A search for radio pulsars in five nearby supernova remnants
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Astronomy & Astrophysics manuscript no. main ©ESO 2021
February 1, 2021
A search for radio pulsars in five nearby supernova remnants
S.Sett1 , R.P.Breton1 , C.J.Clark1 , M.H. Kerkwijk2 , and D.L. Kaplan3,
1
Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, M13 9PL, UK.
e-mail: susmitasett04@gmail.com
2
Department of Astronomy & Astrophysics, University of Toronto, 50 Saint George Street, Toronto, ON, M5S 3H4, Canada.
3
Department of Physics, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA.
arXiv:2101.12486v1 [astro-ph.HE] 29 Jan 2021
received and accepted date
ABSTRACT
Context. Most neutron stars are expected to be born in supernovae, but only about half of supernova remnants (SNRs) are associated
with a compact object. In many cases, a supernova progenitor may have resulted in a black hole. However, there are several possible
reasons why true pulsar-SNR associations may have been missed in previous surveys: The pulsar’s radio beam may not be oriented
towards us; the pulsar may be too faint to be detectable; or there may be an offset in the pulsar position caused by a kick.
Aims. Our goal is to find new pulsars in SNRs and explore their possible association with the remnant. The search and selection of
the remnants presented in this paper was inspired by the non-detection of any X-ray bright compact objects in these remnants when
previously studied.
Methods. Five SNRs were searched for radio pulsars with the Green Bank Telescope at 820 MHz with multiple pointings to cover
the full spatial extent of the remnants. A periodicity search plus an acceleration search up to 500 m/s2 and a single pulse search were
performed for each pointing in order to detect potential isolated binary pulsars and single pulses, respectively.
Results. No new pulsars were detected in the survey. However, we were able to re-detect a known pulsar, PSR J2047+5029, near
SNR G89.0+4.7. We were unable to detect the radio-quiet gamma-ray pulsar PSR J2021+4026, but we do find a flux density limit
of 0.08 mJy. Our flux density limits make our survey two to 16 times more sensitive than previous surveys, while also covering the
whole spatial extent of the same remnants.
Conclusions. We discuss potential explanations for the non-detection of a pulsar in the studied SNRs and conclude that sensitivity is
still the most likely factor responsible for the lack of pulsars in some remnants.
Key words. surveys – stars: pulsars, supernovae – ISM: supernova remnants
1. Introduction lutionary model of a pulsar wind nebula inside an SNR. Con-
firmed associations also help in obtaining independent age and
The association between supernova remnants (SNRs) and neu- distance estimates (Manchester 2004), which in turn can more
tron stars was a key prediction leading to the formal identifica- accurately constrain the birth properties of neutron stars, namely
tion of pulsars as neutron stars following the discovery of the their period, magnetic field, luminosity, and velocity distribu-
Crab pulsar (Staelin & Reifenstein 1968). Since then, numer- tions (Kaplan et al. 2006a). The detection of a pulsar can also
ous deep surveys of SNRs for pulsations from young neutron clarify the unusual morphology of a remnant, as in the previous
stars have been carried out (e.g. Gorham et al. 1996; Kaspi et al. proposed association between PSR B1757− 24 and SNR G5.4−
1996; Biggs & Lyne 1996; Lorimer et al. 1998; Zhang et al. 1.2 (Frail & Kulkarni 1991), where the morphology may other-
2018; Straal & van Leeuwen 2019). The detection and associ- wise lead to misclassification (Becker & Helfand 1985).
ation of pulsars with the remnants has accelerated in the past On the other hand, the non-detection of a pulsar in an SNR
few years and is a result of high-frequency, targeted searches may suggest that the supernova resulted in a black hole rather
for radio and gamma-ray pulsars in SNRs (Camilo et al. 2002; than a neutron star, or that the neutron star received such a large
Gupta et al. 2005; Abdo et al. 2009). kick upon formation that it is no longer within the region of the
Pulsars associated with SNRs are expected to be young SNR (Frail et al. 1994).
(Gaensler & Johnston 1996) and can be important targets for However, several selection effects are present when search-
studying and understanding pulsar properties, such as their brak- ing for pulsars in SNRs. For example, the choice of observing
ing index (Livingstone et al. 2006). The measurement of the pul- frequency connects with a number of underlying factors. On
sar braking index is crucial to understanding the underlying pul- the one hand, most pulsars tend to be brighter at low frequen-
sar spin-down mechanism. Young pulsars have high spin-down cies, which should favour surveys conducted at the lower end of
luminosities (Lyne et al. 1993) and are more likely to be detected the radio spectrum. However, as most SNRs lie on the Galac-
at X-rays and gamma rays, providing observational diagnos- tic plane and may be located at large distances, effects such as
tics for the rotation-powered neutron star energetics (Kaspi et al. pulse scattering, high dispersion measures, Galactic foreground
1996). For example, Gelfand et al. (2014) put constraints on the emission, and emission from the SNRs themselves can hinder
initial spin period of the neutron star in Kes 75 by fitting the such low-frequency surveys (Sanidas et al. 2019). On the other
observed properties of Kes 75 with the predictions of an evo- hand, while higher frequencies may mitigate some of these se-
Article number, page 1 of 5A&A proofs: manuscript no. main
lection effects, the smaller beam size compared to low frequen- searches for the radio counterpart of the pulsar have not yielded
cies is another factor to consider (Gorham et al. 1996). Addi- a positive result, and hence the pulsar is considered to be radio-
tionally, given the large angular size of some SNRs and the quiet (Lin et al. 2013). Our non-detection confirms this (see Sec-
potentially large kick velocities imparted during the supernova tion 4).
explosion, many pointings may be required to survey the full SNR G89.0 + 4.7 (also known as HB 21) is a
vicinity of the associated remnant. For example, Schinzel et al. large, mixed-morphology SNR. It was discovered by
(2019) discovered an association between the SNR CTB 1 and Hanbury Brown & Hazard (1953) at 159 MHz. A pulsar,
PSR J0002+6216 based on the pulsar’s high proper motion and PSR J2047+5029, was detected by Janssen et al. (2009) with
cometary tail, despite a 0.5 degree offset between the two ob- the Westerbork Synthesis Radio Telescope at a frequency of 328
jects. MHz. However, the distance estimates of the pulsar (1.7 kpc,
We performed a sensitive survey of five nearby SNRs with combining all distance estimates from Byun et al. 2006) and the
the 100-m Robert C. Byrd Green Bank Telescope (GBT) in SNR (4.4 kpc, Cordes et al. 2002) differ by a factor of three.
Green Bank, West Virginia, USA, to search for new neutron stars The characteristic age of the pulsar, 1.7 Myr, is also two orders
by their radio pulsations. The SNRs chosen for this survey are all of magnitude greater than the age of the SNR (16 kyr). If we
a result of core collapse supernovae (cc-SNe) and are expected require the age of the pulsar to be consistent with that of the
to produce a compact object after the explosion. Several sim- SNR, its birth spin period would be similar to the current one of
ilar SNRs were searched for X-ray bright compact objects by 0.445s. This is extremely slow when compared to the typically
Kaplan et al. (2004). However, no X-ray bright compact sources estimated pulsar birth periods (Faucher-Giguere & Kaspi 2006),
were detected in the remnants. This survey attempts to search suggesting that the pulsar has to be far older than the estimated
a subset of the remnants studied by Kaplan et al. (2006b) and SNR age. Due to the above reasons, the pulsar is not believed
detect potential neutron stars through their radio pulses. In this to be associated with the remnant (Janssen et al. 2009). Further
paper, we report our results of the survey of five SNRs. searches for an associated radio pulsar by Biggs & Lyne (1996)
and Lorimer et al. (1998), with flux density limits of 13 mJy
(400 MHz) and 1 mJy (606 MHz), respectively, were unsuccess-
2. Targeted supernova remnants ful. However, it should be pointed out that a handful of central
In this section, we discuss the important properties and previous compact objects (CCOs) with clear associations with SNRs
pulsar searches of the five remnants targeted in our survey. Table dubbed ‘anti-magnetars’ have been discovered to be young, but
1 provides the distance, size, and age of the SNRs. These SNRs slowly rotating pulsars (Gotthelf et al. 2013). A measurement
were chosen due to a suitable combination of relatively small of the proper motion of PSR J2047+5029 would help clarify its
distance (less than 3 kpc), low inferred age (less than 20000 possible association.
years), small angular extent, and low sky and remnant fluxes. SNR G116.9 + 0.2 (also known as CTB 1, Wilson & Bolton
These factors imply that full coverage of the remnants using a 1960) is an oxygen-rich, mixed-morphology SNR. It has a
reasonable number of pointings and deep radio luminosity limits complete shell in both optical and radio. The uniform optical
can be achieved for all of them. and radio shells that define CTB 1 are indicative of a blast
SNR G53.6 − 2.2 (also known as 3C 400.2) is a mixed- wave extending into a relatively uniform interstellar medium
morphology SNR, that is, it consists of a radio shell as well (Lazendic & Slane 2006), which may provide a kick velocity to
as centrally brightened X-ray emission (Broersen & Vink 2015). the compact object produced in the explosion. A radio pulsar sur-
G53.6−2.2 was searched for pulsars by Gorham et al. (1996) us- vey by Lorimer et al. (1998) did not yield a positive result. How-
ing the Arecibo 305-m telescope at 430 MHz. They used long- ever, a gamma-ray pulsar, PSR J0002+6216 (Clark et al. 2017),
duration observations (20 minutes to 2 hr) with rapid sampling to was recently detected by the Einstein@Home survey of uniden-
reach a sensitivity of 0.2 mJy. However, no pulsars were detected tified Fermi-LAT sources. Zyuzin et al. (2018) suggested an as-
in the survey. sociation with the remnant due to the consistency between the
SNR G78.2 + 2.1 (also known as γ Cygni) is a shell-type distance of the pulsar and the SNR. This association has recently
SNR that has been imaged in radio waves to gamma rays. The been confirmed by Schinzel et al. (2019), who measured a high
SNR is located in the Cygnus X star-forming region (Leahy et al. proper motion and a long bow-shock pulsar wind nebula that
2013). Radio observations of G78.2+2.1 show that the radio di- both point away from the SNR. While the pulsar is radio-loud
ameter of the remnant is approximately 60 arcmin (Higgs et al. and has been observed in the S and L bands by the Effelsberg
1977). A gamma-ray source, 2CG 078+2, was discovered in the Telescope (Wu et al. 2018), we were unable to detect this pulsar
field of the remnant with the COS B satellite (Swanenburg 1981). in our survey as it is out of our field of view (see also Section 5).
This unidentified source was suspected to be a pulsar or to be the SNR G156.2 + 5.7 (also known as RX04591+5147) was
interactions of accelerated energetic particles with matter and ra- initially discovered in X-ray with an X-ray astronomy satel-
diation (Bykov et al. 2000). Around the time of acquiring our lite, ROSAT (Pfeffermann et al. 1991). G156.2+5.7 has a
observations, a blind search with the Fermi Large Area Tele- spherical shell and is one of the brightest SNRs in X-rays
scope (LAT) established the unidentified source as a radio-quiet (Pfeffermann et al. 1991). Lorimer et al. (1998) searched the
gamma-ray pulsar, PSR J2021+4026 (Abdo et al. 2009). Mea- remnant with the 76-m Lovell telescope, but no pulsars were de-
surement of the neutral hydrogen column density using the X- tected. There are no compact objects associated with the remnant
ray spectrum of PSR J2021+4026 is consistent with that of the to date.
diffused emission located in the central and south-eastern part
of the SNR (Hui et al. 2015). It was also noted that these values 3. Observation and data reduction
agree with the neutral hydrogen column density inferred from
the HI radio absorption spectrum (Leahy et al. 2013). These re- The five SNRs discussed above were observed with the GBT
sults indicate that the pulsar emission, diffuse X-ray emission, (Proposal ID:GBT/10B–044). The survey was conducted with
and the radio shell are at the same distance, and hence they the Prime Focus (PF1) receiver, set to the 680–920 MHz fre-
support the association of the pulsar with the remnant. Targeted quency band with the 200 MHz bandwidth intermediate fre-
Article number, page 2 of 5S.Sett et al.: A search for radio pulsars in five nearby supernova remnants
Total SSNR Average Required Previous
Dist Size Age Number of T
SNR 3 observation sky (Jy/ Smin 2D velocity Smin
(kpc) (’) (10 yr) pointings (K)
time (hr) beam) (mJy) (km/s) (mJy)
G53.6−2.2 2.8a 33 7b 2 4.5 14.11 1.92 0.11 1900 0.2
G78.2+2.1 1.5c 60 7d 5 6.8 39.56 22.13 0.15 1800 2.4
G89.0+4.7 1.7e 105 16 f 10 7.0 16.41 3.71 0.14 1600 1.0
G116.9+0.2 1.6g 34 7h 7 2.3 10.58 1.76 0.19 1100 0.8
G156.2+5.7 1.3i 110 15 j 27 7.3 7.31 0.10 0.16 1300 0.7
Table 1. Supernova remnants targeted in this survey without compact objects and with distances less than 3 kpc. Ages are from Sedov-phase
approximations using X-ray temperatures. The size is the diameter of the remnants. The total observation time is the time each remnant has been
observed. Tsky is the sky temperature calculated from Haslam et al. (1982) assuming a spectral index of −2.6 and an 820 MHz observing frequency.
The SSNR is the flux of the SNR per pointing calculated using data from Green (2017). Also shown are the average flux density thresholds of the
remnants, the required velocity of the pulsar to escape the field of view studied in the survey, and the previous recorded minimum flux densities. All
the SNRs except G53.6−2.2 were observed by Lorimer et al. (1998) down to the minimum flux densities given in the column ’Previous Smin ’. SNR
G53.6−2.2 was observed by (Gorham et al. 1996) down to a sensitivity limit of 0.2 mJy. References are: a) Giacani et al. (1998), b) Goss et al.
(1975), c) Landecker et al. (1980), d) Leahy et al. (2013), e) Byun et al. (2006), f) Lazendic & Slane (2006), g) Yar-Uyaniker et al. (2004), h)
Hailey & Craig (1994), i) Pfeffermann et al. (1991), and j) Borkowski et al. (2001).
quency(IF) filter mode, feeding the Green Bank Ultimate Pulsar low f = 100 Hz. For our shortest (longest) pointings, this range
Processing Instrument(GUPPI) back end (DuPlain et al. 2008). corresponds to z = ±506 m s−2 (z = ±26 m s−2 ) and n = ±160
The field of view of the receiver is 12.5 arcmin. The whole spa- (n = ±704) acceleration steps.
tial extent of the remnants was observed with this configuration We selected candidates from the acceleration search that had
to account for the possible offset position of the compact ob- a PRESTO-reported significance above 6σ and a signal-to-noise
ject due to large kick velocities. The chosen configuration has ratio above 5 and removed duplicate and harmonically related
a large beam size and hence minimises the pointings required candidates. Remaining candidate signals were folded and visu-
for the complete survey. It also arguably provides a balance be- ally inspected to classify them as radio interference or promising
tween background sky temperature and signal, given that pulsars pulsar candidates.
usually have shallower spectral indices than the sky background We estimated the sensitivity of our survey by applying the
(i.e −1.4 vs. −2.6, respectively, Bates et al. 2013; Haslam et al. pulsar version of the radiometer equation to find the limiting flux
1982). Table 1 shows the total number of pointings required to density given by (Lorimer & Kramer 2004),
cover the whole spatial extent of the SNRs and the total obser- √ !
vation time for each remnant. β (S /N)min D T sys + T sky
S min = √ + S snr , (2)
The data were processed with PRESTO (Ransom 2001). The nBT obs G
sampling time was 61.44 µs, and the number of channels was
2048. We used 128 subbands in order to strike a balance be- where β = 1.5 is a predetermined factor due to losses and system
tween computational efficiency and survey sensitivity. The de- imperfections and (S /N)min = 5 is the minimum signal to noise
dispersion plan was created using PRESTO’s ddplan routine. at which the pulsar is expected to be detected. With our observ-
While the NE2001 model (Cordes et al. 2002) predicts a max- ing setup, the number of polarisations was n = 2, the instrument
imum dispersion measure(DM) of 500 pc cm−3 for all the rem- bandwidth B = 200 MHz, the system temperature T sys = 29 K,
nants in this survey, we chose to search DMs up to 2000 pc cm−3 , and gain G = 2 K Jy−1 . The latter two were assumed as per GBT
in steps of 0.03 pc cm−3 below DM = 300 pc cm−3 and in steps specifications1 . The sky temperature, T sky , was calculated from
of 0.05 pc cm−3 above, to account for potential extra contribu- Haslam et al. (1982) assuming a power law spectrum with an
tions from the remnant surroundings. The de-dispersed data was exponent −2.6 and 820 MHz observing frequency. The S SNR is
then fast Fourier transformed to search for periodicity. the flux density of the SNR per beam and was calculated using
An acceleration search was also performed to search for bi- data from Green (2017). Finally, we assumed a pulse duty cy-
nary pulsars. The maximum acceleration of a binary system with cle D = 0.05 and used the appropriate integration time of each
orbital period P is pointing, T obs .
!1/3 !4/3 The de-dispersed time series were also searched for sin-
Gm3c 2π gle pulses using PRESTO’s single_pulse_search.py python rou-
z= , (1) tine (Ransom 2001). No excesses of significant candidate pulses
m2tot P
were detected in the searches towards any SNR.
where mc is the mass of the companion and mtot is the total
mass of the system. A pulsar with spin frequency f experienc-
ing a constant acceleration has an apparent spin-down rate of 4. Results
f˙ = f z/c. For an integration time of T obs , this acceleration range No new pulsars were detected in this survey. However, we were
must be searched with step size ∆ f˙ = 1/T obs 2
, while acceleration able to blindly re-detect PSR J2047+5029 in SNR G89.0+4.7, at
searches lose sensitivity if T obs & P/10 (Ransom 2001). Assum- a DM of 107.104 pc cm−3 , with a significance of 11.8σ, a S/N
ing a canonical pulsar mass of 1.4 M⊙ , we therefore designed of 6.5, and an estimated flux density of 0.2 mJy. As discussed
our search to be sensitive to binaries with companions lighter in Section 2, this pulsar is not believed to be associated with the
than a neutron star (mc < 1.4 M⊙ ), orbital periods at least five
1
times longer than the integration times, and spin frequencies be- https://science.nrao.edu/facilities/gbt/proposing/GBTpg.pdf
Article number, page 3 of 5A&A proofs: manuscript no. main
SNR. The flux density of this pulsar estimated by Janssen et al. and therefore the pulsar could remain undetectable despite hav-
(2009) is 2.5 mJy at a central frequency of 328 MHz. If we as- ing a large beam. However, in this case, it may be possible that
sume a power law spectrum S ( f ) ∝ f α , our detection yields a the pulsar is visible in gamma rays due the gamma-ray beam typ-
spectral index α = −1.9, which is compatible with the typical ically covering a larger range of latitudes. This is the accepted
range for pulsars, −3 < α < −1.3 (Bhat et al. 2018). explanation for the non-detection of the radio-quiet gamma-ray
We were unable to detect the radio-quiet pulsar pulsar PSR J2021+4026 in SNR G78.2+2.1. The other gamma-
PSR J2021+4026, either blindly or by folding using the ray pulsar, PSR J0002+6216 in SNR G116.9+0.2, was not de-
gamma-ray timing ephemeris (Abdo et al. 2013). Using Equa- tected here due to its large angular distance from the SNR, which
tion (2), we find a flux density limit of 0.08 mJy at a S/N of we discuss later. We searched the Fermi-LAT fourth source cata-
6, which is an order of magnitude better than the survey by logue (The Fermi-LAT collaboration 2019) for any unidentified
Trepl et al. (2010). PSR J0002+6216, the gamma-ray, radio- gamma-ray source within a radius of one degree from each of
faint pulsar associated with SNR G116.9+0.2, was also not the five SNRs studied in our survey, but we were unable to find
detected due to its location outside the SNR and our beams (see any such source that could be classified as a pulsar other than the
Section 2). two already known gamma-ray pulsar associations.
We calculated the upper limits on the flux density of the sur- One of the other possibilities is that the pulsar’s magnetic
vey for each SNR using Equation 2. We also calculated the 2D field may take a considerable amount of time to develop. If
minimum velocity required for a pulsar to travel from the cen- the growth timescale is 105 years or more, then even a rapidly
tre of the SNR to the approximate edge of the surveyed region spinning neutron star could still be undetectable (Bonanno et al.
(which provides slightly more coverage than the SNR extent) 2005; Blandford & Romani 1988). In this case, even if a pul-
in a time corresponding to the estimated age of the remnant re- sar were present in the remnant, it would not be emitting radio
ported in Table 1. These limits are also reported in Table 1. waves and should be detected as a CCO. Good examples sup-
porting such a scenario are the young CCOs RX J0822-4300 in
SNR Puppis A (Gotthelf & Halpern 2008) and the faint CCO in
5. Discussion SNR Cas A (Tananbaum 1999). However, these neutron stars are
still hot from their birth and so should be detectable in X-rays.
The average flux density limits of our survey can be com- Of the SNRs searched in this survey that remain unassociated
pared with the previously reported limits given in Table 1 and with a pulsar, SNR G156.2+5.7 has been searched in X-ray for
discussed for each individual SNR in Section 2. Overall, we compact objects down to a limit of 1032 ergs s−1 (Kaplan et al.
achieved sensitivity limits between ∼2 and 16 times deeper than 2006b). However, no obvious X-ray sources that could be neu-
previous surveys. Furthermore, these past surveys only cover the tron stars were detected. In order for neutron stars present in
central regions of the SNRs where the pulsars are expected to be these SNRs to be too faint to be observed in X-rays, they would
born. In comparison, we surveyed the whole spatial extent of the require a cooling process that differs from the predicted cool-
SNRs. ing processes of young pulsars, such as those in Vela (Page et al.
Despite improving on the flux density limit, it is likely that 1996) and 3C 58 (Slane et al. 2002). For example, Kaplan et al.
low radio luminosity is a primary factor accounting for the non- (2006b) suggest that if a neutron star is massive enough to sup-
detection of pulsars in the empty SNRs. Not all pulsars of the port direct Urca (beta decay and electron capture, Potekhin et al.
pulsar population could be detected in our survey. For example, 2015), then the appearance of superfluidity as it cools would lead
the young gamma-ray pulsars J0106+4855 and J1907+0602 to a powerful neutrino emission that accelerates the cooling, al-
are extremely faint in radio, their radio flux density being around lowing it to become invisible within decades.
3 µJy (Abdo et al. 2013). We also computed pseudo-luminosity Another plausible explanation is that the neutron star formed
upper limits (Lmin = S min d2 ) using the values of d and S min in the supernova explosion has undergone a large velocity
provided in Table 1. We compared these to the luminosities kick and is no longer in our viewing field. Such kicks may
of the known pulsars in the Australia National Telescope Fa- make it hard to associate the pulsar with a nearby SNR (Lai
cility(ATNF) catalogue2 (Manchester et al. 2005), extrapolating 2004). The minimum kick velocities that pulsars in the SNRs
the 400 MHz and 1400 MHz luminosities reported therein to our in this survey would have had to experience to escape our
800 MHz observing frequency assuming a spectral index of −2. surveyed regions are given in Table 1. They all exceed 1000
About 10% of the pulsars (in accordance with our most sensitive km/s. Faucher-Giguere & Kaspi (2006) predicted that most pul-
pointing) that have reported luminosities at 400 MHz and 1400 sars should have kick velocities slower than 400 km/s, which is
MHz and are not a part of a binary system would not have been well below the velocities given in Table 1. However, extreme
detected by our survey. Since the low-luminosity pulsar popu- cases are known, such as PSR B2011+38 with a velocity of
lation is severely under-sampled (as they are by nature harder ∼ 1600 km/s (Hobbs et al. 2005), as well as several others with
to discover), we conclude that it is quite likely that these SNRs more poorly constrained distances (e.g. PSR B2224+65 in the
contain pulsars that are simply too faint to be detected by our Guitar Nebula, 1640 km/s; Chatterjee & Cordes 2004). A similar
survey. explanation accounts for the non-detection of PSR J0002+6216
There are other possibilities that could account for the lack in SNR G116.9+0.2, which was recently found to be trav-
of a neutron star in an SNR. For instance, it is possible that a pul- elling at about 1100 km/s away from the remnant’s centre
sar lies within the remnant but that the radiation is not beamed (Schinzel et al. 2019). This is thought to be due to the hydro-
towards the observer. The typical beaming fraction is assumed to dynamic instabilities in the supernova explosion.
be ≈ 20 % in the radio band (Manchester 2007). A larger beam- Even though most of the remnants studied are transparent
ing fraction for the young population of pulsars, as suggested by at radio wavelengths, it is possible that the immediate envi-
Ravi et al. (2010), would make it more difficult to reconcile with ronment of the central star has a relatively high gas density,
our results. Observations show that pulsar beams can be patchy, which would cause unusually large scattering and absorption
(Staveley-Smith et al. 2014). An example of such scattering is
2
https://www.atnf.csiro.au/research/pulsar/psrcat/ that of the Crab Pulsar in the Crab Nebula (Driessen et al. 2019).
Article number, page 4 of 5S.Sett et al.: A search for radio pulsars in five nearby supernova remnants
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