Chandra and Multiwavelength Study of Pevatron Candiate HESS J1702-420A

The gist HESS J1702–420A is a promising Galactic PeVatron candidate whose physical nature remains unknown.

Introduction and Motivation

PeVatrons are Galactic sources responsible for accelerating cosmic rays up to 1015 eV However, despite several plausible associations (i.e., Cao et al. 2021; Tibet ASγ Collaboration et al. 2021), no observation to date has established an unambiguous multi-wavelength (MW) counterpart directly identifying the accelerator responsible for PeV particle acceleration TeV observations have revealed an entirely new population of extended Galactic sources (Aharonian et al. 2005; Chaves 2009; Ong et al. 2009), many of which still have an unknown physical nature It is becoming increasingly apparent that relic (aged) pulsarwind nebulae (PWNe; Kargaltsev et al. 2013) comprise the largest fraction of the population of extended TeV sources found in the Galactic plane (Kargaltsev et al. 2013; Giacinti et al. 2020; H. E. S. S. Collaboration et al. 2018; Tam et al. 2010; Kargaltsev & Pavlov 2010) However, in many cases, the associations are based simply on positional coincidence The observed offsets between the pulsars and TeV sources and the asymmetries of the X-ray PWNe can be explained by either the pulsar proper motion or by the interaction with the reverse shock that had propagated through the interior of the nonhomogeneous supernova remnant (SNR) and reached one side of the PWN sooner than the other side (Blondin et al. 2001) Often an association of a TeV source with a pulsar can be solidified if a PWN and its morphology are resolved in X-rays and/or radio X-ray observations are particularly efficient because PWNe can often be discovered and classified as such at these energies, even in the absence of a known pulsar, due to their distinct properties HESS J1702–420A was discovered in the analysis of deeper TeV observations of HESS J1702–420 The source has a flux of 2.08×10−13 photons cm−2 s−1 above 2 TeV and small size (the 1σ radius is (0.06±0.02stat ±0.03sys)◦), overlapping with an elongated and much more extended companion TeV source HESS J1702–420B dominating the emission below a few tens of TeV (see the white circle for HESS J1702–420A and green ellipse for HESS J1702–420B in Figure 1 and 2) An extremely hard power-law spectrum (Γ = 1.53) and the lack of cut-off up to 100 TeV make HESS J1702–420A one of the hardest VHE γ-ray sources ever detected and a compelling candidate site for the presence of PeV cosmic rays (Abdalla et al. 2021) Abdalla et al. (2021) also analyzed the archival FermiLAT data to constrain the source spectrum at γ-ray energies > 10 GeV with a marginal detection of 4.3σ Several scenarios are proposed to explain the emission from HESS J1702–420 These include cosmic-ray (CR) diffusion from a nearby supernova remnant, SNR G344.7–0.1, and a pulsar, PSR J1702–412 (both ∼ 0.5◦ away from the centroid of the TeV emission; see, e.g., the discussion in Abdalla et al. (2021)) Aharonian et al. (2023) also proposed a model in which HESS J1702–420A is capable of accelerating protons to at least ∼ 100 TeV while HESS J1702-420B is the more extended emission produced as the relativistic protons propagate through that dense molecular cloud The source has been observed several times in X-rays in order to search for a potential MW association to the PeVatron candidate Deep Suzaku X-ray observations did not find a strong counterpart, reinforcing the “dark accelerator" interpretation and supporting a hardronic PeVatron scenario (Fujinaga et al. 2011) Malyshev et al. (2024) searched for diffuse X-ray emission associated with HESS J1702–420 using eROSITA data but found no extended counterpart, placing constraints on both hardronic and leptonic emission scenarios

Chandra Observations and Morphology

The new Chandra observations resolve XMMU J170147.3–421407 into a faint point-like source, Src11 (CXO J170144.5– 421415), and asymmetric diffuse emission comprising eastern and western lobes The X-ray spectral, variability, and MW properties of Src11 and the diffuse emission are consistent with those commonly observed in pulsar/PWN systems, further supported by the ML classifications However, the asymmetric diffuse morphology is unlike a typical bow-shock/tail or jet/torus PWN and the ∼ 7′ offset from HESS J1702–420A makes a firm physical association difficult to establish A possible shell-like radio structure northwest of Src11 may indicate a more complex PWN/SNR environment Other NS candidates are less consistent with a pulsar/PWN interpretation, making Src11 the most compelling candidate in the field

Spectral Analysis and Pulsar Candidate

The spectra of the sources and diffuse emission were jointly fitted using all three Chandra observations The two regions show somewhat different spectral parameters: the western lobe, which is more compact and closer to Src11, has a hard spectrum with Γ = 0.9+0.7−0.6 and poorly constrained NH = 1.4+1.4−1.1× 1022 cm−2, while the more extended and more distant eastern lobe has a softer spectrum (Γ = 2.1+0.8−0.7) with NH = 3+2−1× 1022 cm−2 The combined diffuse emission has an observed 0.5-7 keV flux of (1.5±0.1)×10−13erg cm−2 s−1 with the western and eastern lobes contributing (6.2+0.6−0.5× 10−14) and (5.6+0.7−0.6× 10−14) erg cm-2 s– Src11 is also classified as a NS candidate by the MUWCLASS pipeline and has no convincing MW counterpart Although the model-independent variability analysis initially suggest possible inter-observation variability, the model-dependent fluxes derived from the spectral fits are statistically consistent within their uncertainties and do not provide compelling evidence for significant long-term variability Together with the absence of significant short-term variability, these properties are more naturally consistent with an isolated pulsar than with an accreting compact object and make Src11 the most plausible pulsar candidate physically associated with the diffuse X-ray emission

Morphological Interpretation and Conclusion

The morphology of the extended emission, however, does not readily correspond to a conventional PWN geometry, such as a torus/jet or pulsar-tail structure In particular, the eastern and western lobes are not suggestive of a high-Mach-number bow shock, as would be expected if the ∼ 7′ offset between Src11 and the center of HESS J1702–420A were produced by pulsar motion over several tens of kyr Alternatively, if the putative pulsar remains within its host SNR, the eastern and western lobes could potentially represent a jet and counterjet from a subsonic PWN whose morphology has been distorted by interactions with the surrounding SNR environment In this scenario, the offset TeV source could represent a relic PWN displaced through interactions with the SNR reverse shock The MUWCLASS results and deeper MW imaging also provide important constraints on the other compact-object candidates in the field Taken together, these results make Src11 the most compelling pulsar/PWN candidate among the sources detected in the present Chandra observations Overall, the new Chandra observations strengthen the case that XMMU J170147.3–421407 contains a compact-object/PWN system and identify Src11/CXO J170144.

1. Improve candidate identification in Galactic PeVatron sources by implementing a Multiwavelength Machine Learning pipeline that classifies 18 X-ray sources based on spectral properties, hardness ratios, and variability measures to distinguish between pulsar candidates (Src11) and other compact objects like CVs or YSOs. This allows for the selection of the most plausible candidate as Src11 appears to be the most plausible pulsar candidate because it is located close to the likely non-thermal extended emission.

2. Enhance morphological analysis capabilities by utilizing high angular resolution Chandra observations to resolve X-ray emission into distinct eastern and western lobes, allowing researchers to test interpretations like a relic PWN or a jet and counterjet from a subsonic PWN that are not suggested by lower resolution data.

3. Develop automated cross-matching tools using the NWAY algorithm, incorporating optical/NIR properties from Gaia DR3 and VVV surveys, to calculate probabilities of having a true counterpart among all associations considered for NS candidates. This helps in evaluating whether sources like Src11 have no convincing MW counterpart or if potential counterparts are merely outside the 95% positional uncertainty.

4. Integrate spectral fitting constraints directly into the classification process by tying normalization parameters when no significant long-term variability is found, as described in Section 2.4, to provide a more robust characterization of sources like Src11's photon index when we tied the normalization between the three observations while keeping NH = 2.2 x 10 squared cm−2 fixed.

Aims: HESS J1702-420A is a promising Galactic PeVatron candidate whose physical nature remains unknown. Previous XMM-Newton observations revealed hard, apparently extended X-ray emission from XMMU J170147.3-421407, offset from the center of the TeV source, suggesting a possible pulsar wind nebula (PWN) association. We aim to resolve its X-ray morphology, investigate compact-object candidates in the field, and assess their possible association with HESS J1702-420A. Methods: We obtained three new ACIS-I observations from the Chandra X-ray observatory (CXO) with a total exposure of 50.4 ks. We performed imaging, source detection, spectral and variability analyses, and multiwavelength (MW) machine-learning (ML) source classification. Neutron star (NS) candidates were further investigated using deep near-infrared and radio imaging. Results: The Chandra observations resolve XMMU J170147.3-421407 into a faint point-like source, Src11 (CXO J170144.5-421415), and asymmetric diffuse emission comprising eastern and western lobes. The X-ray spectral, variability, and MW properties of Src11 and the diffuse emission are consistent with those commonly observed in pulsar/PWN systems, further supported by the ML classifications. However, the asymmetric diffuse morphology is unlike a typical bow-shock/tail or jet/torus PWN, and the about7' offset from HESS J1702-420A makes a firm physical association difficult to establish. A possible shell-like radio structure north-west of Src11 may indicate a more complex PWN/SNR environment. Other NS candidates are less consistent with a pulsar/PWN interpretation, making Src11 the most compelling candidate in the field.