<p>Everything rotates at some rate. The rotation-powered pulsars mentioned in ProfRob's answer need to spin fast to generate their radiation. Accretion-powered pulsars do not, so they may be observed at slower rotation rates. The slowest one known is <a href="https://academic.oup.com/mnras/article/469/3/3056/3800702?login=false" rel="noreferrer">AX J1910.7+0917</a>, at a period of about 36,200 seconds. This is extremely slow for such a dense object, with angular momentum/mass of ~0.001% of Earth.</p>
<p>Added in response to comment:</p>
<p>The pulsed emission from an accretion-powered pulsar is thermal. The pulsar accretes matter from a binary companion. The pulsar's magnetic field channels the accreted matter, which impacts and heats the pulsar at hot spots. The changing visibility of the hot spots as the pulsar rotates is observed as pulsating x-rays. If the magnetic field is strong enough, it can also channel the x-rays into beams, enhancing the effect.</p>
<p>Over the short term, accretion-powered pulsars are very regular. Over the long term, the accretion torques the pulsar, changing its period. As an extreme example, <a href="https://articles.adsabs.harvard.edu/pdf/1981apj...243..257d" rel="noreferrer">GX1+4's period decreased at ~2% per year through the 1970's.</a></p>

CC BY-SA 4.0
2022-03-03T15:06:58.183Z
Quite interesting.  Could you provide an explanation of how accretion pulsars generate output, and what, if any kind of regularity these pulses follow (vs the sinus rhythm of rotation pulsars)?
Carl Witthoft
CC BY-SA 4.0
2022-03-03T16:37:03.330Z
@CarlWitthoft See edit.
John Doty
CC BY-SA 4.0
2022-03-03T17:35:19.623Z
Nice - I didn't know about this object. It's hard to accrete without spinning up or down in one direction or the other.
ProfRob
CC BY-SA 4.0
2022-03-03T17:56:11.503Z
@ProfRob That one's extreme, but there are quite a few with periods of minutes, much longer than your 8 seconds. GX1+4, GX301-2, GX304-1, Vela X-1 off the top of my head.
John Doty
CC BY-SA 4.0
2022-03-03T18:07:29.780Z
@JohnDoty the 8s limit applies to *single* neutron stars; as I said. The slow rotators in binaries have been slowed by accretion torques from a close companion.
ProfRob
CC BY-SA 4.0
2022-03-03T18:10:23.340Z
@ProfRob The question was about neutron stars in general, so a limit specific to isolated neutron stars is irrelevant.
John Doty
CC BY-SA 4.0
2022-03-03T18:11:24.737Z
And 36,000s is still rotating as fast as Jupiter.
ProfRob
CC BY-SA 4.0
2022-03-03T18:21:02.047Z
@ProfRob But it's extremely slow for such a compact, dense object. Very low specific angular momentum.
John Doty
CC BY-SA 4.0
2022-03-04T12:40:54.117Z
By the way,  I recommend "Dragon's Egg" by Robert Forward.  It's an enjoyable ride through life on a neutron star.
Carl Witthoft
CC BY-SA 4.0
2022-03-03T14:49:54.580Z
2022-03-03T16:36:47.190Z
2022-03-03T16:36:47.190Z
John Doty
<p>The absence of evidence of spin cannot be evidence for the absence of spin.</p>
<p>We lose the ability to measure the spin of single neutron stars when they slow down below the <a href="https://astrobites.org/2011/03/25/a-pulsar-alone/" rel="noreferrer">pulsar &quot;death line&quot;</a> at rotation periods above about 5-10 seconds.</p>
<p>Neutron stars in binaries are often measured to be fast rotators, either as (millisecond) pulsars or by monitoring the rotation of accretion hot spots. Neutron stars in binaries can also be slowed by accretion torques and these have the slowest <em>measured</em> rotation rates.</p>
<p>The vast majority of neutron stars are not pulsars and are not in close binary systems. They are cool, small and relatively dark. The problem in answering your question is that neutron stars also cool down as they spin down and the slow rotators you seek might be members of this effectively invisible population.</p>

CC BY-SA 4.0
2022-03-03T13:25:09.093Z
*at rotation periods above about 8 seconds*; implying that some stars complete a full rotation *faster* than every 8 seconds - incredible.
stevec
CC BY-SA 4.0
2022-03-03T13:28:06.997Z
@stevec there are neutron stars that spin at a speed close to the speed of light
Kavin Ishwaran
CC BY-SA 4.0
2022-03-03T14:51:40.913Z
Ah, but there are accretion-powered pulsars that rotate much more slowly. See my answer.
John Doty
CC BY-SA 4.0
2022-03-03T16:21:46.287Z
@stevec e.g. see https://astronomy.stackexchange.com/questions/1291/what-is-the-fastest-spinning-rotation-of-a-neutron-star - the term "milli-second pulsar" exists for a reason.
planetmaker
CC BY-SA 4.0
2022-03-04T00:27:12.213Z
@stevec [PSR B1919+21](https://en.wikipedia.org/wiki/PSR_B1919%2B21), the first pulsar to be discovered (by Jocelyn Bell Burnell on 28 November 1967), has a period of 1.3373 seconds. The period of the famous [Crab pulsar](https://en.wikipedia.org/wiki/Crab_Pulsar) PSR B0531+21 (discovered in 1968) is 33.5028583 milliseconds.
PM 2Ring
CC BY-SA 4.0
2022-03-05T02:26:50.433Z
Couldn't it be [tidally locked](https://en.wikipedia.org/wiki/Tidal_locking#Stars)? That is, a tidally locked neutron star would see changes to its orbital period different from one that is not tidally locked? Doppler shift of the radiation from a visible companion star could be used measure the orbital period.
Peter Mortensen
CC BY-SA 4.0
2022-03-06T10:15:32.837Z
@PeterMortensen Tidally locked stars neutron stars would be rapidly rotating. Their orbital periods (and hence rotation periods) would have to be very short to enable tidal locking in a Hubble time.
ProfRob
CC BY-SA 4.0
2022-03-03T08:21:25.617Z
2022-03-03T21:33:50.433Z
2022-03-03T21:33:50.433Z
ProfRob
<p>Neutron stars are one type of remnant of a giant star's core after its collapse. Neutron stars tend to rotate at very high speed and the mismatch between its axis of rotation and magnetic pole make it a &quot;pulsar&quot;. Over time, the rotational energy is lost and may come to a static state, no rotation.</p>
<p>Have we observed any static neutron star(s)?</p>

CC BY-SA 4.0
2022-03-03T18:32:59.367Z
The issue with this question might be similar to asking "Is anything *exactly* 1 meter long? Even when it's right in front of you, let alone thousands of light years away. Maybe draw a line somewhere in rotation?
DKNguyen
CC BY-SA 4.0
2022-03-03T20:23:47.380Z
This might be a duplicate of https://astronomy.stackexchange.com/q/19927/2153.
HDE 226868
CC BY-SA 4.0
2022-03-03T22:41:39.223Z
If the angular deceleration depends on the angular velocity, then only an exponential deceleration will happen and not a complete stall.
peterh
CC BY-SA 4.0
2022-03-04T02:22:01.100Z
@DKNguyen but rotational energy levels are quantized.  Ortho-hydrogen must rotate.  :)
DavePhD
CC BY-SA 4.0
2022-03-04T09:57:10.120Z
@HDE226868 I don't feel so. asking "will it work" is totally different from asking "have you saw it working"
Kavin Ishwaran
<neutron-star>
Have we observed any static neutron stars?
Kavin Ishwaran
# Have we observed any static neutron stars?

(Asked by: username_0 on 2022-03-03)

Neutron stars are one type of remnant of a giant star's core after its collapse. Neutron stars tend to rotate at very high speed and the mismatch between its axis of rotation and magnetic pole make it a "pulsar". Over time, the rotational energy is lost and may come to a static state, no rotation.

Have we observed any static neutron star(s)?

**Tags:** neutron-star
> **Comments:**
> **username_1**: The issue with this question might be similar to asking "Is anything *exactly* 1 meter long? Even when it's right in front of you, let alone thousands of light years away. Maybe draw a line somewhere in rotation?  
> **username_2**: This might be a duplicate of https://astronomy.stackexchange.com/q/19927/2153.  
> **username_3**: If the angular deceleration depends on the angular velocity, then only an exponential deceleration will happen and not a complete stall.  
> **username_4**: @DKNguyen but rotational energy levels are quantized.  Ortho-hydrogen must rotate.  :)  
> **username_0**: @HDE226868 I don't feel so. asking "will it work" is totally different from asking "have you saw it working"  

---

## 2 answers

### Answer by username_7 on 2022-03-03. Score: 25 | Accepted answer

The absence of evidence of spin cannot be evidence for the absence of spin.

We lose the ability to measure the spin of single neutron stars when they slow down below the [pulsar "death line"](https://astrobites.org/2011/03/25/a-pulsar-alone/) at rotation periods above about 5-10 seconds.

Neutron stars in binaries are often measured to be fast rotators, either as (millisecond) pulsars or by monitoring the rotation of accretion hot spots. Neutron stars in binaries can also be slowed by accretion torques and these have the slowest *measured* rotation rates.

The vast majority of neutron stars are not pulsars and are not in close binary systems. They are cool, small and relatively dark. The problem in answering your question is that neutron stars also cool down as they spin down and the slow rotators you seek might be members of this effectively invisible population.

> **Comments:**
> **username_8**: *at rotation periods above about 8 seconds*; implying that some stars complete a full rotation *faster* than every 8 seconds - incredible.  
> **username_0**: @stevec there are neutron stars that spin at a speed close to the speed of light  
> **username_5**: Ah, but there are accretion-powered pulsars that rotate much more slowly. See my answer.  
> **username_9**: @stevec e.g. see https://astronomy.stackexchange.com/questions/1291/what-is-the-fastest-spinning-rotation-of-a-neutron-star - the term "milli-second pulsar" exists for a reason.  
> **username_10**: @stevec [PSR B1919+21](https://en.wikipedia.org/wiki/PSR_B1919%2B21), the first pulsar to be discovered (by Jocelyn Bell Burnell on 28 November 1967), has a period of 1.3373 seconds. The period of the famous [Crab pulsar](https://en.wikipedia.org/wiki/Crab_Pulsar) PSR B0531+21 (discovered in 1968) is 33.5028583 milliseconds.  
> **username_11**: Couldn't it be [tidally locked](https://en.wikipedia.org/wiki/Tidal_locking#Stars)? That is, a tidally locked neutron star would see changes to its orbital period different from one that is not tidally locked? Doppler shift of the radiation from a visible companion star could be used measure the orbital period.  
> **username_7**: @PeterMortensen Tidally locked stars neutron stars would be rapidly rotating. Their orbital periods (and hence rotation periods) would have to be very short to enable tidal locking in a Hubble time.  

---

### Answer by username_5 on 2022-03-03. Score: 14

Everything rotates at some rate. The rotation-powered pulsars mentioned in ProfRob's answer need to spin fast to generate their radiation. Accretion-powered pulsars do not, so they may be observed at slower rotation rates. The slowest one known is [AX J1910.7+0917](https://academic.oup.com/mnras/article/469/3/3056/3800702?login=false), at a period of about 36,200 seconds. This is extremely slow for such a dense object, with angular momentum/mass of ~0.001% of Earth.

Added in response to comment:

The pulsed emission from an accretion-powered pulsar is thermal. The pulsar accretes matter from a binary companion. The pulsar's magnetic field channels the accreted matter, which impacts and heats the pulsar at hot spots. The changing visibility of the hot spots as the pulsar rotates is observed as pulsating x-rays. If the magnetic field is strong enough, it can also channel the x-rays into beams, enhancing the effect.

Over the short term, accretion-powered pulsars are very regular. Over the long term, the accretion torques the pulsar, changing its period. As an extreme example, [GX1+4's period decreased at ~2% per year through the 1970's.](https://articles.adsabs.harvard.edu/pdf/1981apj...243..257d)

> **Comments:**
> **username_6**: Quite interesting.  Could you provide an explanation of how accretion pulsars generate output, and what, if any kind of regularity these pulses follow (vs the sinus rhythm of rotation pulsars)?  
> **username_5**: @CarlWitthoft See edit.  
> **username_7**: Nice - I didn't know about this object. It's hard to accrete without spinning up or down in one direction or the other.  
> **username_5**: @ProfRob That one's extreme, but there are quite a few with periods of minutes, much longer than your 8 seconds. GX1+4, GX301-2, GX304-1, Vela X-1 off the top of my head.  
> **username_7**: @JohnDoty the 8s limit applies to *single* neutron stars; as I said. The slow rotators in binaries have been slowed by accretion torques from a close companion.  
> **username_5**: @ProfRob The question was about neutron stars in general, so a limit specific to isolated neutron stars is irrelevant.  
> **username_7**: And 36,000s is still rotating as fast as Jupiter.  
> **username_5**: @ProfRob But it's extremely slow for such a compact, dense object. Very low specific angular momentum.  
> **username_6**: By the way,  I recommend "Dragon's Egg" by Robert Forward.  It's an enjoyable ride through life on a neutron star.  

---

