Mapping the allowed parameter space for decaying dark matter models

I consider constraints on a phenomenological decaying-dark-matter model, in which two weakly-interacting massive particle (WIMP) species have a small mass splitting, and in which the heavier particle decays to the lighter particle and a massless particle on cosmological timescales. The decay parameter space is parameterized by $v_k$, the speed of the lighter particle in the center-of-mass frame of the heavier particle prior to decay, and the decay time $\tau$. Since I consider the case in which dark-matter halos have formed before there has been significant decay, I focus on the effects of decay in already-formed halos. I show that the $v_k-\tau$ parameter space may be constrained by observed properties of dark-matter halos. I highlight which set of observations is likely to yield the cleanest constraints on $v_k-\tau$ parameter space, and calculate the constraints in those cases in which the effect of decay on the observables can be calculated without N-body simulations of decaying dark matter. I show that for $v_k \gtrsim 5\times 10^3$ km s$^{-1}$, the z=0 galaxy cluster mass function and halo mass-concentration relation constrain $\tau \gtrsim$ 40 Gyr, and that precise constraints on $\tau$ for smaller $v_k$ will require N-body simulations.Comment: 14 pages, 5 figures, references added, replaced to match version published in Phys. Rev.

Dark-Matter Decays and Self-Gravitating Halos

We consider models in which a dark-matter particle decays to a slightly less massive daughter particle and a noninteracting massless particle. The decay gives the daughter particle a small velocity kick. Self-gravitating dark-matter halos that have a virial velocity smaller than this velocity kick may be disrupted by these particle decays, while those with larger virial velocities will be heated. We use numerical simulations to follow the detailed evolution of the total mass and density profile of self-gravitating systems composed of particles that undergo such velocity kicks as a function of the kick speed (relative to the virial velocity) and the decay time (relative to the dynamical time). We show how these decays will affect the halo mass-concentration relation and mass function. Using measurements of the halo mass-concentration relation and galaxy-cluster mass function to constrain the lifetime--kick-velocity parameter space for decaying dark matter, we find roughly that the observations rule out the combination of kick velocities greater than 100 km/s and decay times less than a few times the age of the Universe.Comment: 17 pages, 10 figures, replaced with published versio

Precise Localization of the Soft Gamma Repeater SGR 1627-41 and the Anomalous X-ray Pulsar AXP 1E1841-045 with Chandra

We present precise localizations of AXP 1E1841-045 and SGR 1627-41 with Chandra. We obtained new infrared observations of SGR 1627-41 and reanalyzed archival observations of AXP 1E1841-045 in order to refine their positions and search for infrared counterparts. A faint source is detected inside the error circle of AXP 1E1841-045. In the case of SGR 1627-41, several sources are located within the error radius of the X-ray position and we discuss the likelihood of one of them being the counterpart. We compare the properties of our candidates to those of other known AXP and SGR counterparts. We find that the counterpart candidates for SGR 1627-41 and SGR 1806-20 would have to be intrinsically much brighter than AXPs to have detectable counterparts with the observational limits currently available for these sources. To confirm the reported counterpart of SGR 1806-20, we obtained new IR observations during the July 2003 burst activation of the source. No brightening of the suggested counterpart is detected, implying that the counterpart of SGR 1806-20 remains yet to be identified.Comment: 29 pages, 4 figures, accepted for publication in Ap

A Huge Drop in X-ray Luminosity of the Non-Active Galaxy RXJ1242.6-1119A, and First Post-Flare Spectrum - Testing the Tidal Disruption Scenario

It has been suggested that an unavoidable consequence of the existence of supermassive black holes, and the best diagnostic of their presence in non-active galaxies, would be occasional tidal disruption of stars captured by the black holes. These events manifest themselves in form of luminous flares powered by accretion of debris from the disrupted star into the black hole. Candidate events among optically non-active galaxies emerged in the past few years. For the first time, we have looked with high spatial and spectral resolution at one of these most extreme variability events ever recorded among galaxies. Here, we report measuring a factor ~200 drop in luminosity of the X-ray source RXJ 1242-1119 with the X-ray observatories Chandra and XMM-Newton, and perform key tests of the favored outburst scenario, tidal disruption of a star by a supermassive black hole. We show that the detected `low-state' emission has properties such that it must still be related to the flare. The power-law shaped post-flare X-ray spectrum indicates a `hardening' compared to outburst. The inferred black hole mass, the amount of liberated energy, and the duration of the event favor an accretion event of the form expected from the (partial or complete) tidal disruption of a star (abstract abbreviated).Comment: to appear in March 1 issue of ApJ Letters (submitted Nov. 10, accepted in Dec. 2003); background information available at http://www.xray.mpe.mpg.de/~skomossa

The discovery of a new non-thermal X-ray filament near the Galactic Centre

We report the discovery by XMM-Newton and Chandra of a hard extended X-ray source (XMM J174540-2904.5) associated with a compact non-thermal radio filament (the Sgr A-E `wisp'=1LC 359.888-0.086= G359.88-0.07), which is located within ~4 arcmin of the Galactic Centre. The source position is also coincident with the peak of the molecular cloud, M -0.13-0.08 (the `20 km/s' cloud). The X-ray spectrum is non-thermal with an energy index of 1.0 (+1.1 -0.9) and column density of 38 (+7 -11) x 10^22 H/cm2. The observed 2--10 keV flux of 4 x 10^-13 erg/s/cm2 converts to an unabsorbed X-ray luminosity of 1 x 10^34 erg/s assuming a distance of 8.0 kpc. The high column density strongly suggests that this source is located in or behind the Galactic Centre Region. Taking account of the broad-band spectrum, as well as the source morphology and the positional coincidence with a molecular cloud, we concluded that both the radio and X-ray emission are the result of synchrotron radiation. This is the first time a filamentary structure in the Galactic Centre Region. has been shown, unequivocally, to have a non-thermal X-ray spectrum.Comment: 5 pages, 4 figures, Accepted for publication in MNRAS, also found in http://www.star.le.ac.uk/~mas/research/paper/#Sakano2002mnra

A detailed look at the thermal and non-thermal X-ray emission from the Vela supernova remnant with SRG/eROSITA

Aims. Our goal is the characterization of the hot ejecta and shocked interstellar medium (ISM) associated to the Vela supernova remnant (SNR), as well as the relativistic electrons injected into the ambient medium by its central pulsar. To achieve this, we analyze the X-ray data set of Vela acquired by SRG/eROSITA during its first four all-sky surveys. Methods. Apart from multi-band imaging, a quantitative view of the physical parameters affecting the observed thermal and non-thermal emission is obtained by performing spatially resolved X-ray spectroscopy of over 500 independent regions using multi-component spectral models. Results. Imaging demonstrates that the X-ray emission of the Vela SNR consists of at least three morphologically distinct components, with shell-like structures dominating below 0.6 keV, radial outward-directed features becoming apparent at medium energies, and the pulsar wind nebula (PWN) dominating the hard emission above 1.4 keV. Our spectroscopy reveals a highly structured distribution of X-ray absorption column densities, which intriguingly appears anticorrelated with optical extinction measurements. We find evidence for multiple ejecta clumps inside and outside the shell, within which we find a strongly supersolar concentration of neon and magnesium relative to oxygen. This includes the bright shrapnel D, in which we separate shocked ISM in the soft bow-shock from a hot, ejecta-rich clump at its apex, based on the new data. Finally, we find an extremely extended, smoothly decreasing distribution of synchrotron emission from the PWN, which extends up to 14 pc from the pulsar, with a total X-ray luminosity of $1.5\times10^{-3}$ of the pulsar's spin-down power. The extended emission likely traces a relativistic electron population in an ISM-level magnetic field, which requires the existence of a TeV counterpart powered by inverse Compton radiation.Comment: 24+3 pages, 13+4 figures, accepted for publication in Astronomy & Astrophysics; Abstract abridge

The X-ray Halo of GX5-1

Using Chandra observations we have measured the energy-resolved dust-scattered X-ray halo around the low-mass X-ray binary GX5-1, which shows signs of both singly- and multiply-scattered X-rays. We compared the observed X-ray halo at various energies to predictions from a range of dust models. These fits used both smoothly-distributed dust as well as dust in clumped clouds, with CO and 21 cm observations helping to determine the position of the clouds along the line of sight. We found that the BARE-GR-B model of Zubko, Dwek & Arendt (2004 generally led to the best results, although inadequacies in both the overall model and the data limit our conclusions. We also found that the composite dust models of Zubko, Dwek & Arendt (2004), especially the ``no carbon'' models, gave uniformly poor results. Although models using cloud positions and densities derived naively from CO and 21 cm data gave generally poor results, plausible adjustments to the distance of the largest cloud and the mass of a cloud in the expanding 3 kpc Arm lead to significantly improved fits. We suggest that combining X-ray halo, CO, and 21 cm observations will be a fruitful method to improve our understanding of both the gas and dust phases of the interstellar medium.Comment: 10 pages, accepted by Ap

Infrared Spectroscopy of Symbiotic Stars. IV. V2116 Ophiuchi/GX 1+4, The Neutron Star Symbiotic

We have computed, based on 17 infrared radial velocities, the first set of orbital elements for the M giant in the symbiotic binary V2116 Ophiuchi. The giant's companion is a neutron star, the bright X-ray source GX 1+4. We find an orbital period of 1161 days by far the longest of any known X-ray binary. The orbit has a modest eccentricity of 0.10 with an orbital circularization time of less than 10^6 years. The large mass function of the orbit significantly restricts the mass of the M giant. Adopting a neutron-star mass of 1.35M(Sun), the maximum mass of the M giant is 1.22M(Sun), making it the less massive star. Derived abundances indicate a slightly subsolar metallicity. Carbon and nitrogen are in the expected ratio resulting from the red-giant first dredge-up phase. The lack of O-17 suggests that the M-giant has a mass less than 1.3M(Sun), consistent with our maximum mass. The red giant radius is 103R(Sun), much smaller than the estimated Roche lobe radius. Thus, the mass loss of the red giant is via a stellar wind. Although the M giant companion to the neutron star has a mass similar to the late-type star in low-mass X-ray binaries, its near-solar abundances and apparent runaway velocity are not fully consistent with the properties of this class of stars.Comment: In press to The Astrophysical Journal (10 April 2006 issue). 23 page