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Register a new userA strong excess in the 20-100 keV emission of NGC 1365
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We present a new Suzaku observation of the obscured AGN in NGC 1365, revealing an unexpected excess of X-rays above 20 keV of at least a factor ~2 with respect to the extrapolation of the best-fitting 3-10 keV model. Additional Swift-BAT and Integral-IBIS observations show that the 20-100 keV is concentrated within ~1.5 arcmin from the center of the galaxy, and is not significantly variable on time scales from days to years. A comparison of this component with the 3-10 keV emission, which is characterized by a rapidly variable absorption, suggests a complex structure of the circumnuclear medium, consisting of at least two distinct components with rather different physical properties, one of which covering >80% of the source with a column density NH~3-4x10^24 cm^(-2). An alternative explanation is the presence of a double active nucleus in the center of NGC 1365.
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The morphology and characteristics of the so-called GeV gamma-ray excess detected in the Milky Way lead us to speculate about a possible common origin with the 511 keV line mapped by the SPI experiment about ten years ago. In the previous version of our paper, we assumed 30 GeV dark matter particles annihilating into $b bar{b}$ and obtained both a morphology and a 511 keV flux (phi_{511 keV} ~ 10^{-3} ph/cm^2/s) in agreement with SPI observation. However our estimates assumed a negligible number density of electrons in the bulge which lead to an artificial increase in the flux (mostly due to negligible Coulomb losses in this configuration). Assuming a number density greater than $n_e > 10^{-3} cm^{-3}$, we now obtain a flux of 511 keV photons that is smaller than phi_{511 keV} ~ 10^{-6} ph/cm^2/s and is essentially in agreement with the 511 keV flux that one can infer from the total number of positrons injected by dark matter annihilations into $b bar{b}$. We thus conclude that -- even if 30 GeV dark matter particles were to exist-- it is impossible to establish a connexion between the two types of signals, even though they are located within the same 10 deg region in the galactic centre.
Context. Recent findings of line emission at 3.5 keV in both individual and stacked X-ray spectra of galaxy clusters have been speculated to have dark matter origin. Aims. If the origin is indeed dark matter, the emission line is expected to be detectable from the Milky Way dark matter halo. Methods. We perform a line search in public Chandra X-ray observations of the region near Sgr A*. We derive upper limits on the line emission flux for the 2.0-9.0 keV energy interval and discuss their potential physical interpretations including various scenarios of decaying and annihilating dark matter. Results. While find no clear evidence for its presence, the upper flux limits are not inconsistent with the recent detections for conservative mass profiles of the Milky Way. Conclusions. The results depends mildly on the spectral modelling and strongly on the choice of dark matter profile.
We study the extended nuclear emission of the starburst galaxy NGC 1365. A weak obscured AGN and a strong starburst both contribute to the observed X-ray, optical, infrared, and radio emission in the inner 2kpc. The X-ray emission is spatially resolved, allowing comparison with multiwavelength data that highlights the structures dominating the nuclear region: the AGN, the nuclear spiral, the circumnuclear starburst ring, and nuclear outflow. The ultrasoft X-ray emission below 0.5keV is spatially coincident with the conical outflow traced by higher excitation optical emission lines like [O III] and [Ne III]. The strong starburst concentrated in super-star clusters in a circumnuclear ring with radius ~1kpc dominates the 0.5-1.5keV emission and is visible in radio, molecular CO, and infrared maps of the central kiloparsec. The hard (2-10keV) emission is dominated by the obscured AGN, but also contributes to the emission from relatively old (~7Myr) but still enshrouded in dust and extremely massive (10^7Msun) super-star clusters (Galliano 2008), hidden from view in the optical and soft X-ray bands. In the Appendix we present the X-ray spectroscopy and photometry of BL Lac MS 0331.3-3629, a high-energy peaked BL Lac candidate at z=0.308, serendipitously detected in one Chandra and five XMM-Newton observations of NGC1365.
The Fermi-LAT Galactic Center excess and the 511 keV positron-annihilation signal from the inner Galaxy bare a striking morphological similarity. We propose that both can be explained through a scenario in which millisecond pulsars produce the Galactic Center excess and their progenitors, low-mass X-ray binaries, the 511 keV signal. As a proof-of-principle we study a specific population synthesis scenario from the literature involving so-called ultracompact X-ray binaries. Moreover, for the first time, we quantitatively show that neutron star, rather than black hole, low-mass X-ray binaries can be responsible for the majority of the positrons. In this particular scenario binary millisecond pulsars can be both the source of the Fermi-LAT $gamma$-ray excess and the bulge positrons. Future avenues to test this scenario are discussed.
We present the first Chandra/ACIS imaging study of the circumnuclear region of the nearby Seyfert galaxy NGC 1365. The X-ray emission is resolved into point-like sources and complex, extended emission. The X-ray morphology of the extended emission shows a biconical soft X-ray emission region extending ~5 kpc in projection from the nucleus, coincident with the high excitation outflow cones seen in optical emission lines particularly to the northwest. Harder X-ray emission is detected from a kpc-diameter circumnuclear ring, coincident with the star-forming ring prominent in the Spitzer mid-infrared images; this X-ray emission is partially obscured by the central dust lane of NGC 1365. Spectral fitting of spatially separated components indicates a thermal plasma origin for the soft extended X-ray emission (kT=0.57 keV). Only a small amount of this emission can be due to photoionization by the nuclear source. Detailed comparison with [OIII]5007 observations shows the hot interstellar medium (ISM) is spatially anticorrelated with the [OIII] emitting clouds and has thermal pressures comparable to those of the [OIII] medium, suggesting that the hot ISM acts as a confining medium for the cooler photoionized clouds. The abundance ratios of the hot ISM are fully consistent with the theoretical values for enrichment from Type II supernovae, suggesting that the hot ISM is a wind from the starburst circumnuclear ring. X-ray emission from a ~450 pc long nuclear radio jet is also detected to the southeast.
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