Why have searched for supernova remnants

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Latest Partners. Similar Articles. The ESA astronaut Read more. Photographing the M87 galaxy black hole Space 5th November Corresponding author: A. Greve, greve iram. Received: 20 April Accepted: 22 October The absence of RSNe and SNRs in and near A and B may, however, also be due to a violent and turbulent outflow of stellar winds and supernova ejected material, which does not provide a quiescent environment for the development of SNRs within and near the SSCs.

Data correspond to usage on the plateform after The current usage metrics is available hours after online publication and is updated daily on week days. Introduction 2. General structure 4. The threshold above which a shell hypothesis is accepted should be treated with caution, since the limited range of tested null hypothesis morphologies may lead to an increased false-positive rate the effect of which has not been numerically quantified.

Adopting the assumption that the chosen morphological models for shell H 1 and null hypothesis H 0 represent the true TeV source populations sufficiently well, the improvement in the fit quality i.

However, there is the purely analytical issue that the two compared models are non-nested, i. In this case, a likelihood ratio test LRT cannot be applied Protassov et al. For a given model, AIC is computed as 2. Testing a set of models on the same data set, 3. In order to quantify if and how translates into a probability that the improvement obtained with the shell fit over the Gaussian model is due to statistical fluctuations, a limited number of simulations have been performed using the parameters of the H.

The number of false-positives type I errors , i. While the correspondence to a chance probability was not verified for these two sources with analogous simulations, the resulting values ensure to sufficient degree of certainty a low probability of a chance identification as a shell for these two sources as well. To perform an as unbiased as possible search for new shell morphologies in the HGPS data set, a shell H 1 versus Gaussian H 0 morphology test has been performed on a grid of Galactic sky coordinate test positions covering the HGPS area with equidistant spacings of 0.

To be computationally efficient, also a grid of tested parameters was defined for both H 1 and H 0 ; the parameters for H 1 radius and width of the shell broadly encompass the parameters of the known TeV SNR shells.

The parameters are listed in Table 2. At each test position x 0 , y 0 , the test statistics difference between the best-fitting shell and the best-fitting Gaussian has been derived and stored into a sky map. Insuch a map, the signature of a shell candidate is an isolated peak surrounded by a broad ring-like artifact 4. At further four positions, significant signatures for shell morphologies were revealed. Three positions are clearly identified with known H. Collaboration The grid search for new shells described above has several limitations.

The limited number of tested null-hypothesis models and the restriction of keeping the same centroid for the null-hypothesis model as for the shell may lead to a nonoptimum H 0 fit and therefore to overestimating the likelihood of the H 1 versus H 0 improvement.

Final likelihoods for the individual candidates are therefore estimated with an improved method as described in the following Sect. This is not the morphology the search is targeting. The search has been designed to be independent of the HGPS source identification mechanism, which could have also been used to define test positions and regions of interest.

In principle, shell morphologies could be present that cover two or more emission regions identified as independent sources by the HGPS procedure. To overcome the limitations of the grid search see Sect. Morphological fits were performed on uncorrelated on-counts i. Entries in these maps have pure Poissonian statistical errors.

The model fit function On i is constructed as 4. Bkg i is the estimated background event map derived from the ring-background method Berge et al. Collaboration f for a detailed explanation , while i runs over the bins. The employed fitting routines are based on the cstat implementation of the Cash statistics 6 Cash available in the Sherpa 7 package. To quantify the improvement of the fit quality between two models, the Akaike Information Criterion as discussed in Sect. Table 3 lists the results for the three new TeV shells.

During the evaluation of the fit improvement when applying the shell model, the assumption of spherical symmetry of the respective shell model was preserved. Nevertheless, after identification of the shell sources, the symmetry of the TeV sources Fig.

The AIC was used again to quantify the improvement of the goodness of fit. The parameters of the additional Gaussian component are however not consistent within statistical errors when modifying analysis configurations or using the cross-check analysis. Therefore, fit results for models including these additional components are not given here. The maps were calculated with a correlation radius of 0.

The inlets show the point spread function of the specific observations after applying the same correlation radius and smoothing, respectively. The green ellipse denotes the outer boundary of the radio SNR G The green circle denotes the position and extent of 3FGL J The morphological fits presented in Sect.

For the visualization of sky maps, depending on the properties of the data set, it may be necessary to correct acceptance changes in the sky maps themselves, specifically in the case of large extended sources. For the sources presented in this paper, however, all available observations of the respective source were used, whether they were part ofan observation of the particular source or a nearby source.

Therefore, surface brightness maps were constructed for the three sources. For this, flux maps were derived following the procedure described in H. To derive the surface brightness, the flux is divided by the area of the correlation circle for every bin of the sky map. The grid size is 0. From these maps, the integral flux of a source can be obtained by integrating over the radius of a region of interest, provided that the integration region is large compared to the psf. The surface brightness maps were checked for each of the three sources by comparing the integral flux from the maps with the respective result of the spectral analysis.

In Fig. In fact, the assumed spectral index does not influence the appearance of the map. Radial and azimuthal profile representations of these surface brightness maps including the morphological fit results can be found in Appendix A. TeVenergy spectra for each of the new shells were derived using a forward-folding technique.

Data were selected and analyzed according to the description in Sect. Source, background, and effective area spectra with equidistant binning in log-space are derived runwise and are summed up. Power-law models 5. The lower fit boundary results from the image amplitude cut and corresponding energy bias cut. The upper fit boundary is dominated by the energy bias cut. The value E 0 is the decorrelation energy of the fit, i. All spectral parameters are listed in Table 4.

All three spectra are statistically compatible with power laws. The systematic errors that have been discussed in Sect.

With the current spectral analysis, a more detailed description beyond a power law is not justified. Upper boxes show the H. Systematic errors do not permit the application of more complex models to describe the data. Confusion with thermal emission and Galactic background variations might therefore hamper the detection of radio counterparts of the new SNR candidates as well.

We searched publicly available radio catalogs and survey data for counterparts of the new TeV sources. In , Green et al. The newly detected radio SNR candidate G The extension and shell appearance of the two sources are in excellent agreement as well. To compare the two sources on more quantitative basis, radial profiles using elliptical annuli were extracted from the radio and TeV data.

Two additional uncataloged source candidates and two extended sources were also removed. The RMS in the resulting source-subtracted image is 1. The flux density of G The flux density amounts to 0. The difference is mostly due to one of the two uncataloged sources cited above that lie within the SNR candidate. The profile of the MHz emission toward G The TeV profile was derived following the procedure described in Appendix A , using elliptical annuli with the same parameters as used for the profile of the radio emission.

The resulting TeV and radio radial profiles are shown in Fig. The two profiles are statistically compatible with each other, confirming the association of the two sources as being due to the same object. The total flux of G Moreover, as explained earlier, the radio surface brightness of G A two times higher radio flux would reduce the distance estimate to 15 kpc. The ratio of minor and major axes of the ellipse and the center position were both taken from Green et al.

Since Green et al. Both profiles were normalized to have the same integral value. The SNR candidate status was confirmed through polarization detected at 2. However, as already discussed in Aharonian et al. The data suffer to some extent from side lobes from bright emission in the W49A region. No obvious counterpart to the TeV source was found by inspecting these data. It is possible that the radio SNR candidate G The source is classified in both catalogs as disk-like.

Together with the spectral match see Fig. However, the identification currently does not add enough information to improve the astrophysical classification of the object. Triggered by preliminary H. The GeV source position and size are in good morphological agreement with G Since the TeV source is extended and located in the Galactic plane, source confusion and emission from the Galactic plane might so far have prevented discovery of the source using the LAT instrument information alone.

An analysis of all three objects using the TeV sources as prior information for a LAT analysis is ongoing. The integral fluxes given in the Fermi -LAT catalogs were converted using the power-law models given in the catalogs.

The energy of the flux points was determined by calculating the geometrical center of the energy boundaries in log-space. In this section, all MWL searches are reported that have not resulted in firm positive identifications with the new TeV sources.

The following section provides details on these searches; in Sect. Cas A Pavlov et al. While the detection of possibly associated pulsars may not help in confirming the SNR nature of the new TeV shells, they may be used to elaborate possible SNR scenarios. Also, since energetic pulsars may drive TeV PWNe, it is important to check for these possible alternative object scenarios that may explain part of the TeV emission.

In relic PWN scenarios, significant angular offsets between the powering pulsar and bulk of the TeV emission can be expected e. Collaboration d ; Aharonian et al. An association of that pulsar with the TeV source has been discussed in the context of a possible PWN scenario to explain the lower-statistics TeV source at the time of its discovery Aharonian et al. None of the pulsars show known radio or X-ray PWNe. Also in this case, none of the pulsars show known radio or X-ray PWNe.

However, there is currently no positive evidence to support such a hypothesis. All well-established TeV SNRs display strong extended nonthermal X-ray synchrotron emission from TeV electrons, typically in filamentary morphologies tracing the forward and possibly reverse shocks.

In the following, published and unpublished X-ray observations are reviewed that cover the TeV sources, in view of possible shell-like X-ray emission. Further pointings to complete the coverage of the TeV source with Suzaku had already been approved, but could not be performed because of the failure of the satellite and subsequent decommissioning of the observatory. Particle-background subtracted, vignetting- and exposure-corrected mosaic images in the full band and in the harder band of 2 keV—12 keV were created.

The harder band is expected to be more sensitive specifically to a nonthermal component of the potential X-ray counterpart because of Galactic absorption. No significant emission is detected from the source region in both mosaics. To estimate an X-ray upper limit from the area of the TeV source, spectra from a limited on- and an off-source region are derived; see Fig.

In both spectra, there is a soft emission component whose characteristics are consistent with emission from hot thermal interstellar gas, and which is likely due to local X-ray foreground. In order to estimate an X-ray flux upper limit from the SNR, an additional absorbed power-law component was included in the on-spectrum model. Point sources were not removed from the image. Contours denote the TeV surface brightness. The large solid ellipse denotes the outer boundary of the radio SNR. The small solid ellipse is the extraction region to derive an X-ray upper limit estimate from the SNR; the dashed ellipse is the corresponding background extraction region.

Observations did not cover the entire TeV source but focused on the northeastern and southwestern components as well as on the central position. Matsumoto et al. Concerning point sources specifically in the central area, a possibly relevant source for this study is XMMU J First, mosaic images of the extended emission were created with ESAS. To this end, images for each observation were created and source detection was performed. Point sources detected by this procedure were masked out of the data.

Then the quiet particle background and the soft proton contamination were modeled for each observation. All these images were combined and used to create background-subtracted, exposure-corrected mosaic images.

Figure 6 shows these mosaics in two different energy bands, 0. The soft-band image demonstrates that the field specifically in the northeast is significantly contaminated by soft stray light from the nearby SNR RCW To estimate the flux of the diffuse X-ray emission component, spectra were extracted for the entire hard emission region seen in the northwest above 3 keV see circle in Fig.

Nearby background regions were selected to be representative of the expected background in the respective source extraction regions, and their spectra were fitted simultaneously. The emission — if confirmed — fills a large portion of the FoVs of the EPIC instruments, and spectral analysis requires a detailed modeling of all background components.

However, the results are not sufficient to significantly improve the astrophysical classification of the object at this time. Point sources were removed from the images. The soft-band image is dominated by stray light from RCW northeastern arc feature.

The hard-band image is dominated by a putative diffuse X-ray emission region coincident with the north component of the TeV source. The solid circle indicates an extraction area used to assess the spectrum for this diffuse component. Different background control regions not shown were used to estimate the systematic error induced by the background estimate. As already discussed in Aharonian et al.

None of the detected nine point sources seem particularly outstanding. No diffuse emission was detected. In view of the improved TeV morphology derived in this work, we reanalyzed the Chandra data ObsId Such emission regions could consist of stellar wind material e. A morphological correlation between the IR and the TeV maps is not seen in the images and would also not necessarily be expected even if the TeV sources were associated with the HII emission regions.

It is also interesting to evaluate the energy from the cluster as a whole. The total cluster mass is unconstrained, since most of the stars in the vicinity of Pismis 22, which are likely to be member stars, lack a spectral type determination. The expected kinetic energy in the system was estimated using the Starburst 99 cluster evolution model Leitherer , and references therein. The model results scale with the initial cluster mass M SC.

The total kinetic energy including stellar winds and SNe over a time span of 40 Myr is. Such a system with its output in kinetic energy would certainly leave its imprint in the cluster surroundings.

Models such as that by Silich et al. Interestingly, the candidate HII region G Lacking any observational evidence, this possibility remains hypothetical for the moment. Archival infrared images toward the fields of the three TeV sources. Top panel : MSX Price et al. Color scales were adjusted individually to emphasize the structures in the images. Contours denote the TeV surface brightness of the respective source. Voids in HI data would be suggestive of stellar wind bubbles blown by massive progenitor stars, while arcs of HI emission or asymmetric spectral line profiles may be attributable to gas shocked by a SNR.

Such features would deliver a SNR kinematic distance solution. Nanten CO data Matsunaga et al. To trace atomic gas as opposed to the aforementioned molecular gas tracers toward the new H. Longitude-velocity plots of Columbia CO data Dame et al.

In general, there are often ambiguities in associating specific gas components with specific Galactic arms, but associations indicated in Fig.

Multiple line-of-sight gas components may potentially be associated with the TeV sources. For these comparisons, CO and HI sky maps were produced in a series of velocity bands covering the entire velocity range shown in Fig. The HI emission appears as a partial shell toward the rim of the TeV source, hinting at the possibility of a blown-out bubble related to a shell-type SNR from the progenitor star or to an alternative central energy source.

This distance range encompasses that of the Pismis 22 open cluster cf. Since no unambiguous association was found for any of the three sources, two representative distances with a possible gas match have been chosen per source. No attempt was made to quantify whether the matches themselves are statistically significant, from the gas data alone.

The associations are detailed in Appendix B. The way gas densities are derived is explained in Appendix B. Results are shown in Table 7 and are discussed further in the following discussion section. The longitudinal extent of each of these SNRs is indicated by dashed lines. The CS feature at 9. From the presented morphological studies using H. The nondetection of radio synchrotron emission from the other two sources is not in conflict with the SNR hypothesis for these objects. The GeV counterpart situation i.

For the new SNR candidates, Table 7 lists derived parameters diameter, luminosity for three different assumed distances, namely a generic 1 kpc distance and the distances listed in Table 5. On average, the photon indices of the new shells seem slightly softer than those of the known SNR shells. However, for individual sources the respective errors are too large to draw any conclusion. Distances at a 8 kpc…10 kpc scale are either disfavored or would indicate a different, substantially more luminous new TeV SNR source population.

If this cutoff is indeed attributed to a lower limit in the ambient density distribution, as argued by Badenes et al. Parameters of the new TeV SNR candidates, assuming a generic 1 kpc distance and two other distances coming from possible association scenarios. The energy share between relativistic hadrons and leptons and the maximum particle energy determine whether SNRs contribute significantly to the generation of Galactic CRs up to the knee in the CR particle spectrum.

The lack of thermal X-rays may be interpreted as a signature for low density environment and therefore leptonically dominated TeV emission processes. However, the absorption column to the new sources is not constrained from the TeV data. Depending on the actual distance to the source, and on the temperature and density of the emitting plasma, foreground absorption might have prevented detection of soft thermal X-rays, for example, in the ROSAT survey.

There is also sufficient uncertainty about which level of thermal heating could be expected, for example, in a clumpy environment e. As discussed in Sect. To establish a sensitive upper limit indicating that the TeV emission stems from protons is however challenging even for current X-ray instruments because of the large extension of the source. Table 6. The distance estimate to this stellar cluster, 1. Another association was suggested by Sakai et al. Appendix B. At a distance of 4.

As for all sources presented in the paper, the TeV shell morphology implies that particle acceleration to supra-TeV energies is likely still ongoingor has been ongoing until the recent past; otherwise, diffusion of the no longer confined particles would have washed out the current morphology. To maintain the association, the birth spin period of the pulsar therefore needs to have been close to the current spin period, which is in principle possible.

In the delta-function approximation Kelner et al. Following the arguments in Aharonian et al. It is assumed that the proton energy spectrum can be described by a broken power law with a break energy at 10 TeV, 9. To illustrate the possible energy contents in accelerated protons, three scenarios are given in Table 7 for each source.

The other two are derived from the possible gas association scenarios as introduced in Sect. Table 5 , where error ranges are propagated from the estimated ranges of gas densities.

The table lists both the energy contents of the protons in the TeV-emitting energy range and for an extrapolated spectrum down to 1 GeV. Distances at a 8 kpc…10 kpc scale and beyond are disfavored in hadronic emission scenarios.