Abstract
We present an in-depth study of the large scale, western jet of the microquasar XTE J1550−564, based on X-ray and radio observations performed in 2002–2003. The jet is spatially resolved in ...both observing windows. The X-ray jet is expanding in time along the axis of the jet's propagation: we observe the formation of a tail (∼2.25 arcsec), which appears to extend backwards with an apparent velocity ∼−0.10c. The origin of this feature is discussed in the framework of scenarios of energy dissipation. A single power law adequately describes the broad-band spectra, supporting a synchrotron origin of the X-ray emission. However, a spectral break at ≈1015 Hz is necessary in coincidence with a re-flare at 8.64 GHz in 2002 September. This finding may be indicative of emission from newly accelerated low-energy particles. The first detection of the jet is in 2001 February (F
8.64 GHz = 0.25 ± 0.09 mJy) in the flux rising phase. A phase of stable emission is followed by a rapid decay (t
decay = 167 ± 5 d). The decay at radio frequencies is significantly shorter than in X-rays (t
decay = 338 ± 14 d). We detected a high fraction (up to ∼9 per cent) of linearly polarized radiation at 4.8 and 8.6 GHz. The orientation of the electric vector is consistent with the picture of a shock-compressed magnetic field, and there are hints of variations on month-time-scales, possibly connected with the evolution of the jet structure.
In this paper we report on Expanded Very Large Array radio and Chandra and Swift X-ray observations of the outburst decay of the transient black hole candidate MAXI J1659-152 in 2011. We discuss the ...distance to the source taking the high inclination into account and conclude that the source distance is probably 6 ± 2 kpc. The lowest observed flux corresponds to a luminosity of
erg s−1. This, together with the orbital period of 2.4 h reported in the literature, suggests that the quiescent X-ray luminosity is higher than predicted on the basis of the orbital period-quiescent X-ray luminosity relationship. It is more in line with that expected for a neutron star, although the outburst spectral and timing properties reported in the literature strongly suggest that MAXI J1659-152 harbours a black hole. This conclusion is subject to confirmation of the lowest observed flux as the quiescent flux. The relation between the accretion and ejection mechanisms can be studied using the observed correlation between the radio and X-ray luminosities as these evolve over an outburst. We determine the behaviour of MAXI J1659-152 in the radio-X-ray diagram at low X-ray luminosities using the observations reported in this paper and at high X-ray luminosities using values reported in the literature. At high X-ray luminosities, the source lies closer to the sources that follow a correlation index steeper than 0.6-0.7. However, when compared to other sources that follow a steeper correlation index, the X-ray luminosity in MAXI J1659-152 is also lower. The latter can potentially be explained by the high inclination of MAXI J1659-152 if the X-ray emission comes from close to the source and the radio emission is originating in a more extended region. However, it is probable that the source was not in the canonical low-hard state during these radio observations and this may affect the behaviour of the source as well. At intermediate X-ray luminosities, the source makes the transition from the radio underluminous sources in the direction of the relation traced by the 'standard' correlation similar to what has been reported for H 1743−322 in the literature. However, MAXI J1659-152 remains underluminous with respect to this 'standard' correlation.
We constrain the LMC’s global parameters by modeling the first soft X-ray (0.4–6.8 keV) observations of the entire Large Magellanic Cloud (LMC) produced in a single pointing with moderate spectral ...resolution by the HaloSat CubeSat. These data are best fit with the sum of two thermal plasma components in collisional ionization equilibrium and a power-law. We find cool (0.210±0.014keV) and hot (0.89±0.14keV) components. The LMC’s total X-ray luminosity is(1.08±0.14)×1039erg s−1.X-ray binaries make up a large fraction of the emission with a luminosity of(6.0±0.8)×1038ergs−1, followed by cool gas from superbubbles, supernovae, and diffuse emission with a luminosity of(3.0±0.3)×1038erg s−1. The hot gas from star formation contributes the smallest fraction, with a luminosity of(1.9±0.5)×1038erg s−1. We estimate the total volume of the cool gas to be between(0.2–1.2)×1010pc3and the hot gas to be between(1.0–5)×107pc3for filling factors off= 1and0.2. These volumes result in a total thermal energy for the cool gas between (1.4–3)×1054ergs for electron densities of 0.017–0.04 cm−3, and a thermal energy for the hot gas between (1.7–4)×1053ergs for electron densities of 0.12–0.3 cm−3. This yields cooling timescales for the cool and hot gas of(1.5–3)×108years and(1.8–6)×107years, respectively
Recently, Nyland et al. argued that the radio emission observed in the center of the dwarf galaxy NGC 404 originates in a low-luminosity active galactic nucleus powered by a massive black hole (MBH, ...M lap 10 super(6) M sub(odot)). High-resolution radio detections of MBHs are rare. Here we present sensitive, contemporaneous Chandra X-ray, and very long baseline interferometry (VLBI) radio observations with the European VLBI Network. The source is detected in the X-rays, and shows no long-term variability. If the hard X-ray source is powered by accretion, the apparent low accretion efficiency would be consistent with a black hole (BH) in the hard state. Hard state BHs are known to show radio emission compact on the milliarcsecond scales. However, the central region of NGC 404 is resolved out on 10 mas (0.15-1.5 pc) scales. Our VLBI non-detection of a compact, partially self-absorbed radio core in NGC 404 implies that either the BH mass is smaller than 3 super(+5) sub(-2) x 10 super(5)M sub(odot ), or the source does not follow the fundamental plane of BH activity relation. An alternative explanation is that the central BH is not in the hard state. The radio emission observed on arcsecond (tens of parsecs) scales may originate in nuclear star formation or extended emission due to AGN activity, although the latter would not be typical considering the structural properties of low-ionization nuclear emission-line region galaxies with confirmed nuclear activity.
Abstract
Dark matter is a key piece of the current cosmological scenario, with weakly interacting massive particles (WIMPs) a leading dark matter candidate. WIMPs have not been detected in their ...conventional parameter space (100 GeV ≲
M
χ
≲ 100 TeV), a mass range accessible with current Imaging Atmospheric Cherenkov Telescopes. As ultraheavy dark matter (UHDM;
M
χ
≳ 100 TeV) has been suggested as an underexplored alternative to the WIMP paradigm, we search for an indirect dark matter annihilation signal in a higher mass range (up to 30 PeV) with the VERITAS
γ
-ray observatory. With 216 hr of observations of four dwarf spheroidal galaxies, we perform an unbinned likelihood analysis. We find no evidence of a
γ
-ray signal from UHDM annihilation above the background fluctuation for any individual dwarf galaxy nor for a joint-fit analysis, and consequently constrain the velocity-weighted annihilation cross section of UHDM for dark matter particle masses between 1 TeV and 30 PeV. We additionally set constraints on the allowed radius of a composite UHDM particle.
We present quasi-simultaneous, multi-epoch radio and X-ray measurements of Holmberg II X-1 using the European VLBI Network (EVN), the Karl G. Jansky Very Large Array (VLA), and the Chandra and Swift ...X-ray telescopes. The X-ray data show apparently hard spectra with steady X-ray luminosities four months apart from each other. In the high-resolution EVN radio observations, we have detected an extended milliarcsecond scale source with unboosted radio emission. The source emits non-thermal, likely optically thin synchrotron emission, and its morphology is consistent with a jet ejection. The 9-GHz VLA data show an arcsecond-scale triple structure of Holmberg II X-1 similar to that seen at lower frequencies. However, we find that the central ejection has faded by at least a factor of 7.3 over 1.5 yr. We estimate the dynamical age of the ejection to be higher than 2.1 yr. We show that such a rapid cooling can be explained with simple adiabatic expansion losses. These properties of Holmberg II X-1 imply that ULX radio bubbles may be inflated by ejecta instead of self-absorbed compact jets.
Although Galactic cosmic rays (protons and nuclei) are widely believed to be mainly accelerated by the winds and supernovae of massive stars, definitive evidence of this origin remains elusive nearly ...a century after their discovery. The active regions of starburst galaxies have exceptionally high rates of star formation, and their large size—more than 50 times the diameter of similar Galactic regions—uniquely enables reliable calorimetric measurements of their potentially high cosmic-ray density. The cosmic rays produced in the formation, life and death of massive stars in these regions are expected to produce diffuse -ray emission through interactions with interstellar gas and radiation. M82, the prototype small starburst galaxy, is predicted to be the brightest starburst galaxy in terms of -ray emission. Here we report the detection of >700-GeV -rays from M82. From these data we determine a cosmic-ray density of 250 eV cm-3 in the starburst core, which is about 500 times the average Galactic density. This links cosmic-ray acceleration to star formation activity, and suggests that supernovae and massive-star winds are the dominant accelerators.
Holmberg IX X-1 is an archetypal ultraluminous X-ray source (ULX). Here we study the properties of the optical counterpart and of its stellar environment using optical data from SUBARU/Faint Object ...Camera and Spectrograph, GEMINI/GMOS-N and Hubble Space Telescope (HST)/Advanced Camera for Surveys, as well as simultaneous Chandra X-ray data. The V ~ 22.6 spectroscopically identified optical counterpart is part of a loose cluster with an age 20 Myr. Consequently, the mass upper limit on individual stars in the association is about 20 M . The counterpart is more luminous than the other stars of the association, suggesting a non-negligible optical contribution from the accretion disk. An observed UV excess also points to non-stellar light similar to X-ray active low-mass X-ray binaries. A broad He II Delta *l4686 emission line identified in the optical spectrum of the ULX further suggests optical light from X-ray reprocessing in the accretion disk. Using stellar evolutionary tracks, we have constrained the mass of the counterpart to be 10 M , even if the accretion disk contributes significantly to the optical luminosity. Comparison of the photometric properties of the counterpart with binary models show that the donor may be more massive, 25 M , with the ULX system likely undergoing case AB mass transfer. Finally, the counterpart exhibits photometric variability of 0.14 mag between two HST observations separated by 50 days which could be due to ellipsoidal variations and/or disk reprocessing of variable X-ray emission.
We observed the Ultraluminous X-ray Source (ULX) IC 342 X-1 simultaneously in X-ray and radio with Chandra and the JVLA to investigate previously reported unresolved radio emission coincident with ...the ULX. The Chandra data reveal a spectrum that is much softer than observed previously and is well modelled by a thermal accretion disc spectrum. No significant radio emission above the rms noise level was observed within the region of the ULX, consistent with the interpretation as a thermal state though other states cannot be entirely ruled out with the current data. We estimate the mass of the black hole using the modelled inner disc temperature to be
$30 \,\mathrm{M_{{\odot }}} \lesssim M\sqrt{\mathrm{cos}i}\lesssim 200 \,\mathrm{M_{{\odot }}}$
based on a Shakura–Sunyaev disc model. Through a study of the hardness and high-energy curvature of available X-ray observations, we find that the accretion state of X-1 is not determined by luminosity alone.
We obtained multi-epoch Very Large Telescope optical spectroscopic data in 2011 and 2012 on the ultraluminous X-ray source NGC 5408 X-1. We confirm that the He ii λ4686 line has a broad component ...with an average full width at half-maximum of v = 780 ± 64 km s−1 with a variation of ∼13 per cent during observations spanning over four years, and is consistent with the origin in the accretion disc. The deepest optical spectrum does not reveal any absorption line from a donor star. Our aim was to measure the radial velocity curve and estimate the parameters of the binary system. We find an upper limit on the semi-amplitude of the radial velocity of K = 132 ± 42 km s−1. A search for a periodic signal in the data resulted in no statistically significant period. The mass function and constraints on the binary system imply a black hole mass of less than ∼510 M. Whilst, a disc irradiation model may imply a black hole mass smaller than ∼431-1985 M, depending on inclination. Our data can also be consistent with an unexplored orbital period range from a couple of hours to a few days, thus with a stellar-mass black hole and a subgiant companion.
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