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A&A
Volume 689, September 2024
Article Number A1
Number of page(s) 6
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/202449762
Published online 27 August 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

Rotating radio transients (RRATs) were discovered in 2006 as dispersed pulses in archived data from the 64-meter telescope in the Parkes (McLaughlin et al. 2006). Unlike conventional pulsars, which emit a pulse at each or almost every revolution of a neutron star, RRATs do not emit pulses regularly and hours can pass between two recorded pulses (McLaughlin et al. 2006).

The search for RRAT pulses (Cordes & McLaughlin 2003; Fridman 2010; Swinbank et al. 2015; Zhang et al. 2020) and the search for pulsars (Burns & Clark 1969; Staelin 1969; Hankins & Rickett 1975; Eatough et al. 2010) are conducted in different ways. Unlike conventional pulsars, in which it is possible to accumulate a signal by adding data with a known period over long time intervals, for the search for dispersed pulses, it is necessary to have a high instantaneous sensitivity. That is, the search for pulsars can be carried out when their individual pulses are not visible in the raw data after compensation for the dispersion measure (DM). To search for a RRAT, it is necessary for the ratio of the signal (pulse height; A) to noise (root mean square deviation, RMS, with the symbol σn, in the noise track) to be greater than 6–7: S /N = A/σn > 6–7. In addition, due to the random appearance of RRAT pulses and unpredictable waiting time, long-term observations of each point in the sky are needed to find them.

There is no generally accepted definition for a RRAT. In this paper, we adhere to the definition of RRATs as a special group of pulsars that are detected by individual irregular pulses (Burke-Spolaor & Bailes 2010). It is assumed that RRATs can be ordinary pulsars with individual strong pulses (Zhou et al. 2023), pulsars with a very wide distribution of pulses by energy (Weltevrede et al. 2006), pulsars with giant pulses (Brylyakova & Tyul’bashev 2021; Tyul’bashev et al. 2021b), or pulsars with nulling (Zhang et al. 2007; Wang et al. 2007).

To date, several hundred RRATs have been found in the currently available catalogs1,2,3,4 (Manchester et al. 2005; Han et al. 2021), but there has been no general consensus on the nature of RRAT established thus far. Considerations of their specific nature question whether they are a special selection (a new type) of pulsars or simply ordinary pulsars that had been included in long-known samples. It is plausible these different samples are not related to each other and observers may be seeing only a general manifestation of neutron star activity in the form of rare dispersed pulses.

Telescopes that currently have the highest sensitivity in their ranges are used in the search for RRATs. In the meter wavelength range, RRATs have been found or investigated on three antenna arrays: Low Frequency Array (LOFAR; van Haarlem et al. 2013), Murchison Widefield Array (MWA; Tingay et al. 2013), and Large Phased Array (LPA; Shishov et al. 2016). The search for RRATs on LOFAR was carried out on recordings, the duration of which was 1 hour for each direction in the sky (Sanidas et al. 2019). The search for RRATs on MWA with duration 1.33 hour for each direction was reported in the paper by Bhat et al. (2023), but no new RRATs have been discovered thus far. The RRAT search on the LPA was performed several times. Most of the RRATs were discovered when processing semi-annual round-the-clock observations (Tyul’bashev et al. 2018a). For six months, the accumulation at each point in the sky amounted to about ten hours, making it possible to discover 25 new RRATs and the pulses of almost a hundred known pul-sars5. Since the time intervals between consecutive pulses of previously detected RRATs can be several hours (McLaughlin et al. 2006), we expected that observations within ten hours would allow us to detect all or almost all of the RRATs available for observations on the LPA. However, subsequent studies (Logvinenko et al. 2020; Tyul’bashev et al. 2022b) have shown that there are RRATs in which tens of hours can pass between consecutive detections.

The main purpose of this study is to search for new RRATs in the three-year observation interval on the LPA radio telescope. The accumulation at each point in the sky is approximately 65 hours. The daily survey covers 17 000 sq.deg.

2 Observations and data processing

The observations were carried out on the LPA radio telescope of Lebedev Physical Institute (LPI) (Shishov et al. 2016; Tyul’bashev et al. 2016). LPA is a full-power antenna array consisting of 16 384 dipoles. Several radio telescopes have been created on the basis of an antenna field measuring approximately 200 × 400 m. In the project Pushchino multibeams pulsar search (PUMPS; Tyul’bashev et al. 2022a) LPA3 radio telescope is used, which has 128 stationary beams. The beams are located in the plane of the meridian. They overlap at the 0.405 level and cover declinations −9° < δ < +55°. The dimensions of the receiving beam are about 0.5° × 1°. The passage of a source through the meridian occurs once a day (one observation session per day) and takes about 3.5 time minutes at half power. The instantaneous viewing area is about 50 sq.deg. The observations are carried out around the clock on the central frequency of 110.25 MHz, in the 2.5 MHz band. The band is divided into 32 frequency channels, with a width of 78 kHz. The sampling of a point is 12.5 ms. By the end of 2014, observations had been initiated in 96 beams, covering declinations of −9° < δ < +42°, while the remaining 32 beams were connected to the recorders in 2021-2022.

There are several standard steps in a regular RRAT search: subtracting the baseline, sorting through different DMs, estimating the RMS deviations of the noise, and searching for pulses having a S/N value greater than the set value. During the search, the data were chopped into sections, followed by a check to detect the presence of a pulse Tyul’bashev et al. (2018a).

To estimate the peak flux density (Sp) of pulses, well-known antenna parameters were used, with the effective area depending on the height of the source above the antenna, the position of the source relative to the center of the beam, the frequency band, the sampling time, and the background temperature in the direction of the transient. The background temperature was recalculated from a frequency of 178 MHz (see maps in Turtle & Baldwin 1962) to a frequency of 111 MHz based on the dependence Tb ~ v−2.55.

To obtain the result reported in this work, we processed data for the period between August 2014 and December 2017, obtained on declinations −9° < δ < +42° and checked ~1012 of chopped sections of raw data. The search was conducted for transients with DM < 100 pc cm−3. With such a large number of data slices, we were able to detect a few thousands (S /N = 6) of false transients. However, the probability of re-detecting a transient with the same coordinate and on the same DM is negligible. In this paper, we have chosen the detection criterion for the reliable discovery of three pulses with S /N > 6.

The processing program selected 4.5 × 106 RRAT candidates. A strong pulse in the proposed treatment can be detected on many DMs, but with knowledge of its exact time coordinate, it is possible to then mark the candidate as RRAT-only for the DM where it shows the maximum S/N. To reduce the amount of interference, we carried out an additional filtering of candidates using a recurrent neural network (RNN) and using LSTM layers (Hochreiter & Schmidhuber 1997). As a result, we had ~106 RRAT candidates remaining. The vast majority of the remaining candidates are pulses of known pulsars observed in the main, along with the side and back lobes of LPA3. Their coordinates and DMs are known, because previously the RRAT search was conducted on a semi-annual interval (Tyul’bashev et al. 2018a). Tens of thousands of pulses were observed for some pulsars. From the search, we excluded the directions in which pulses from known pulsars are visible. Such pulses account for approximately 98% of all remaining pulses for verification. Thus, approximately 106 × 0.02 = 20 000 pulses were visually checked. The analysis showed that most of the detected 20 000 pulses also belong to known pulsars observed in the side and back lobes of LPA3; otherwise, they are considered to be an interference.

3 Results

After the initial screening, 104 candidates were selected for further research. Between one and more than ten pulses were recorded for the selected candidates over a three-year interval. For all candidates, the DM was specified according to the strongest pulses, and an additional search for new pulses was carried out using monitoring data recorded from August 2014 to August 2023. Taking into account 3.5 min of observations per day for each direction in the sky, approximately 190 hours were accumulated over 9 yr for each RRAT candidate. Therefore, even if the transient emits a pulse once every two days, we would have to register between three and four pulses from it. In this work, we adopted the criterion of reliable detection of RRAT as three detected pulses.

Out of 104 sources of pulsed dispersed radiation, 19 turned out to be new RRATs. The dynamic spectra and pulse profiles of these RRATs have no special features and are posted on our website6. In Table 1, Cols. 1–3 show the name of the transient, along with the coordinates for right ascension and declination for the year 2000. The accuracy of coordinates in right ascension is ±1.5m. The accuracy of the declination coordinates is defined as half the distance to neighboring LPA3 beams, located in beams with declinations above and below the beam where the pulse is detected, and is approximately ±15′. Pulsars J1530+00 and J1830+18 are available on the AO327 (Deneva et al. 2013) and CHIME/FRB (Dong et al. 2023) websites7,8 under the names J1532+00 and J1830+17, but have not been officially published. For these, we managed to independently found theirs pulses. Columns 4–8 show the period (P), when it was possible to determine, as well as the DM and the error in determining the DM, the observed value of S/N of the strongest pulse, its visible half-width (W05 - is the width of the pulse at half of its height), and Sp. Column 9 shows the number of pulses (n; S/N > 6) found over an interval of 9 yr. Column 10 shows the distance (D) to the RRAT. An estimate of the distance to the discovered RRATs can be made using the calculator located on the ATNF website9 and using the YMW16 (Yao et al. 2017) model for calculating. The accuracy of the DM determination depends on the observed S/N of a pulse and on a value of DM. The higher the DM, the greater the detection error due to pulse broadening associated with dispersion smoothing inside the frequency channel and due to scattering (Kuz’min et al. 2007). Furthermore, Sp values were determined with a large error, since the pulse coordinate can be shifted relative to the center of the receiving beam pattern, both in right ascension and declination. This displacement of the coordinate relative to the center of the antenna beam is unknown, so we cannot make an adjustment that takes into account the transient hitting the edge of the receiving beam. The actual Sp can be up to 1.5–2 times higher than that indicated in Col. 8. For open transients, a cross-search with the ATNF catalog, as well as with the RRAT search papers for 2023–2024, was carried out.

We note three additional sources that are not included in Table 1. First, J1556+01 is the RRAT J1555+0108 we found earlier (Tyul’bashev et al. 2018b). In the new search, there are several sessions lasting several minutes, when 2–3 pulses are observed. This allows us to make the period estimate as 0.577 s. J1953+30 and J2333+20, for which 6 and 19 pulses were found. Their DM and coordinates coincide with the pulsars J1953+30 and J2333+20 previously discovered in PUMPS (Tyul’bashev et al. 2017, 2022a). We detected the apparent individual pulses of these pulsars.

There are 17 sources that appear to be RRATs. We were unable to show that the pulses found could be a manifestation of the known strong pulsars observed in the side and back lobes of LPA3. There are between one and two pulses are recorded for these RRAT candidates at an interval of 9 yr. The existence of transients with one and two pulses found in the three-year observation interval probably indicates the existence of new transients in the available nine-year observation interval. Thus, we plan to carry out this processing.

Since in this work cites the formal criterion for detecting at least three pulses to confirm a RRAT, these sources were not included in Table 1. In Fig. 1, we present the dynamic spectra of the strongest RRAT candidates and we briefly discuss them in the next section. Light bands are clearly visible on some dynamic spectra. Their appearance is related to the way dynamic spectra are presented. The weakest signals on the spectra are drawn in white, the strongest are marked in black. In places where light bands are visible on the dynamic spectra, the signals forming the dispersion delay line were the strongest.

For 21 sources on dynamic spectra, the dispersion delay line is invisible or poorly visible, while the pulse profile is well visible (S/N > 6), as seen in Fig. 2. Some of these sources have three or more pulses. Since these RRAT candidates did not pass visual verification, they were not included in Table 1. On the one hand, the detection of several pulses with close coordinates of right ascension and declination for different dates, as well as with close DM, indicates that the probability of detecting noise signals is low. At the same time, the absence of a pronounced dispersion delay line indicates the likely interference nature of the pulses. For an unambiguous answer to the question of whether the detected dispersed signals are new RRATs, or interference instead, additional search criteria are needed; alternatively, an independent test on telescopes with sensitivity higher than LPA3 sensitivity would be required. There are 13 sources that are known pulsars visible in the side lobes of LPA3. For 34 sources, it was possible to show that they are, in fact, interference.

Table 1 contains 19 new RRATs, but the number of detected transients may grow significantly. Seventeen RRAT candidates having between one and two pulses. Some candidates with more than three pulses and a poorly distinguishable dispersion delay line may increase the number of RRATs found with further research up to ~45–50.

Table 1

Characteristics of the found RRATs.

thumbnail Fig. 1

Dynamic spectra of some RRAT candidates, for which one to two pulses were detected at an interval of 9 yr.
thumbnail Fig. 2

Pulse profile (bottom panel) and dynamic spectrum (upper panel) of J1718+2538. Six pulses were detected in the transient. The two red dots on the dynamic spectrum are the expected beginning and end of the line along which the dispersion delay line should be located.

4 Discussion

4.1 The nature of RRAT

We started by considering the nature of RRATs. As noted in the introduction, the main hypotheses dealing with the nature of RRATs state that rotating radio transients can be part of pulsar samples with a wide distribution of pulses by energy (Weltevrede et al. 2006), with giant pulses (Brylyakova & Tyul’bashev 2021; Tyul’bashev et al. 2021b), with sporadic strong pulses (Zhou et al. 2023), and with nulling (Zhang et al. 2007; Wang et al. 2007).

Thus, in the paper by Zhou et al. (2023) examining the RRAT search on the FAST telescope, it was shown that 43 out of the 48 (~90%) transients previously detected in the decimeter wavelength range are ordinary pulsars with sporadic strong pulses. For RRATs discovered on FAST, a regular radiation was detected in only for 24 out of 76 (~31.6%) transients. A smaller percentage of detected pulsars with regular radiation may indicate a lack of FAST sensitivity and it is likely that regular radiation will be detected in the future with prolonged data accumulation.

In the search for slow (P ~ 1 s) pulsars in the meter wavelength range on the LPA3 radio telescope and using power spectra summarized over an interval of several years, it was possible to detect regular weak pulsar radiation in eight previously discovered RRATs (Tyul’bashev et al. 2024). The RRATs search on declinations +56° < δ < +87° using the LPA1 radio telescope made it clear that for known pulsars, the peak flux densities in average profiles and in individual pulses in a single observation session can range from tens to hundreds of times (Tyul’bashev et al. 2021a).

We note that pulsars with a wide energy distribution of pulses, pulsars with sporadically appearing strong pulses and pulsars with giant pulses will seem similar in the context of a search. All these types of pulsars have randomly appearing strong pulses and exhibit regular pulsar radiation. In order to determine the specific type that the transient belongs to, additional research is needed. For example, it has been shown that a power-law energy distribution of pulses may signal the detection of pulsars with giant pulses (Brylyakova & Tyul’bashev 2021).

Despite the fact that 90% of known RRATs have been observed by FAST near the Galactic plane as pulsars with regular radiation, it cannot be claimed that all RRATs will be detected as ordinary slow pulsars when a high enough level of sensitivity is achieved. Thus, in LPA3 observations, the sensitivity in the search for pulsars using summed power spectra can reach 0.1-0.2 mJy (Tyul’bashev et al. 2022a) and, at the same time, regular pulsar radiation was detected for only 10% of new RRATs (Tyul’bashev et al. 2024). There are also RRATs where pulsar radiation is not detected in the summed power spectra, but regular radiation is recorded in individual sessions lasting 3.5 min (Tyul’bashev et al. 2023). The peak pulse flux density of many RRATs exceeds tens of Jy at a frequency of 111 MHz (Tyul’bashev et al. 2018a, 2023) and this paper. Assuming that regular radiation can be detected for all RRATs during accumulation, the ratio between the peak flux densities of the average profiles and the individual pulses recorded should be more than several thousand.

Therefore, for some RRATs, the hypothesis that they can be pulsars with large nulling fraction appears more natural. Thus, in the paper mentioned earlier in this work, Zhou et al. (2023), it was reported that in FAST observations at a central frequency of 1.25 GHz and a frequency band of 500 MHz, regular radiation could not be detected for 10% of known RRATs detected in the GPPS survey. Since standard sessions on FAST last 5 min, we can assume a nulling fraction reaching 99%. In the paper Tyul’bashev et al. (2023), based on observations at 111 MHz, it was reported that RRAT J1312+39 was detected, where only three pulses with peak flux densities from 33 to 165 Jy were detected in daily sessions of 3.5 min over an interval of 8 yr. These flux densities are many times higher than the pulse detection limit at LPA3, which is about 2 Jy for S/N = 7 (Tyul’bashev et al. 2018b). For J1312+39, a nulling fraction can reach 99.999%. Henceforth, it is assumed that all RRAT have the same period, namely, P = 1s.

The previous section provides examples of sources (see Fig. 1) with between one and two pulses registered. These pulses are no different from the pulses of previously detected by RRATs. If they belong to the new RRATs, this means that it is necessary to recognize the existence of transients emitting one pulse per 100-200 hours of observations. The nulling fraction of these pulsars can reach 99.9999%.

The analysis of the properties of RRATs carried out in the paper Abhishek et al. (2022) shows that transients are not statistically related to nulling pulsars and are not pulsars at a late stage of their life. Observations of known RRATs on FAST with a sensitivity of up to ten times higher than in the works with the discovery of these RRATs confirm that pulsars with nulling are not the main part of RRATs (Zhou et al. 2023). For a small number of RRATs where it was possible to determine the P and P˙$\dot P$ periods, it is shown that, on average, P and P˙$\dot P$ have higher values than for ordinary pulsars (see Fig. 10 in Cui et al. 2017). Usually, long periods are associated with the age of a pulsar and with its approach to the death line on the P/P˙$P/\dot P$ plane (Chen & Ruderman 1993). However, for RRATs, the points on the P/P˙$P/\dot P$ plane are generally not pushed against the line of death. This supports the assumption that the observed properties of RRATs are related to some internal processes in the magnetosphere of the neutron star itself (Abhishek et al. 2022).

4.2 Detection of close transients

We go on to consider the detection of close transients. Out of the 19 transients presented in Table 1, three RRATs are located at a distance of less than 200 pc: J0408+28 (134 pc), J0440+35 (136 pc), and J0630+23 (160 pc). A search in the ATNF catalog for February 2024 shows 22 equally close pulsars, of which 3 are located in the northern hemisphere.

If the peak flux density in the average profile and the distance to a pulsar are known, its pseudo-luminosity can be estimated (L = S × d2, where S is the average pulsar flux density in mJy, and d is the distance to the pulsar in kpc; Lorimer & Kramer 2012). The usual definition of the average flux density when calculating pseudo-luminosity for transients is not applicable. An insignificant part of the pulses was detected for these sources and, with a formal approach, when all the periods in the observation interval are summed up, the determined average flux density will be close to zero. However, it is possible to estimate the flux density at one period when a pulse is observed. Assuming that the observed pulses have a triangular shape and the pulse parameters are known (values of P, W0.5, Sp from Table 1), we have estimated S111 = 35.6 and 131.8 mJy, L111=0.64, and 2.44 mJy kpc2 for J0408+28 and J0440+35, accordingly.

The ATNF catalog shows pseudo-luminosities for about 600 pulsars at 400 MHz. For convenience, we have made estimates of pseudo-luminosity at a frequency of 400 MHz using the obtained flux density at 111 MHz. We converted the flux density into 400 MHz, assuming that it is a power-law spectrum and a power-law exponent α = l.8(S ~ ν-α) (Maron et al. 2000); then, we have S400 = 3.54 and 13.12 mJy, and L400 = 0.064 and 0.244 mJy kpc2 for J0408+28 and J0440+35. There are only three pulsars in the ATNF catalog (i.e., 0.5% of those having a pseudo-luminosity evaluation) with comparable values of L400. These are pulsars J0307+7443, J0613+3731, and B1014-53, located at distances from 116 to 386 pc and having pseudo-luminosities from 0.04 to 0.06 mJy kpc2. Thus, J0408+28 are included in a short list of pulsars closest to the Sun with minimal pseudo-luminosities.

5 Conclusion

First, when checking 104 candidates for rotating radio transients discovered in daily observations lasting 3 yr, 19 new RRATs were discovered. There are 13 RRAT candidates that are known pulsars observed in the side and back lobes of LPA3. Then, 17 sources are very similar to conventional RRATs, but with only one or two pulses detected in daily observations lasting 9 yr. A further 21 sources failed visual check, and their nature is unknown, 34 candidates were associated with interference. Taking into account previously discovered RRATs10. the total number of rotating radio transients discovered at PUMPS has reached ≃80.

Second, for some of the candidates, only one pulse was detected over a time interval equivalent to continuous observations of more than fifty and possibly more than a hundred hours. If these candidates belong to a sample of pulsars with nulling, then the share of the nulling fraction can reach 99.9999%. Finally, the distances to three discovered RRATs (J0408+28; J0440+35; J0630+23) are less than 200 pc, which indicates the location of the sources in the immediate vicinity of the Sun.

Acknowledgements

The study was carried out at the expense of a grant Russian Science Foundation 22-12-0023611. The authors thank L.B. Potapova for her help in execution of the paper. The authors thank the anonymous referee for the comments that made it possible to improve the readability of the paper.

Data availability

The PUMPS survey is not finished yet. The raw data underlying this paper will be shared on reasonable request to the corresponding author.

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All Tables

Table 1

Characteristics of the found RRATs.

In the text

All Figures

thumbnail Fig. 1

Dynamic spectra of some RRAT candidates, for which one to two pulses were detected at an interval of 9 yr.
In the text
thumbnail Fig. 2

Pulse profile (bottom panel) and dynamic spectrum (upper panel) of J1718+2538. Six pulses were detected in the transient. The two red dots on the dynamic spectrum are the expected beginning and end of the line along which the dispersion delay line should be located.
In the text

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