Nancay Radio Telescope Research Articles

HI line observations of 290 evolved stars made with the Nancay Radio Telescope. I. Data

We present a compendium of 21-cm line observations of circumstellar envelopes (CSEs) of 290 evolved stars, mostly (sim 84<!PCT!>) on the asymptotic giant branch (AGB), made with the 100m-class, single-dish Radio Telescope. The observational and data reduction procedures were optimised to separate genuine CSE emission from surrounding Galactic line features. For most targets (254), the results have not been previously published. Clear detections were made of 34 objects, for 33 of which the total flux and the size of the CSE could be determined. Possible detections were made of 21 objects, and upper limits could be determined for 95 undetected targets, while for 140 objects confusion from Galactic emission along the line of sight precluded meaningful upper limits. The collective results of this survey can provide guidance on the detectability of circumstellar gas for future mapping and imaging studies.

A Nançay Radio Telescope study of the hyperactive repeating FRB 20220912A

Abstract The repeating fast radio burst source FRB 20220912A was remarkably active in the weeks after its discovery. Here we report 696 bursts detected with the Nançay Radio Telescope (NRT) as part of the Extragalactic Coherent Light from Astrophysical Transients (ÉCLAT) monitoring campaign. We present 68 observations, conducted from October 2022 to April 2023, with a total duration of 61 hours and an event rate peaking at $75^{+10}_{-9}$ bursts per hour above a fluence threshold of 0.59 Jy ms in the 1.2 − 1.7-GHz band. Most bursts in the sample occur towards the bottom of the observing band. They follow a bimodal wait-time distribution, with peaks at 33.4 ms and 67.0 s. We find a roughly constant dispersion measure (DM) over time (δDM ≲ 2 pc cm−3) when taking into account ‘sad-trombone’ drift, with a mean drift rate of −8.8 MHz ms−1. Nonetheless, we confirm small ∼0.3 pc cm−3 DM variations using microshot structure, while finding that microstructure is rare in our sample – despite the 16 μs time resolution of the data. The cumulative spectral energy distribution shows more high-energy bursts (Eν ≳ 1031 erg Hz−1) than would be expected from a simple power-law distribution. The burst rate per observation appears Poissonian, but the full set of observations is better modelled by a Weibull distribution, showing clustering. We discuss the various observational similarities that FRB 20220912A shares with other (hyper)active repeaters, which as a group are beginning to show a common set of phenomenological traits that provide multiple useful dimensions for their quantitative comparison and modelling.

First upper limits on the 21 cm signal power spectrum from cosmic dawn from one night of observations with NenuFAR (Corrigendum)

The redshifted 21 cm signal from neutral hydrogen is a direct probe of the physics of the early universe and has been an important science driver of many present and upcoming radio interferometers. In this study we use a single night of observations with the New Extension in Nan cay Upgrading LOFAR (NenuFAR) to place upper limits on the 21 cm power spectrum from cosmic dawn at a redshift of $z$ = 20.3. NenuFAR is a new low-frequency radio interferometer, operating in the 10--85 MHz frequency range, currently under construction at the Nan cay Radio Observatory in France. It is a phased array instrument with a very dense $u coverage at short baselines, making it one of the most sensitive instruments for 21 cm cosmology analyses at these frequencies. Our analysis adopts the foreground subtraction approach, in which sky sources are modeled and subtracted through calibration and residual foregrounds are subsequently removed using Gaussian process regression. The final power spectra are constructed from the gridded residual data cubes in the $u plane. Signal injection tests are performed at each step of the analysis pipeline, the relevant pipeline settings are optimized to ensure minimal signal loss, and any signal suppression is accounted for through a bias correction on our final upper limits. We obtain a best 2sigma upper limit of $2.4 mK $ at $z$ = 20.3 and $k$ = 0.041 $h\ cMpc $. We see a strong excess power in the data, making our upper limits two orders of magnitude higher than the thermal noise limit. We investigate the origin and nature of this excess power and discuss further improvements to the analysis pipeline that can potentially mitigate it and consequently allow us to reach thermal noise sensitivity when multiple nights of observations are processed in the future.

First upper limits on the 21 cm signal power spectrum from cosmic dawn from one night of observations with NenuFAR

The redshifted 21 cm signal from neutral hydrogen is a direct probe of the physics of the early universe and has been an important science driver of many present and upcoming radio interferometers. In this study we use a single night of observations with the New Extension in Nançay Upgrading LOFAR (NenuFAR) to place upper limits on the 21 cm power spectrum from cosmic dawn at a redshift of z = 20.3. NenuFAR is a new low-frequency radio interferometer, operating in the 10–85 MHz frequency range, currently under construction at the Nançay Radio Observatory in France. It is a phased array instrument with a very dense uv coverage at short baselines, making it one of the most sensitive instruments for 21 cm cosmology analyses at these frequencies. Our analysis adopts the foreground subtraction approach, in which sky sources are modeled and subtracted through calibration and residual foregrounds are subsequently removed using Gaussian process regression. The final power spectra are constructed from the gridded residual data cubes in the uv plane. Signal injection tests are performed at each step of the analysis pipeline, the relevant pipeline settings are optimized to ensure minimal signal loss, and any signal suppression is accounted for through a bias correction on our final upper limits. We obtain a best 2σ upper limit of 2.4 × 107 mK2 at z = 20.3 and k = 0.041 h cMpc−1. We see a strong excess power in the data, making our upper limits two orders of magnitude higher than the thermal noise limit. We investigate the origin and nature of this excess power and discuss further improvements to the analysis pipeline that can potentially mitigate it and consequently allow us to reach thermal noise sensitivity when multiple nights of observations are processed in the future.

Dense forests of microshots in bursts from FRB 20220912A

ABSTRACT We report on exceptionally bright bursts (>400 Jy ms) detected from the repeating fast radio burst source FRB 20220912A using the Nançay radio telescope (NRT), as part of the ECLAT (Extragalactic Coherent Light from Astrophysical Transients) monitoring campaign. These bursts exhibit extremely luminous, broad-band, short-duration structures (∼16 μs), which we term ‘microshots’ and which can be especially well studied in the NRT data given the excellent signal-to-noise and dynamic range (32-bit samples). The estimated peak flux density of the brightest microshot is 450 Jy. We show that the microshots are clustered into dense ‘forests’ by modelling them as Weibull distributions and obtaining Weibull shape parameters of approximately 0.5. Our polarimetric analysis reveals that the bursts are nearly 100 per cent linearly polarized; have ≲10 per cent circular polarization fractions; a near-zero average rotation measure of 0.10(6) rad m−2; and varying polarization position angles over the burst duration. For one of the bursts, we analyse raw voltage data from simultaneous observations with the Westerbork RT-1 single 25-m dish. These data allow us to measure the scintillation bandwidth, 0.30(3) MHz, and to probe the bursts on (sub-)microsecond time-scales. Some important nuances related to dedispersion are also discussed. We propose that the emission mechanism for the broad-band microshots is potentially different from the emission mechanism of the broader burst components, which still show a residual drift of a few hundred MHz ms−1 after correcting for dispersion using the microshots. We discuss how the observed emission is phenomenologically analogous to different types of radio bursts from the Sun.

Low NH3/H2O ratio in comet C/2020 F3 (NEOWISE) at 0.7 au from the Sun

Context. A lower-than-solar elemental nitrogen content has been demonstrated for several comets, including 1P/Halley and 67P/Churyumov-Gerasimenko (67P/C-G) with independent in situ measurements of volatile and refractory budgets. The recently discovered semi-refractory ammonium salts in 67P/C-G are thought to be the missing nitrogen reservoir in comets. Aims. The thermal desorption of ammonium salts from cometary dust particles leads to their decomposition into ammonia (NH3) and a corresponding acid. The NH3/H2O ratio is expected to increase with decreasing heliocentric distance with evidence for this in near-infrared observations. NH3 has been claimed to be more extended than expected for a nuclear source. Here, the aim is to constrain the NH3/H2O ratio in comet C/2020 F3 (NEOWISE) during its July 2020 passage. Methods. OH emission from comet C/2020 F3 (NEOWISE) was monitored for 2 months with the Nançay Radio Telescope (NRT) and observed from the Green Bank Telescope (GBT) on 24 July and 11 August 2020. Contemporaneously with the 24 July 2020 OH observations, the NH3 hyperfine lines were targeted with GBT. From the data, the OH and NH3 production rates were derived directly, and the H2O production rate was derived indirectly from the OH. Results. The concurrent GBT and NRT observations allowed the OH quenching radius to be determined at (5.96 ± 0.10) × 104 km on 24 July 2020, which is important for accurately deriving Q(OH). C/2020 F3 (NEOWISE) was a highly active comet with Q(H2O) ≈ 2 × 1030 mol s−1 one day before perihelion. The 3σ upper limit for QNH3 / QH2O is < 0.29% at 0.7 au from the Sun. Conclusions. The obtained NH3/H2O ratio is a factor of a few lower than measurements for other comets at such heliocentric distances. The abundance of NH3 may vary strongly with time depending on the amount of water-poor dust in the coma. Lifted dust can be heated, fragmented, and super-heated; whereby, ammonium salts, if present, can rapidly thermally disintegrate and modify the NH3/H2O ratio.

The SPAN512 mid-latitude pulsar survey at the Nançay Radio Telescope

Context. The large number of ongoing surveys for pulsars and transients at various radio observatories is motivated by the science obtained from these sources. Timing and polarisation analysis of relativistic binaries can place strong constraints on theories of gravity. The observation of a growing number of millisecond pulsars (MSPs) spread over the celestial sphere may allow the detection of a stochastic gravitational wave background arising from supermassive black hole binaries. A more complete sample of young pulsars improves our knowledge of neutron star birth and evolution. Transients such as fast radio bursts can serve to probe the intergalactic medium. Aims. The SPAN512 pulsar survey covers intermediate Galactic latitudes using the L-band receiver of the Nançay Radio Telescope (NRT). The survey covers 224 sq. deg. of the sky for a total exposure time of 2200 h. Population syntheses predict the discovery of 3 to 19 new normal pulsars and a few MSPs. Methods. We present detailed modelling of the NRT beam with its L-band receiver and its sensitivity which we used to precisely assess the expected survey yield. We used the flexible Pulsar Arecibo L-band Feed Array data processing pipeline to search the 47 TB of SPAN512 data for pulsars and transients. Results. The SPAN512 survey discovered two new MSPs and one new middle-aged pulsar. We focus on the analysis of the 2.4-ms spin period pulsar J2205+6012 for which we also report the detection of gamma-ray pulsations. Its narrow pulse width (35µs at an observing frequency of 2.55 GHz) allows for sub-microsecond timing precision over 8 yr, with exciting prospects for pulsar timing array programs.

Evolution of OH Maser Emission in the Active Star-Forming Region W75N. I. Research in the 1665 MHz Line

The monitoring results of the OH maser source in W75 N in the main line at 1665 MHz performed in 2007–2020 using the Nancay Radio Telescope (France) are presented. Generally, the maser in 2007–2009 is characterized by a high activity. During the monitoring, time variations of the Stokes parameters were found for all spectral features in the radial velocity range from –1 to 14 km/s. We found that its positional angle $$\chi $$ and the degree of linear polarization $${{m}_{{\text{L}}}}$$ , as well as the degree of circular polarization $${{m}_{{\text{C}}}}$$ of most features, change with time according to certain regularities, not chaotically. Or they can change very weakly and there is a correlation with the variability of the flux density. In this case, the radial velocities of the features change very little. It is shown that the main feature at 12 km/s during 50 years since the discovery of the OH maser in W75 N had two maxima of activity with an interval of about 30 years between them. The identification of spectral features in the 1665 MHz line with maser spots on VLBA maps has been carried out. We showed that a strong flare of the main features in 2007–2009 occurred in the VLA 2 source. During this flare, small changes in the angle $$\chi $$ and significant changes in the degree of both linear and circular polarization of features at 1.75, 2.1, and 3.05 km/s were observed. For the 0.65 km/s feature, significant changes in all emission parameters were observed. The emission in the 1612 MHz satellite line is broadband. The broadening of the line to 1.9 km/s indicates that, in addition to thermal motion of matter, there is also turbulent motion of matter in the medium.

The Nançay H i Zone of Avoidance survey of 2MASS bright galaxies

To complement the 2MASS Redshift Survey (2MRS) and the 2MASS Tully-Fisher survey (2MTF) a search for 21cm HI line emission of 2MASS bright galaxy candidates has been pursued along the dust-obscured plane of the Milky Way with the 100m Nancay Radio Telescope. For our sample selection we adopted an isophotal extinction-corrected K-band magnitude limit of $K_s^o = 11.25$mag, corresponding to the first 2MRS data release and 2MTF, for which the 2MASX completeness level remains fairly constant deep into the Zone of Avoidance (ZoA). About one thousand galaxies without prior redshift measurement accessible from Nancay (Dec > -40\deg) were observed to an rms noise level of ~3 mJy for the velocity range -250 to 10'600 km/s. This resulted in 220 clear and 12 marginal detections of the target sample. Only few detections have redshifts above 8000 km/s due to recurring radio frequency interference (RFI). A further 29 detections and 6 marginals have their origin in non-target galaxies in the telescope beam. The newly detected galaxies are on average considerably more \HI-rich (mostly $10^9 - 10^{10}$M$_\odot$) compared to systematic (blind) HI surveys. The HI detections reveal various new filaments crossing the mostly uncharted northern ZoA (e.g. at $\ell \sim 90\deg, 130\deg, 160\deg$), whilst consolidating galaxy agglomerations in Monoceros and Puppis ($\ell \sim 220\deg, 240\deg$). Considerably new insight has been gained about the extent of the Perseus-Pisces Supercluster through the confirmation of a ridge ($\ell \sim 160\deg$) encompassing the 3C129 cluster that links Perseus-Pisces to Lynx, and the continuation of the second Perseus-Pisces arm ($\ell \sim 90\deg$) across the ZoA.

The loss of large amplitude pulsations at the end of AGB evolution

AbstractSince 2013, we are performing with the Nancay Radio Telescope (NRT) a monitoring program of > 100 Galactic disk OH/IR stars, having bright 1612-MHz OH maser emission. The variations of the maser emission are used to probe the underlying stellar variability. We wish to understand how the large-amplitude variations are lost during the AGB – post-AGB transition. The fading out of pulsations with steadily declining amplitudes seems to be a viable process.

OH masers as probes: How does the variability fade away during the AGB - post-AGB transition?

AbstractWe are currently performing a monitoring program of the 1612 MHz OH maser emission of several dozen Galactic disk OH/IR stars with the Nancay Radio Telescope (NRT). They are complemented by several OH/IR stars toward the Galactic center, which were monitored with the Hartebeesthoek radio telescope. We use the maser variations to probe the underlying stellar variability. As early monitoring programs already have shown, some stars are large amplitude variables with periods up to 7 years, others show small or even no amplitude variations. This dichotomy in the variability behaviour is assumed to mark the border between the AGB and the post-AGB stages. With the current program, we wish to find objects in transition and to describe their variability properties. We consider the fading out of pulsations with steadily declining amplitudes as a viable process. Promising candidates in the disk are the small-amplitude variables OH 138.0+7.2 and OH 51.8−0.2. ’Non-variable’ OH/IR stars in the Galactic center region may be as frequent as in the disk.

Distances of Stars by mean of the Phase-lag Method

AbstractVariable OH/IR stars are Asymptotic Giant Branch (AGB) stars with an optically thick circumstellar envelope that emit strong OH 1612 MHz emission. They are commonly observed throughout the Galaxy but also in the LMC and SMC. Hence, the precise inference of the distances of these stars will ultimately result in better constraints on their mass range in different metallicity environments. Through a multi-year long-term monitoring program at the Nancay Radio telescope (NRT) and a complementary high-sensitivity mapping campaign at the eMERLIN and JVLA to measure precisely the angular diameter of the envelopes, we have been re-exploring distance determination through the phase-lag method for a sample of stars, in order to refine the poorly-constrained distances of some and infer the currently unknown distances of others. We present here an update of this project.

Kinematics of the Local Universe

Aims.This paper presents 828 new 21 cm neutral hydrogen line measurements carried out with the FORT receiver of the meridian transit Nançay radio telescope (NRT) in the years 2000–2007.Methods.This observational program was part of a larger project aimed at collecting an exhaustive and magnitude-complete HI extragalactic catalog for Tully-Fisher applications. Through five massive data releases, the KLUN series has collected a homogeneous sample of 4876 HI-spectra of spiral galaxies, complete down to a flux of 5 Jy km s-1and with declinationδ> −40°.Results.We publish here the last release of the KLUN HI observational program, corresponding to the faint end of the survey, with HI masses ranging from 5 × 108to 5 × 1010solar masses. The size of this final sample is comparable to the catalogs based on the Arecibo and Parkes radio telescope campaigns, and it allows general HI mass distribution studies from a set of homogeneous radio measurements.

Recurring OH Flares towards o Ceti: I. location and structure of the 1990s’ and 2010s’ events

We present the analysis of the onset of the new 2010s' OH flaring event detected in the OH ground-state main line at 1665~MHz towards o Ceti and compare its characteristics with those of the 1990s' flaring event. This is based on a series of complementary single-dish and interferometric observations both in OH and H2O obtained with the Nancay Radio telescope (NRT), the Medicina and Effelsberg Telescopes, the European VLBI Network (EVN), and (e)Multi-Element Radio Linked Interferometer Network ((e)MERLIN). We compare the overall characteristics of o Ceti's flaring events with those which have been observed towards other thin-shell Miras, and explore the implication of these events with respect to the standard OH circumstellar-envelope model. The role of binarity in the specific characteristics of o Ceti's flaring events is also investigated. The flaring regions are found to be less than ~400$ +/- 40 mas (i.e., ~40 +/- 4$ AU) either side of o Ceti, with seemingly no preferential location with respect to the direction to the companion Mira B. Contrary to the usual expectation that the OH maser zone is located outside the H2O maser zone, the coincidence of the H2O and OH maser velocities suggests that both emissions arise at similar distances from the star. The OH flaring characteristics of Mira are similar to those observed in various Mira variables before, supporting the earlier results that the regions where the transient OH maser emission occurs are different from the standard OH maser zone.

Recent results from CODALEMA and the Nançay radio facilities related to cosmic-ray measurements

Since 2003, the Nancay Radio Observatory hosts the CODALEMA experiment, dedicated to radio detection of cosmic ray induced extensive air showers. CODALEMA also features the R&D EXTASIS project, aiming at detecting the low- frequency signal ([2-6] MHz) produced at the sudden disappearance of the air shower particles hitting the ground. The 3 current antenna arrays present different antenna den- sity and extent, and can be operated in a joint mode to record simultaneously the radio signal coming from air showers. Therefore, the Nancay facilities may offer a complete description of the air shower induced electric field at small, medium and large scale, and over an unique and very wide frequency band (from ∼ 2 to 200 MHz).

Study of galaxies in the Lynx-Cancer void

Context. Void population consists mainly of late-type and low surface brightness (LSB) dwarf galaxies whose atomic hydrogen is the main component of their baryonic matter. Therefore, observations of void galaxy HI are mandatory in order to understand their evolution and dynamics. Aims. Our aim was to obtain integrated HI parameters for a fainter part of the nearby Lynx-Cancer void galaxy sample (total of 45 objects) with the Nancay Radio Telescope (NRT) and to conduct the comparative analysis of all the 103 void galaxies with known HI data with a sample of similar galaxies residing in denser environments of the Local Volume. Methods. For HI observations we used the NRT with its sensitive antenna/receiver system FORT and standard processing. The comparison of the void and control samples on the parameter M(HI)/L_B is conducted with the non-parametric method `The 2x2 Contingency Table test'. Results. We obtained new HI data for about 40% of the Lynx-Cancer galaxy sample. Along with data from the literature, we use these new data for further analysis of 103 void objects. The proxy of the evolutional parameter M(HI)/L_B of the void sample is compared with that of 82 galaxies of morphological types 8--10 residing in the Local Volume (LV) groups and aggregates. Conclusions. At the confidence level of P = 0.988, we conclude that for the same luminosity, these void galaxies are systematically gas-richer, on average by ~39%. This result is consistent with the authors' earlier conclusion on the smaller gas metallicities and evidence for the slower low-mass galaxy evolution in voids.

A model for distortions of polarisation-angle curves in radio pulsars

Some radio pulsar profiles (in particular those of millisecond pulsars contain wide emission structures which cover large intervals of pulse phase. Local distortions of an average curve of polarisation angle (PA) can be identified in such profiles, and they are often found to be associated with absorption features or narrow emission components. The features may be interpreted as a convolution of a lateral profile of an emitter with a microscopic radiation pattern of a non-negligible angular extent. We study a model which assumes that such an extended microbeam of the X-mode curvature radiation is spreading the radiation polarised at a fixed position angle within an interval of pulse phase. The model is capable of interpreting the strongly dissimilar polarisation of double notches in PSR B1821-24A (for which we present new polarisation data from the Nancay Radio Telescope) and PSR J0437-4715. It also explains a step-like change in PA observed at the bifurcated trailing component in the profile of J0437-4715. A generic form of the modelled PA distortion is a zigzag-shaped wiggle, which in the presence of the second polarisation mode (O mode) can be magnified or transformed into a W- or U-shaped deflection of a total net PA. The model's efficiency in interpreting dissimilar polarisation effects provides further credence to the stream-based (fan-beam) geometry of pulsar emission. It also suggests that the microbeam width may not always be assumed negligible in comparison with the angular scale of emissivity gradients in the emission region.

On sky characterization of the BAORadio wide band digital backend

We have observed regions of three galaxy clusters at z$\sim$ [0.06, 0.09] (Abell85, Abell1205, Abell2440), as well as calibration sources with the Nancay radiotelescope (NRT) to search for 21 cm emission and fully characterize the FPGA based BAORadio digital backend. The total observation time of few hours per source have been distributed over few months, from March 2011 to January 2012, due to scheduling constraints of the NRT, which is a transit telescope. Data have been acquired in parallel with the NRT standard correlator (ACRT) back-end, as well as with the BAORadio data acquisition system. The latter enables wide band instantaneous observation of the [1250, 1500]MHz frequency range, as well as the use of powerful RFI mitigation methods thanks to its fine time sampling. A number of questions related to instrument stability, data processing and calibration are discussed. We have obtained the radiometer curves over the integration time range [0.01,10 000] seconds and we show that sensitivities of few mJy over most of the wide frequency band can be reached with the NRT. It is clearly shown that in blind line search, which is the context of HI intensity mapping for Baryon Acoustic Oscillations, the new acquisition system and processing pipeline outperforms the standard one. We report a positive detection of 21 cm emission at 3 sigma-level from galaxies in the outer region of Abell85 at 1352 MHz (14 400 km/s) corresponding to a line strength of 0.8 Jy km/s. We observe also an excess power around 1318 MHz (21 600 km/s), although at lower statistical significance, compatible with emission from Abell1205 galaxies. Detected radio line emissions have been cross matched with optical catalogs and we have derived hydrogen mass estimates.

Magnetic fields in methanol maser condensations based on data for related regions. Seven sources: Astrophysical parameters

The astrophysical parameters of seven OH maser condensations are estimated based on magnetic fields obtained from polarization observations carried out on the Nan cay Radio Telescope (France) in the 1665 and 1667 MHz lines in four Stokes parameters. Regions in the studied sources containing the observed clusters of maser condensations, as well as clusters of Class I and II methanol masers, have been identified. The associations of the masers are real; i.e., the magnetic field in the clusters can also extend to groups of methanol masers. The linear dimensions of these associations have been found. The ratio of the mass to the magnetic flux, ratio of the thermal to the magnetic pressure, and virial relationships between energies (kinetic, magnetic, and gravitational) in the regions containing the OH andmethanol masers have been obtained. In sources whose magnetic fields have been determined fairly reliably, the ratio of the mass to the magnetic flux exceeds a critical value, and the energies of chaotic motions and of the magnetic field are considerably smaller than the gravitational binding energy. On the other hand, in all cases, the ratio of the thermal to the magnetic pressure is <1, suggesting that the clouds may be in amagnetically dominated regime. This inconsistency is related to probable uncertainties in the the magnetic field values and the estimated distances to the sources, which may lead to overestimation of the sizes of the regions studied.

Properties of the most metal-poor gas-rich LSB dwarf galaxies SDSS J0015+0104 and J2354−0005 residing in the Eridanus void

SDSS J0015+0104 is the lowest metallicity low surface brightness dwarf (LSBD) galaxy known. The oxygen abundance in its HII region SDSS J001520.70+010436.9 (at ~1.5 kpc from the galaxy centre) is 12+log(O/H)=7.07 (Guseva et al.). This galaxy, at the distance of 28.4 Mpc, appears to reside deeply in the volume devoid of luminous massive galaxies, known as the Eridanus void. SDSS J235437.29-000501.6 is another Eridanus void LSBD galaxy, with parameter 12+log(O/H)=7.36 (also Guseva et al.). We present the results of their HI observations with the Nancay Radio Telescope revealing their high ratios of M(HI)/L_B ~2.3. Based on the Sloan Digital Sky Survey images, we derived for both galaxies their radial surface brightness profiles and the main photometric parameters. Their colours and total magnitudes are used to estimate the galaxy stellar mass and ages. The related gas mass-fractions, f_g ~0.98 and ~0.97, and the extremely low metallicities (much lower than for their more typical counterparts with the same luminosity) indicate their unevolved status. We compare these Eridanus void LSBDs with several extreme LSBD galaxies residing in the nearby Lynx-Cancer void. Based on the combination of all their unusual properties, the two discussed LSBD galaxies are similar to the unusual LSBDs residing in the closer void. This finding presents additional evidence for the existence in voids of a4 figures, sizable fraction of low-mass unevolved galaxies. Their dedicated search might result in the substantial increase of the number of such objects in the local Universe and in the advancement of understanding their nature.