arXiv subjects

A. Wolter

Publications and source records attributed to A. Wolter.

At least 37 records · Page 2Linked to original sources

arXiv2019

Te-REX: a sample of extragalactic TeV-emitting candidates

The REX (Radio-Emitting X-ray sources) is a catalogue produced by cross-matching X-ray data from the ROSAT-PSPC archive of pointed observations and radio data from the NRAO VLA Sky Survey, aimed at the selection of blazars. From the REX catalogue, we select a well defined and statistically complete sample of high-energy peaked BL Lac (HBL). HBL are expected to be the most numerous class of extragalactic TeV emitting sources. Specifically, we have considered only the REX sources in the currently planned CTA extragalactic survey area satisfying specific criteria and with an optical spectroscopic confirmation. We obtain 46 HBL candidates that we called Te-REX (TeV-emitting REX). We estimate the very high-energy gamma-ray emission, in the TeV domain, using an empirical approach i.e. using specific statistical relations between gamma-rays (at GeV energies) and radio/X-rays properties observed in bright HBL from the literature. We compare the spectral energy distributions (SEDs) with the sensitivities of current and upcoming Cherenkov telescopes and we predict that 14 Te-REX could be detectable with 50 hours of observations of CTA and 7 of them also with current Cherenkov facilities in 50 hours. By extrapolating these numbers on the total extragalactic sky, we predict that about 800 HBL could be visible in pointed CTA observations and about 400 with current Cherenkov telescopes in 50 hours. Interestingly, our predictions show that a non-negligible fraction (about 30%) of the HBL that will be detectable by CTA is composed of relatively weak objects whose optical nuclear emission is swamped by the host-galaxy light and not (yet) detected by Fermi-LAT.

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arXiv2019

GASP. XXII The molecular gas content of the JW100 jellyfish galaxy at z~0.05: does ram pressure promote molecular gas formation?

Within the GASP survey, aimed at studying the effect of the ram-pressure stripping on the star formation quenching in cluster galaxies, we analyze here ALMA observations of the jellyfish galaxy JW100. We find an unexpected large amount of molecular gas ($\sim 2.5 \times 10^{10} M_{\odot}$), 30\% of which is located in the stripped gas tail out to $\sim$35 kpc from the galaxy center. The overall kinematics of molecular gas is similar to the one shown by the ionized gas, but for clear signatures of double components along the stripping direction detected only out to 2 kpc from the disk. The line ratio $r_{21}$ has a clumpy distribution and in the tail can reach large values ($\geq 1$), while its average value is low (0.58 with a 0.15 dispersion). All these evidence strongly suggest that the molecular gas in the tail is newly born from stripped HI gas or newly condensed from stripped diffuse molecular gas. The analysis of interferometric data at different scales reveals that a significant fraction ($\sim 40\%$) of the molecular gas is extended over large scales ($\geq 8$ kpc) in the disk, and this fraction becomes predominant in the tail ($\sim 70\%$). By comparing the molecular gas surface density with the star formation rate surface density derived from the \Ha emission from MUSE data, we find that the depletion time on 1 kpc scale is particularly large ($5-10$ Gyr) both within the ram-pressure disturbed region in the stellar disk, and in the complexes along the tail.

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arXiv2018

The two Ultraluminous X-ray sources in the galaxy NGC 925

NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC 925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality, simultaneous XMM-Newton and NuSTAR data of both sources. Although at a first glance ULX-1 resembles an intermediate mass black hole candidate (IMBH) because of its high X-ray luminosity ($(2$$-$$4)\times10^{40}$ erg s$^{-1}$) and its spectral/temporal features, a closer inspection shows that its properties are more similar to those of a typical super-Eddington accreting stellar black hole and we classify it as a `broadened disc' ultraluminous X-ray source. Based on the physical interpretation of this spectral state, we suggest that ULX-1 is seen at small inclination angles, possibly through the evacuated cone of a powerful wind originating in the accretion disc. The spectral classification of ULX-2 is less certain, but we disfavour an IMBH accreting at sub-Eddington rates as none of its spectral/temporal properties can be associated to either the soft or hard state of Galactic accreting black hole binaries.

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arXiv2018

A new Ultraluminous X-ray source in the galaxy NGC 5907

We report on the serendipitous discovery of a new transient in NGC 5907, at a peak luminosity of 6.4x10^{39} erg/s. The source was undetected in previous 2012 Chandra observations with a 3 sigma upper limit on the luminosity of 1.5x10^{38} erg/s, implying a flux increase of a factor of >35. We analyzed three recent 60ks/50ks Chandra and 50ks XMM-Newton observations, as well as all the available Swift observations performed between August 2017/March 2018. Until the first half of October 2017, Swift observations do not show any emission from the source. The transient entered the ULX regime in less than two weeks and its outburst was still on-going at the end of February 2018. The 0.3-10 keV spectrum is consistent with a single multicolour blackbody disc (kT~1.5 keV). The source might be a ~30 solar mass black hole accreting at the Eddington limit. However, although we did not find evidence of pulsations, we cannot rule-out the possibility that this ULX hosts an accreting neutron star.

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arXiv2018

A multi-wavelength study of the evolution of Early-Type Galaxies in Groups: the ultraviolet view

ABRIDGED- The UV-optical color magnitude diagram (CMD) of rich galaxy groups is characterised by a well developed Red Sequence (RS), a Blue Cloud (BC) and the so-called Green Valley (GV). Loose, less evolved groups of galaxies likely not virialized yet may lack a well defined RS. This is actually explained in the framework of galaxy evolution. We are focussing on understanding galaxy migration towards the RS, checking for signatures of such a transition in their photometric and morphological properties. We report on the UV properties of a sample of ETGs galaxies inhabiting the RS. The analysis of their structures, as derived by fitting a Sersic law to their UV luminosity profiles, suggests the presence of an underlying disk. This is the hallmark of dissipation processes that still must have a role in the evolution of this class of galaxies. SPH simulations with chemo-photometric implementations able to match the global properties of our targets are used to derive their evolutionary paths through UV-optical CDM, providing some fundamental information such as the crossing time through the GV, which depends on their luminosity. The transition from the BC to the RS takes several Gyrs, being about 3-5 Gyr for the the brightest galaxies and more long for fainter ones, if it occurs. The photometric study of nearby galaxy structures in UV is seriously hampered by either the limited FoV of the cameras (e.g in HST) or by the low spatial resolution of the images (e.g in the GALEX). Current missions equipped with telescopes and cameras sensitive to UV wavelengths, such as Swift-UVOT and Astrosat-UVIT, provide a relatively large FoV and better resolution than the GALEX. More powerful UV instruments (size, resolution and FoV) are obviously bound to yield fundamental advances in the accuracy and depth of the surface photometry and in the characterisation of the galaxy environment.

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arXiv2017

An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907

Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any X-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43~s in 2003 to 1.13~s in 2014. It has an isotropic peak luminosity of about 1000 times the Eddington limit for a NS at 17.1~Mpc. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity > 10^{41} erg/s) might harbor NSs.

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arXiv2017

Investigating early-type galaxy evolution with a multiwavelength approach. II. The UV structure of 11 galaxies with Swift-UVOT

GALEX detected a significant fraction of early-type galaxies showing Far-UV bright structures. These features suggest the occurrence of recent star formation episodes. We aim at understanding their evolutionary path[s] and the mechanisms at the origin of their UV-bright structures. We investigate with a multi-lambda approach 11 early-types selected because of their nearly passive stage of evolution in the nuclear region. The paper, second of a series, focuses on the comparison between UV features detected by Swift-UVOT, tracing recent star formation, and the galaxy optical structure mapping older stellar populations. We performed their UV surface photometry and used BVRI photometry from other sources. Our integrated magnitudes have been analyzed and compared with corresponding values in the literature. We characterize the overall galaxy structure best fitting the UV and optical luminosity profiles using a single Sersic law. NGC 1366, NGC 1426, NGC 3818, NGC 3962 and NGC 7192 show featureless luminosity profiles. Excluding NGC 1366 which has a clear edge-on disk , n~1-2, and NGC 3818, the remaining three have Sersic's indices n~3-4 in optical and a lower index in the UV. Bright ring/arm-like structures are revealed by UV images and luminosity profiles of NGC 1415, NGC 1533, NGC 1543, NGC 2685, NGC 2974 and IC 2006. The ring/arm-like structures are different from galaxy to galaxy. Sersic indices of UV profiles for those galaxies are in the range n=1.5-3 both in S0s and in Es. In our sample optical Sersic indices are usually larger than the UV ones. (M2-V) color profiles are bluer in ring/arm-like structures with respect to the galaxy body. The lower values of Sersic's indices in the UV bands with respect to optical ones, suggesting the presence of a disk, point out that the role of the dissipation cannot be neglected in recent evolutionary phases of these early-type galaxies.

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arXiv2017

Pulsator-like spectra from Ultraluminous X-ray sources and the search for more ultraluminous pulsars

Ultraluminous X-ray sources (ULXs) are a population of extragalactic objects whose luminosity exceeds the Eddington limit for a 10 Msun black hole (BH). Their properties have been widely interpreted in terms of accreting stellar-mass or intermediate-mass BHs. However at least three neutron stars (NSs) have been recently identified in ULXs through the discovery of periodic pulsations. Motivated by these findings we studied the spectral properties of a sample of bright ULXs using a simple continuum model which was extensively used to fit the X-ray spectra of accreting magnetic NSs in the Galaxy. We found that such a model, consisting of a power-law with a high-energy exponential cut-off, fits very well most of the ULX spectra analyzed here, at a level comparable to that of models involving an accreting BH. On these grounds alone we suggest that other non-pulsating ULXs may host NSs. We found also that above 2 keV the spectrum of known pulsating ULXs is harder than that of the majority of the other ULXs of the sample, with only IC 342 X-1 and Ho IX X-1 displaying spectra of comparable hardness. We thus suggest that these two ULXs may host an accreting NS and encourage searches for periodic pulsations in the flux.

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arXiv2015

The connection between radio and high energy emission in black hole powered systems in the SKA era

Strong evidence exists for a highly significant correlation between the radio flux density and gamma-ray energy flux in blazars revealed by Fermi. However, there are central issues that need to be clarified in this field: what are the counterparts of the about 30% of gamma-ray sources that are as yet unidentified? Are they just blazars in disguise or they are something more exotic, possibly associated with dark matter? How would they fit in the radio-gamma ray connection studied so far? With their superb sensitivity, SKA1-MID and SKA1-SUR will help to resolve all of these questions. Even more, while the radio-MeV/GeV connection has been firmly established, a radio-VHE connection has been entirely elusive so far. The advent of CTA in the next few years and the expected CTA-SKA1 synergy will offer the chance to explore this connection, even more intriguing as it involves the opposite ends of the electromagnetic spectrum and the acceleration of particles up to the highest energies. We are already preparing to address these questions by exploiting data from the various SKA pathfinders and precursors. We have obtained 18 cm European VLBI Network observations of E>10 GeV sources, with a detection rate of 83%. Moreover, we are cross correlating the Fermi catalogs with the MWA commissioning survey: when faint gamma-ray sources are considered, pure positional coincidence is not significant enough for selecting counterparts and we need an additional physical criterion to pinpoint the right object. It can be radio spectral index, variability, polarization, or compactness, needing high angular resolution in SKA1-MID; timing studies can also reveal pulsars, which are often found from dedicated searches of unidentified gamma-ray sources. SKA will be the ideal instrument for investigating these characteristics in conjunction with CTA. (abridged)

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arXiv2015

Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1

NGC 55 ULX1 is a bright Ultraluminous X-ray source located 1.78 Mpc away. We analysed a sample of 20 Swift observations, taken between 2013 April and August, and two Chandra observations taken in 2001 September and 2004 June. We found only marginal hints of a limited number of dips in the light curve, previously reported to occur in this source, although the uncertainties due to the low counting statistics of the data are large. The Chandra and Swift spectra showed clearly spectral variability which resembles those observed in other ULXs. We can account for this spectral variability in terms of changes in both the normalization and intrinsic column density of a two-components model consisting of a blackbody (for the soft component) and a multicolour accretion disc (for the hard component). We discuss the possibility that strong outflows ejected by the disc are in part responsible for such spectral changes.

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arXiv2013

X-ray variability and energy spectra from NGC5408 X-1 with XMM-Newton

The notion of source states characterizing the X-ray emission from black hole binaries has revealed to be a very useful tool to disentangle the complex spectral and aperiodic phenomenology displayed by those classes of accreting objects. We seek to use the same tools for Ultra-Luminous X-ray (ULX) sources. We analyzed the data from the longest observations obtained from the ULX source in NGC 5408 (NGC 5408 X-1) taken by XMM-Newton. We performed a study of the timing and spectral properties of the source. In accordance with previous studies on similar sources, the intrinsic energy spectra of the source are well described by a cold accretion disc emission plus a curved high-energy emission component. We studied the broad-band noise variability of the source and found an anti-correlation between the root mean square variability in the 0.0001-0.2Hz and intensity, similarly to what is observed in black-hole binaries during the hard states. We discuss the physical processes responsible for the X-ray features observed and suggest that NGC 5408 X-1 harbors a black hole accreting in an unusual bright hard-intermediate state.

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arXiv2013

Radio afterglows of a complete sample of bright Swift GRBs: predictions from present days to the SKA era

Radio observations of Gamma Ray Bursts afterglows are fundamental in providing insights into their physics and environment, and in constraining the true energetics of these sources. Nonetheless, radio observations of GRB afterglows are presently sparse in the time/frequency domain. Starting from a complete sample of 58 bright Swift long bursts (BAT6), we constructed a homogeneous sub-sample of 38 radio detections/upper limits which preserves all the properties of the parent sample. One half of the bursts have detections between 1 and 5 days after the explosion with typical fluxes F>100 muJy at 8.4 GHz. Through a Population SYnthesis Code coupled with the standard afterglow Hydrodynamical Emission model (PSYCHE) we reproduce the radio flux distribution of the radio sub-sample. Based on these results we study the detectability in the time/frequency domain of the entire long GRB population by present and future radio facilities. We find that the GRBs that typically trigger Swift can be detected at 8.4 GHz by JVLA within few days with modest exposures even at high redshifts. The final SKA can potentially observe the whole GRB population provided that there will be a dedicated GRB gamma-ray detector more sensitive than Swift. For a sizable fraction (50%) of these bursts, SKA will allow us to perform radio-calorimetry, after the trans-relativistic transition (occurring ~100 d), providing an estimate of the true (collimation corrected) energetics of GRBs.

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arXiv2012

Chandra Observations of the Collisional Ring Galaxy NGC 922

In this paper we report on Chandra observations of the starburst galaxy NGC 922. NGC 922 is a drop-through ring galaxy with an expanding ring of star formation, similar in many respects to the Cartwheel galaxy. The Cartwheel galaxy is famous for hosting 12 ULX, most of which are in the star forming ring. This is the largest number of ULX seen in a single system, and has led to speculation that the low metallicity of the Cartwheel (0.3 solar) may optimize the conditions for ULX formation. In contrast, NGC 922 has metallicity near solar. The Chandra observations reveal a population of bright X-ray sources, including 7 ULX. The number of ULX in NGC 922 and the Cartwheel scales with the star formation rate: we do not find any evidence for an excess of sources in the Cartwheel. Simulations of the binary population in these galaxies suggest that the ULX population in both systems is dominated by systems with strong wind accretion from supergiant donors onto direct-collapse BHs. The simulations correctly predict the ratio of the number of sources in NGC 922 and the Cartwheel. Thus it would appear that the the metallicity of the Cartwheel is not low enough to see a difference in the ULX population compared to NGC 922.

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arXiv2012

The aperiodic variability of the Ultraluminous X-ray source in NGC 5408

Ultra-Luminous X-ray sources are accreting black holes that might represent strong evidence of the Intermediate Mass Black Holes (IMBH), proposed to exist by theoretical studies but with no firm detection (as a class) so far. We analyze the best X-ray timing and spectral data from the ULX in NGC 5408 provided by XMM-Newton. The main goal is to study the broad-band noise variability of the source. We found an anti-correlation of the fractional root-mean square variability versus the intensity of the source, similar to black-hole binaries during hard states.

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arXiv2012

Hot gas in groups: NGC 5328 and the intriguing case of NGC 4756 with XMM-Newton

[Abridged] NGC 5238 and NGC 4756 are the brightest unperturbed elliptical galaxies in their respective loose groups. In the present study we aim at characterizing the properties of the hot gas in the halos of the brightest members and in the environment. In NGC 4756 we are also interested in the properties of a substructure identified to the SW and the region connecting the two structures, to search for a physical connection between the two. However, we have to take into account the fact that the group is projected against the bright, X-ray emitting cluster A1361, which heavily contaminates and confuses the emission from the foreground structure. We present a careful analysis of XMM-Newton data of the groups to separate different components. We also present a re-evaluation of the dynamical properties of the systems and . SPH simulations to interpret the results. We find that the X-ray source associated with NGC 4756 indeed sits on top of extended emission from the background cluster A1361, but can be relatively well distinguished from it as a significant excess over it out to r\sim150"\ (~40 kpc). NGC 4756 has an X-ray luminosity of ~10^41 erg/s due to hot gas, with an average temperature of kT\sim0.7 keV. We measure a faint diffuse emission also in the region of the subclump to the SW, but more interestingly, we detect gas between the two structures, indicating a possible physical connection. The X-ray emission from NGC 5328 is clearly peaked on the galaxy, also at 10^41 erg/s, and extends to r\sim110 kpc. Simulations provide an excellent reproduction of the SED and the global properties of both galaxies, which are caught at two different epochs of the same evolutionary process, with NGC 5328 ~2.5 Gyr younger than NGC 4756.

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arXiv2012

Seyfert's Sextet: where is the gas?

Seyfert's Sextet (a.k.a HCG 79) is one of the most compact and isolated galaxy groups in the local Universe. It shows a prominent diffuse light component that accounts for ~50% of the total observed light. This likely indicates that the group is in an advanced evolutionary phase, which would predict a significant hot gaseous component. Previous X-ray observations had suggested a low luminosity for this system, but with large uncertainties and poor resolution. We present the results from a deep (70 ks), high resolution Chandra observation of Seyfert's Sextet, requested with the aim of separating the X-ray emission associated with the individual galaxies from that of a more extended inter-galactic component. We discuss the spatial and spectral characteristics of this group we derive with those of a few similar systems also studied in the X-ray band. The high resolution X-ray image indicates that the majority of the detected emission does not arise in the compact group but is concentrated towards the NW and corresponds to what appears to be a background galaxy cluster. The emission from the group alone has a total luminosity of ~1x10^40 erg/s in the (0.5-5) keV band. Most of the luminosity can be attributed to the individual sources in the galaxies, and only ~2x10^39 erg/s is due to a gaseous component. However, we find that this component is also mostly associated with the individual galaxies of the Sextet, leaving little or no residual in a truly IGM component. The extremely low luminosity of the diffuse emission in Seyfert's Sextet might be related to its small total mass.

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arXiv2012

Studying the asymmetry of the GC population of NGC 4261

We present an analysis of the Globular Cluster (GC) population of the elliptical galaxy NGC 4261 based on HST WFPC2 data in the B, V and I bands. We study the spatial distribution of the GCs in order to probe the anisotropy in the azimuthal distribution of the discrete X-ray sources in the galaxy revealed by Chandra images (Zezas et al. 2003). The luminosity function of our GC sample (complete at the 90% level for V_mag = 23.8 mag) peaks at V_mag = 25.1 (-0.6)(+1.0) mag, which corresponds to a distance consistent with previous measurements. The colour distribution can be interpreted as being the superposition of a blue and red GC component with average colours V-I = 1.01 (-0.06)(+0.06) mag and 1.27 (-0.08)(+0.06) mag, respectively. This is consistent with a bimodal colour distribution typical of elliptical galaxies. The red GC's radial profile is steeper than that of the galaxy surface brightness, while the profile of the blue subpopulation looks more consistent with it. The most striking finding is the significant asymmetry in the azimuthal distribution of the GC population about a NE-SW direction. The lack of any obvious feature in the morphology of the galaxy suggests that the asymmetry could be the result of an interaction or a merger.

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arXiv2010

Galaxies and the Local Universe with WFXT

Galaxies are essential building blocks in the Universe. However they are faint and complex X-ray sources and require high performance instrumentation to be properly studied. Yet they are fundamental for our understanding of the Universe, and a detailed knowledge of the local structures is mandatory to explain the deep and far Universe. We make a few examples, and discuss how well suited WFXT is to address this issue.

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