 - 10 keV spectra of these objects are well characterized by an absorbed power-law model with photon indices, Gamma, in the range of 0.6 < Gamma_PSR < 2.1 and 1.3 < Gamma_PWN < 2.3, for the pulsars and their nebulae, respectively. A linear regression fit relating these two sets of indexes yields, Gamma_PWN = 0.86 +/- 0.20 + (0.72 +/- 0.13) X Gamma_PSR, with a correlation coefficient of r = 0.96. The spectra of these pulsars are found to steepen as Gamma = Gamma_max + alpha / sqrt(Edot), with Gamma_max providing an observational limit on the spectral slopes of young rotation-powered pulsars. These results reveal basic properties of young pulsar systems, allow new observational constraints on models of pulsar wind emission, and provide a means of predicting the energetics of such systems when lacking detected pulsations.Comment: 6 pages, 3 figures, 1 table, LaTex, emulateapj5.sty. To appear in the June 20, 2003 issue of the Astrophysical Journa